CBSE Class 12 Physics Ray Optics Optical Instruments Worksheet

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. How can the chromatic aberration be corrected    

(a) By providing different suitable curvature to its two surface
(b) By combining it with another lens of opposite nature
(c) By reducing its aperture
(d) By providing proper polishing of its two surfaces

Answer :   B

Question. When a beam of light from air enters into the water, the characteristics of light will not be changed, is :     
(a) frequency
(b) speed
(c) colour
(d) amplitude

Answer :   A

Question. If two mirrors are kept at 45° to each other and a body is placed in the middle then total number of images formed is :   
(a) 7
(b) 8
(c) 14
(d) 4

Answer :   A

Question. An astronomical telescope of ten fold angular magnification has a length of 44 cm. The focal length of the objective is :   
(a) 44 cm
(b) 440 cm
(c) 40 cm
(d) 4 cm

Answer :   C

Question. The refractive index of diamond is 2.0, velocity of light in diamond in cm per second is approximately :    
(a) 1.5 × 10¹⁰
(b) 2.0 × 10¹⁰
(c) 6 × 10¹⁰
(d) 3 × 10¹⁰

Answer :   A

Question. The critical angle for the material of a prism is 45°and its refracting angle is 30°. A monochromatic ray goes out perpendicular to the surface of emergence from the prism. Then the angle of incidence on the prism will be :   
(a) 60°
(b) 75° 
(c) 45°
(d) 30°

Answer :   C

Question. Mirage is a phenomenon due to :     
(a) refraction of light
(b) diffraction of light
(c) total internal reflection of light
(d) none of these

Answer :   C

Question. In an astronomical microscope, the focal length of the objective is made :  
(a) shorter than that of the eye piece
(b) greater than that of the eye piece
(c) half of the eye piece
(d) equal to that of the eye piece

Answer :   B

Question. A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens.
Then its focal length will
(a) become zero
(b) become infinite
(c) become small, but non-zero
(d) remain unchanged.

Answer :  B

Question. An astronomical telescope of tenfold angular magnification has a length of 44 cm. The focal length of the objective is
(a) 44 cm
(b) 440 cm
(c) 4 cm
(d) 40 cm

Answer :  D

Question. Exposure time of camera lens at f/2.8 setting is 1/200 second. The correct time of exposure at f/5.6 is
(a) 0.20 second
(b) 0.40 second
(c) 0.02 second
(d) 0.04 second. 

Answer :  C

Question. For a plano convex lens (m = 1.5) has radius of curvature 10 cm. It is silvered on its plane surface.
Find focal length after silvering.
(a) 10 cm
(b) 20 cm
(c) 15 cm
(d) 25 cm

Answer :  A

Question. The focal length of converging lens is measured for violet, green and red colours. It is respectively fv, fg, fr . We will get
(a) fv < fr
(b) fg > fr
(c) fv = fg
(d) fg < fr 

Answer :  A

Question. If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power?
(a) + 7.5 D
(b) –0.75 D
(c) + 6.5 D
(d) –6.5 D 

Answer :  B

Question. If fV and fR are the focal lengths of a convex lens for violet and red light respectively and FV and FR are the focal lengths of a concave lens for violet and red light respectively, then we must have
(a) fV > fR and FV > FR
(b) fV < fR and FV > FR
(c) fV > fR and FV < FR
(d) fV < fR and FV < FR

Answer :  B

Question. A plano convex lens is made of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is    
(a) 200 cm
(b) 100 cm
(c) 50 cm
(d) 400 cm 

Answer   B

Question. A luminous object is placed at a distance of 30 cm from the convex lens of focal length 20 cm. On the other side of the lens, at what distance from the lens a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it ?   
(a) 50 cm
(b) 30 cm
(c) 12 cm
(d) 60 cm

Answer   A

Question. The focal length of converging lens is measured for violet, green and red colours. It is respectively fv, fg, fr . We will get   
(a) fv < fr
(b) fg > fr
(c) fv = fg
(d) fg < fr 

Answer   A

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. An equiconvex lens is cut into two halves along     
(i) XOX′ and
(ii) YOY′ as shown in the figure.
Let f, f ′, f ′′ be the focal lengths of the complete lens, of each half in case (i), and of each half in case (ii), respectively. Choose the correct  statement from the following.
(a) f ′ = f, f ′′ = 2f
(b) f ′ = 2f, f ′′ = f
(c) f ′ = f, f ′′ = f
(d) f′ = 2f, f ′′ = 2f
Answer   A

Question. The angle of a prism is A. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2A on the first surface returns back through the same path after suffering reflection at the silvered surface. The refractive index m, of the prism is   
(a) 2sinA
(b) 2cosA
(c) 1/2cos A 
(d) tanA 
Answer   B

Question. For the angle of minimum deviation of a prism to be equal to its refracting angle, the prism must be made of a material whose refractive index    
(a) lies between 2 and 1
(b) lies between 2 and 2
(c) is less than 1
(d) is greater than 2 
Answer   B

Question. The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are   
(a) 10 cm, 10 cm
(b) 15 cm, 5 cm
(c) 18 cm, 2 cm
(d) 11 cm, 9 cm. 
Answer   C

Question. Four lenses of focal length ±15 cm and ±150 cm are available for making a telescope. To produce the largest magnification, the focal length of the eyepiece should be   
(a) +15 cm
(b) +150 cm
(c) –150 cm
(d) –15 cm
Answer   A

Question. A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens.   
Then its focal length will
(a) become zero
(b) become infinite
(c) become small, but non-zero
(d) remain unchanged. 
Answer   B

Question. A bulb is located on a wall. Its image is to be obtained on a parallel wall with the help of convex lens. The lens is placed at a distance d ahead of second wall, then required focal length will be    
(a) only d/4
(b) only d/2
(c) more than d/4 but less than d/2
(d) less than d /4
Answer   B

Question. An astronomical telescope of tenfold angular magnification has a length of 44 cm. The focal length of the objective is   
(a) 44 cm
(b) 440 cm
(c) 4 cm
(d) 40 cm 
Answer   D

Question. A lens is placed between a source of light and a wall.    
It forms images of area A1 and A2 on the wall, for its two different positions. The area of the source of light is
(a) A₁ - A₂ /2
(b) 1/A₁ + 1/A₂
(c) √A₁ A₂
(d) A₁ + _A2 /2
Answer   C

Question. Exposure time of camera lens at f/2.8 setting is 1/200 second. The correct time of exposure at f/5.6 is    
(a) 0.20 second
(b) 0.40 second
(c) 0.02 second
(d) 0.04 second. 
Answer   C

Question. For a plano convex lens (m = 1.5) has radius of curvature 10 cm. It is silvered on its plane surface.   
Find focal length after silvering.
(a) 10 cm
(b) 20 cm
(c) 15 cm
(d) 25 cm  
Answer   A

Question. A lens of refractive index 1.5 is dipped in to a medium of refractive index 1.5. Then the new refractive index of lens
a) 1.5
b) 1.5x1.5
c) 1
d) 1.8
Answer. C

Question. A convex lens has focal length f. It is cut in to two equal parts perpendicular to its principal axis. The focal length of each part is
a) f/2
b) f
c) 3/2 f
d) 2f
Answer. D

Question. The brilliance of diamond is due to
a) Reflection
b) Refraction
c) TIR
d) Scattering
Answer. C

Question. The angle of the prism is 300 .The rays incident at 600 at one refracting face suffer a deviation of 300. The angle of emergence is
a) 0
b) 30
c) 60
d) 90
Answer. A

Question. Light travelling from a transparent medium to air undergoes TIR at an angle of incidence 450. Then the refractive index of the medium may be
a) 1.5
b) 1.4
c) 1.1
d) 1/√2
Answer. B

Question. A convex lens is made of three different materials as shown in figure. For a point object on the principal axis, the number of images formed are
a) 5
b) 1
c) 3
d) 4
Answer. C

Question. A compound microscope has two lenses. The magnifying power of one is 5 and the combined magnifying power is 100. The magnifying power of the other lens is
a) 10
b) 20
c) 50
d) 25
Answer. B

Question. A convex lens of focal length 20 cm produces a real image twice the size of the object. Then the distance of the object from the lens is
a) 10 cm
b) 20 cm
c) 30 cm
d) 60 cm
Answer. C

Question. An astronomical telescope has two lenses of powers 0.5 D and 20 D. Then its magnifying power will be
a) 8
b) 20
c) 30
d) 40
Answer. D

1 MARK QUESTIONS

Question. When a converging and diverging lens of equal focal length are combined what is the power of the combination?
Answer. P=0, P=1/f1+1/f2

Question. How does the angle of minimum deviation of a glass prism vary if the incident violet light is replaced by red light?
Answer. Decreases. dR<dV

Question. For the same angle of incidence, the angle of refraction in two media A and B are 25° and 35° respectively. In which one of the two media is the speed of light lesser?
Answer. 𝑣𝐴<𝑣𝐵

Question. An unsymmetrical double convex thin lens forms the image of a point object on its axis. Will the position of the image change if the lens is reversed?
Answer. No change. Principle of reversibility of light.

Question. State the condition under which a large magnification can be achieved in an astronomical telescope.
Answer. focal length of objective must be greater than focal length of eyepiece. M=fo/fe

Question. A lens of power of –4.0 D what is its focal length? Identify the nature of the lens.
Answer. f=-25cm,concave

2 MARKS QUESTIONS

Question. Focal length of a convex lens of refractive index 1.5 is 2 cm. What will be the Focal length of lens, when immersed in a liquid of refractive index of 1.25 ?
Answer. 1/𝑓 =(n-1)2/𝑅 ,1/2=(1.5-1)*2/R, R=2cm
1/𝑓=(1.5/1.25-1)*2/R, f=5cm

Question. A ray of light passes through an equilateral glass prism such that the angle of incidence is equal to angle of emergence and each of these angles is equal to 3/4 of angle of prism. What is the value of angle of deviation?
Answer. In prism i + e = A + D = and i= e=3/4A
A+D=3/4A+3/4A, D=A/2=30°

Question. A spherical convex surface of radius of curvature 20 cm, made of glass (n = 1.5) is placed in air. Find the position of the image formed, if a point object is placed at 30 cm in front of the convex surface on the principal axis.
Answer. 𝑛₂/𝑣 -𝑛₁/𝑢=(𝑛₂−𝑛₁)/𝑅
𝑉=−180𝑐𝑚

Question. Calculate the speed of light in a medium whose critical angle is 45°. Does critical angle for a given pair of media depend on wave length of incident light? Give reason.
Answer. n=1/sinic=1/sin45=√2
Yes, critical angle for a pair of media depends on wavelength, because n=a+b/λ² where a and b are constants of the media.

Question. Write two conditions necessary for total internal reflection to take place.
Answer. (a) The incident ray should travel from the denser to the rarer medium.
(b) The angle of incidence, in the denser medium, should be greater than the critical angle for the given pair of media.

CASE BASED QUESTION

1.An optical fibre is extremely thin and long strand of very fine quality glass or quartz coated with the thin layer of a material of refractive index less than refractive index of strand. They are used as a guided medium for transmission of signals over long distances. The inner layer is called core and outer layer is called cladding. In order to provide the safety and strength the core cladding of optical fibre, it is enclosed in a plastic jacket called as buffer coating.

Question. Optical fibres are used extensively to transmit
a) Optical Signal
b) current
c) Sound waves
d) None of the above
Answer. A

Question. Which of the following statement is not true.
a) Optical fibres is based on the principle of total internal reflection.
b) The refractive index of the material of the core is less than that of the cladding.
c) An optical fibre can be used to act as an optical pipe.
d) There is no appreciable loss in the intensity of the light signal while propagating through an optical fibre
Answer. B

Question. In which of the following the principle of total internal reflection is not used.
a) Endoscope
b) Mirage
c) Total reflection prism
d) Periscope
Answer. D

Question. The basic reason for the extraordinary sparkle of suitably cut diamond is that
(a) It has low refractive index
(b) It has high transparency
(c)It has high refractive index
(d) It is very hard
Answer. C

Question. A substance has critical angle 45° for yellow light. What is its refractive index?
a) √2
b) 1/√2
c) 2
d) ½
Answer. A

2.Power of a lens is given by the reciprocal of focal length (f) of the lens ie P= 1/f where f is in metre power in dioptre. For a convex lens power is positive and for concave lens power is negative. When a number of thin lenses of powers P1, P2,P3…. Are held in contact with each other, power of the combination is given by the algebraic sum of the powers of all the lenses . ie P= P1 + P2+ P3+………..

Question. When two thin lenses of equal and opposite focal lengths are placed in contact then what is the power of the combination
a) 0
b) α
c) P1 + P2
d) P1 -P2
Answer. A

Question. Two thin lenses are in contact and focal length of the combination is 80 cm. If the focal length of one is 20 cm , what would be the power of the other lens
a) 2 D
b) 3.75 D
c) -3.75 D
d) 0
Answer. C

Question. What is the focal length and power of a plane mirror
a) 0, α
b) 0, 1
c) α, 0
d) α, 1
Answer. C

Question. Three lenses of powers 3D, -0.5 D and 1.5 D are placed in contact. Will the combination produce converging or diverging effect
a) converging effect
b) Diverging effect
c) no effect
d) sometimes converging sometimes diverging
Answer. A

3.The criteria for image formation by a mirror or a lens is , mimimum two reflected or refracted light rays actually meet at a point or appears to meet at a point. The image formed by actal meeting of light rays is called real image. When two light rays appears to meet at point , the image formed is called virtual image. Real image can be focussed on screen but virtual image cannot be focussed on screen. Real image always inveted while virtual image is erect.

Question. A convex lens is of focal length 20 cm is cut in to two equal halves horizotally parallel to principal axis. Then the focal length of each halves
a) 10 cm, 10 cm
b) 5 cm, 5 cm
c) 20 cm , 20 cm
d) 20 cm , 10 cm
Answer. C

Question. The half portion ( both sides) of a convex lens is painted with black colour, the what happens to the image
a) the image will not be formed
b) the size of the image will be half
c)image will be formed but its intensity will be half
d) blurred image will be formed
Answer. C

Question. A concave mirror and a convex lens are immerced in water, then what happens to focal length
a) focal length of both will not change
b) focal length of both will change
c) mirror focal length will not change but lens focal length will increases
d) ) mirror focal length will not change but lens focal length will decreases
Answer. C

Question. A convex lens of focal length 15 cm and concave lens of focal length 15 cm combined together then the combination will acts as
a) converging
b) diverging
c) neither converging nor diverging
d) it will not pass light rays
Answer. C

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. Light appears to travel in a straight line, because  
(a) its wavelength is very small 
(b) its velocity is large
(c) it is not absorbed by surroundings
(d) it is reflected by surroundings

Answer :   A

Question. Sky appears to be blue in clear atmosphere due to which property of light :   
(a) Scattering
(b) Polarization
(c) Diffraction
(d) Dispersion

Answer :   A
 
Question. A doctor prescribes spectacles to a patient with a combination of a convex lens of focal length 40 cm, and concave lens of focal length 25 cm then the power of spectacles will be :  
(a) – 6.5 D
(b) 1.5 D
(c) – 1.5 D
(d) – 8.5 D

Answer :   C

Question. When a ray of light enters a glass slab, then    
(a) its frequency and wavelength changes
(b) its frequency does not change
(c) only frequency changes
(d) its frequency and velocity changes

Answer :   B

Question. An equilateral prism is made of a material of refractive index 3 . The angle of minimum deviation for the prism is     
(a) 90°
(b) 60°
(c) 45°
(d) 30°

Answer :   B

Question. A concave mirror having the focal length 15 cm, forms an image having twice of the linear dimensions of the object. If the image is virtual, then the position of the object will be :  
(a) 7.5 cm
(b) 22.5 cm
(c) 40 cm
(d) 30 cm

Answer :   A

Question. An air bubble in a glass slab with refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3 cm deep when viewed from the opposite face. The thickness (in cm) of the slab is
(a) 8
(b) 10
(c) 12
(d) 16. 

Answer :   C

Question. A microscope is focussed on a mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again?
(a) 2 cm upward
(b) 1 cm upward
(c) 4.5 cm downward
(d) 1 cm downward

Answer :   B

Question. Four lenses having the focal length of + 15 cm, 20 cm, +150 cm, and +250 cm respectively are provided to make an astronomical telescope. The focal length of the eyepiece to produce the largest magnification, should be :    
(a) + 250 cm
(b) + 155 cm
(c) 25 cm
(d) + 15 cm

Answer :   D

Question. The Cauchy’s dispersion formula is    
(a) h = A + B l2 + Cl4
(b) μ = A + B l–2 + Cl4
(c) μ = A + B l2 + Cl–4
(d) μ = A + B l–2 + Cl–4

Answer :   D

Question. Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be

(a) 10 cm
(b) 2.5 cm
(c) 5 cm
(d) 7.5 cm

Answer :  C

Question. The refracting angle of a prism is A, and refractive index of the material of the prism is cot (A/2). The angle of minimum deviation is
(a) 90° – A
(b) 180° + 2A
(c) 180° – 3A
(d) 180° – 2A

Answer :  D

Question. The angle of a prism is A. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2A on the first surface returns back through the same path after suffering reflection at the silvered surface. The refractive index m, of the prism is
(a) 2sinA
(b) 2cosA
(c) 1/2 cos A
(d) tanA

Answer :  B

Question. For the angle of minimum deviation of a prism to be equal to its refracting angle, the prism must be made of a material whose refractive index
(a) lies between √2 and 1
(b) lies between 2 and √2
(c) is less than 1
(d) is greater than 2

Answer :  B

Question. A ray of light is incident on a 60° prism at the minimum deviation position. The angle of refraction at the first face (i.e., incident face) of the prism is
(a) zero
(b) 30°
(c) 45°
(d) 60°

Answer :  B

Question. If the refractive index of a material of equilateral prism is √3, then angle of minimum deviation of the prism is
(a) 60°
(b) 45°
(c) 30°
(d) 75°

Answer :  A

Question. Pick the wrong answer in the context with rainbow.
(a) Rainbow is a combined effect of dispersion, refraction and reflection of sunlight.
(b) When the light rays undergo two internal reflections in a water drop, a secondary rainbow is formed.
(c) The order of colours is reversed in the secondary rainbow.
(d) An observer can see a rainbow when his front is towards the sun. 

Answer :  D

Question. If the focal length of objective lens is increased then magnifying power of    
(a) microscope will increase but that of telescope decrease
(b) microscope and telescope both will increase
(c) microscope and telescope both will decrease
(d) microscope will decrease but that of telescope will increase 

Answer   D

Question. A thin prism having refracting angle 10° is made of glass of refractive index 1.42. This prism is combined with another thin prism of glass of refractive index 1.7. This combination produces dispersion without deviation. 
The refracting angle of second prism should be    
(a) 6°
(b) 8°
(c) 10°
(d) 4°

Answer   A

Question. The reddish appearance of the sun at sunrise and sunset is due to    
(a) the scattering of light
(b) the polarisation of light
(c) the colour of the sun
(d) the colour of the sky. 

Answer   A

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. An object is immersed in a fluid. In order that the object becomes invisible, it should :    
(a) behave as a perfect reflector
(b) absorb all light falling on it
(c) have refractive index one
(d) have refractive index exactly matching with that of the surrounding fluid

Answer :   D

Question. Sodium lamps are used in foggy conditions because :    
(a) yellow light is scattered less by the fog particles
(b) yellow light is scattered more by the fog particles
(c) yellow light is unaffected during its passage through the fog
(d) Wavelength of yellow light is the mean of the visible part of the spectrum

Answer :   A

Question. In refraction, light waves are bent on passing from one medium to the second medium, because, in the second medium:    
(a) the frequency is different
(b) the coefficient of elasticity is different
(c) the speed is different
(d) the amplitude is smaller

Answer :   C

Question. 1 wire mesh consisting of very small squares is viewed at a distance of S cm through a magnifying converging lens of focal length 10 cm, kept close to the eye. The magnification produced by the lens is:    
(a) 5
(b) 8
(c) 10
(d) 20

Answer :   A

Question. An achromatic convergent lens of focal length 20 cms is made of two lenses (in contact) of materials having dispersive powers in the ratio of 1 : 2 and having focal lengths f1 and f2. Which of the following is true ?  
(a) f1 = 10 cms, f2 = –20 cms
(b) f1 = 20 cms, f2 = 10cms
(c) f1 = –10 cms, f2 = –20 cms
(d) f1 = 20 cms, f2 = –20 cms

Answer :   A

Question. A ray of light is incident at an angle of incidence, i, on one face of prism of angle A (assumed to be small) and emerges normally from the opposite face. If the refractive index of the prism is m, the angle of incidence i, is nearly equal to  
(a) μA
(b) μA/2
(c) μ/A
(d) A/2μ

Answer :   A

Question. A lens is made of flint glass (refractive index = 1.5). When the lens is immersed in a liquid of refractive index 1.25, the focal length:    
(a) increases by a factor of 1.25
(b) increases by a factor of 2.5
(c) increases by a factor of 1.2
(d) decreases by a factor of 1.2

Answer :   B

Question. A leaf which contains only green pigments, is illuminated by a laser light of wavelength 0.6328 mm. It wound appear to be:    
(a) brown
(b) black
(c) red
(d) green

Answer :   B

Question. In total internal reflection when the angle of incidence is equal to the critical angle for the pair of media in contact, what will be angle of refraction?
(a) 90°
(b) 180°
(c) 0°
(d) equal to angle of incidence 

Answer :   A

Question. Which of the following is not due to total internal reflection?
(a) Working of optical fibre
(b) Difference between apparent and real depth of a pond
(c) Mirage on hot summer days
(d) Brilliance of diamond 

Answer :   B

Question. The focal length of the objective and eye lenses of a microscope are 1.6 cm and 2.5 cm respectively.    
The distance between the two lenses is 21.7 cm.
If the final image is formed at infinity, what is the linear magnification? 
(a) 11
(b) 110
(c) 1.1
(d) 44

Answer :   B

Question. The camera lens has an aperture of f and the exposure time is (1/60)s. What will be the new exposure time if the aperture become 1.4 f ?    
(a) 1/42
(b) 1/56
(c) 1/72
(d) 1/31

Answer :   D

Question. Which colour of the light has the longest wavelength?
(a) violet
(b) red
(c) blue
(d) green 

Answer :  B

Question. A thin prism having refracting angle 10° is made of glass of refractive index 1.42. This prism is combined with another thin prism of glass of refractive index 1.7. This combination produces dispersion without deviation.
The refracting angle of second prism should be
(a) 6°
(b) 8°
(c) 10°
(d) 4°

Answer :  A

Question. The reddish appearance of the sun at sunrise and sunset is due to
(a) the scattering of light
(b) the polarisation of light
(c) the colour of the sun
(d) the colour of the sky. 

Answer :  A

Question. A thin prism of angle 15° made of glass of refractive index m1 = 1.5 is combined with another prism of glass of refractive index m2 = 1.75. The combination of the prisms produces dispersion without deviation.
The angle of the second prism should be
(a) 5°
(b) 7°
(c) 10°
(d) 12°

Answer :  C

Question. Rainbow is formed due to
(a) scattering and refraction
(b) internal reflection and dispersion
(c) reflection only
(d) diffraction and dispersion. 

Answer :  B

Question. The blue colour of the sky is due to the phenomenon of
(a) scattering
(b) dispersion
(c) reflection
(d) refraction.

Answer :  A

Question. A plano convex lens is made of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is
(a) 200 cm
(b) 100 cm
(c) 50 cm
(d) 400 cm 

Answer :  B

Question. A luminous object is placed at a distance of 30 cm from the convex lens of focal length 20 cm. On the other side of the lens, at what distance from the lens a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it ?
(a) 50 cm
(b) 30 cm
(c) 12 cm
(d) 60 cm

Answer :  A

Question. A thin prism of angle 15° made of glass of refractive index m₁ = 1.5 is combined with another prism of glass of refractive index m₂ = 1.75. The combination of the prisms produces dispersion without deviation.

The angle of the second prism should be   
(a) 5°
(b) 7°
(c) 10°
(d) 12°

Answer   C

Question. Rainbow is formed due to    
(a) scattering and refraction
(b) internal reflection and dispersion
(c) reflection only
(d) diffraction and dispersion. 

Answer   B

Question. The blue colour of the sky is due to the phenomenon of    
(a) scattering
(b) dispersion
(c) reflection
(d) refraction.

Answer   A

Question. A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use, will be    
(a) convex, +2.25 dioptre
(b) concave, –0.25 dioptre
(c) concave, –0.2 dioptre
(d) convex, +0.15 dioptre. 

Answer   B

Question. A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance    
(a) 50.0 cm
(b) 54.0 cm
(c) 37.3 cm
(d) 46.0 cm

Answer   B

Question. For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is 20 D. Using this information, the distance between the retina and the cornea-eye lens can be estimated to be    
(a) 1.67 cm
(b) 1.5 cm
(c) 5 cm
(d) 2.5 cm 

Answer   A

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. A thin glass (refractive index 1.5) lens has optical power of – 5 D in air. Its optical power in a liquid medium with refractive index 1.6 will be    
(a) – 1D
(b) 1 D
(c) – 25 D
(d) 25 D

Answer :   B

Question. A fish looking up through the water sees the outside world contained in a circular horizon. If the refractive index of water is 4/3 and the fish is 12 cm below the surface, the radius of this circle in cm is   
(a) 36/√7
(b) 36 √7
(c) 4√ 5
(d) 36 √5

Answer :   A

Question. A fish in an aquarium, 30 cm deep in water can see a light bulb kept 50 cm above the surface of water. The fish can also see the image of this bulb in the reflecting bottom surface of the aquarium. Total depth of water is 60 cm. Then the apparent distance between the two images seen by the fish is (mw = 4/3)     
(a) 140 cm
(b) 760/3 cm
(c) 280/3 cm
(d) 380/3 cm

Answer :   B

Question. The focal length of the objective and eye piece of a telescope are respectively 200 cm and 5 cm.  The maximum magnifying power of the telescope will be 
(a) – 100
(b) – 60
(c) – 48
(d) – 40

Answer :   C

Question. A thin prism P1 with angle 6° and made froM glass of refractive index 1.54 is combined with another thin prism P2 of refractive index 1.72 to produce dispersion without deviation. The angle of prism P2 will be    
(a) 4° 30'
(b) 8.5°
(c) 6.5°
(d) none of these

Answer :   A

Question. The focal length of a converging lens are fV and fR for violet and red light respectively. Then   
(a) fV > fR
(b) fV = fR
(c) fV < fR
(d) any of the three is
possible depending on the value of the average refractive index m

Answer :   C

Question. A plano-convex lens of focal length 30 cm has its plane surface silvered. An object is placed 40 cm from the lens on the convex side. The distance of the image from the lens is  
(a) 18 cm
(b) 24 cm
(c) 30 cm
(d) 40 cm

Answer :   B

Question. An endoscope is employed by a physician to view the internal parts of a body organ. It is based on the principle of :    
(a) refraction
(b) reflection
(c) total internal reflection
(d) dispersion

Answer :   C

Question. A telescope has an objective lens of focal length 200 cm and an eye piece with focal length 2 cm.
If this telescope is used to see a 50 metre tall building at a distance of 2 km, what is the height of the image of the building formed by the objective lens?    
(a) 5 cm
(b) 10 cm
(c) 1 cm
(d) 2 cm

Answer :   A

Question. A man 160 cm high stands in front of a plane mirror. His eyes are at a height of 150 cm from the floor. Then the minimum length of the plane mirrorfor him to see his full length image is  
(a) 85 cm
(b) 170 cm
(c) 80 cm
(d) 340 cm

Answer :   C

Question. A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use, will be
(a) convex, +2.25 dioptre
(b) concave, –0.25 dioptre
(c) concave, –0.2 dioptre
(d) convex, +0.15 dioptre. 

Answer :  B

Question. A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance
(a) 50.0 cm
(b) 54.0 cm
(c) 37.3 cm
(d) 46.0 cm.

Answer :  B

Question. If the focal length of objective lens is increased then magnifying power of 
(a) microscope will increase but that of telescope decrease
(b) microscope and telescope both will increase
(c) microscope and telescope both will decrease
(d) microscope will decrease but that of telescope will increase

Answer :  D

Question. For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is 20 D. Using this information, the distance between the retina and the cornea-eye lens can be estimated to be
(a) 1.67 cm
(b) 1.5 cm
(c) 5 cm
(d) 2.5 cm

Answer :  A

Question. The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are
(a) 10 cm, 10 cm
(b) 15 cm, 5 cm
(c) 18 cm, 2 cm
(d) 11 cm, 9 cm. 

Answer :  C

Question. Four lenses of focal length ±15 cm and ±150 cm are available for making a telescope. To produce the largest magnification, the focal length of the eyepiece should be
(a) +15 cm
(b) +150 cm
(c) –150 cm
(d) –15 cm

Answer :  A

Question. The refractive index of the material of a prism is √2 and the angle of the prism is 30°. One of the two refracting surfaces of the prism is made a mirror inwards, by silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is
(a) 60°
(b) 45°
(c) 30°
(d) zero

Answer   B

Question. The angle of incidence for a ray of light at a refracting surface of a prism is 45°. The angle of prism is 60°. If the ray suffers minimum deviation through the prism, the angle of minimum deviation and refractive index of the material of the prism respectively, are   
(a) 45 ; √2
(b) 30; 1/√2
(c) 45; 1/√2
(d) 30;   √2

Answer   D

Question. The refracting angle of a prism is A, and refractive index of the material of the prism is cot (A/2). The angle of minimum deviation is   
(a) 90° – A
(b) 180° + 2A
(c) 180° – 3A
(d) 180° – 2A

Answer   D

Question. A ray of light is incident on a 60° prism at the minimum deviation position. The angle of refraction at the first face (i.e., incident face) of the prism is    
(a) zero
(b) 30°
(c) 45°
(d) 60°

Answer   B

Question. If the refractive index of a material of equilateral prism is √3, then angle of minimum deviation of the prism is    
(a) 60°
(b) 45°
(c) 30°
(d) 75°

Answer   A

Question. Pick the wrong answer in the context with rainbow.    
(a) Rainbow is a combined effect of dispersion, refraction and reflection of sunlight.
(b) When the light rays undergo two internal reflections in a water drop, a secondary rainbow is formed.
(c) The order of colours is reversed in the secondary rainbow.
(d) An observer can see a rainbow when his front is towards the sun. 

Answer   D

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. A green light is incident from the water to the air - water interface at the critical angle (q). Select the correct statement.    

(a) The entire spectrum of visible light will come out of the water at an angle of 90º to the normal.
(b) The spectrum of visible light whose frequency is less than that of green light will come out to the air medium.
(c) The spectrum of visible light whose frequency is more than that of green light will come out to the air medium.
(d) The entire spectrum of visible light will come out of the water at various angles to the normal.

Answer :   B

Question. An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be
(a) 30 cm away from the mirror
(b) 36 cm away from the mirror
(c) 30 cm towards the mirror
(d) 36 cm towards the mirror. 

Answer :   B

Question. A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source. The beam is reflected back as a spot on a scale placed just above the source L. When the mirror is rotated through a small angle q, the spot of the light is found to move through a distance y on the scale. The angle q is given by
(a) y/x
(b) x/2y
(c) x/y
(d) y/2x

Answer :   D

Question. Two plane mirrors are inclined at 70°. A ray incident on one mirror at angle, q after reflection falls on second mirror and is reflected from there parallel to first mirror. The value of q is
(a) 45°
(b) 30°
(c) 55°
(d) 50°

Answer :   D

Question. A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such a way that its end closer to the pole is 20 cm away from the mirror. The length of the image is
(a) 10 cm
(b) 15 cm
(c) 2.5 cm
(d) 5 cm. 

Answer :   D

Question. A tall man of height 6 feet, want to see his full image. Then required minimum length of the mirror will be
(a) 12 feet
(b) 3 feet
(c) 6 feet
(d) any length

Answer :   B

Question. A beam of light composed of red and green ray is incident obliquely at a point on the face of rectangular glass slab. When coming out on the opposite parallel face, the red and green ray emerge from
(a) two points propagating in two different non parallel directions
(b) two points propagating in two different parallel directions
(c) one point propagating in two different directions
(d) one point propagating in the same directions.

Answer :   B

Question. A ray of light travelling in air have wavelength l, frequency n, velocity v and intensity I. If this ray enters into water then these parameters are l′, n′, v′ and I′ respectively. Which relation is correct from following?
(a) l = l′
(b) n = n′
(c) v = v′
(d) I = I′ 

Answer :   B

Question. A bubble in glass slab (m = 1.5) when viewed from one side appears at 5 cm and 2 cm from other side, then thickness of slab is
(a) 3.75 cm
(b) 3 cm
(c) 10.5 cm
(d) 2.5 cm 
9.4 Total Internal Reflection

Answer :   C

Question. A ray of light travelling in a transparent medium of refractive index m, falls on a surface separating the medium from air at an angle of incidence of 45°.
For which of the following value of m the ray can undergo total internal reflection?
(a) m = 1.33
(b) m = 1.40
(c) m = 1.50
(d) m = 1.25

Answer :   C

Question. If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power?   
(a) + 7.5 D
(b) –0.75 D
(c) + 6.5 D
(d) –6.5 D 

Answer   B

Question. Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be    
(a) 10 cm
(b) 2.5 cm
(c) 5 cm
(d) 7.5 cm

Answer   C

Question. If fV and fR are the focal lengths of a convex lens for violet and red light respectively and FV and FR are the focal lengths of a concave lens for violet and red light respectively, then we must have    
(a) fV > fR and FV > FR
(b) fV < fR and FV > FR
(c) fV > fR and FV < FR
(d) fV < fR and FV < FR

Answer   B

Important Questions for NCERT Class 12 Physics Ray Optics and Optical Instruments

Question. Optical fibre are based on
(a) total internal reflection
(b) less scattering
(c) refraction
(d) less absorption coefficient. 

Answer :  A

Question. A ray is incident at an angle of incidence i on one surface of a small angle prism (with angle of prism A) and emerges normally from the opposite surface. If the refractive index of the material of the prism is m,  then the angle of incidence is nearly equal to
(a) A/2m
(b) 2A/m
(c) mA
(d) mA/2 

Answer :  C

Question. The refractive index of the material of a prism is 2 and the angle of the prism is 30°. One of the two refracting surfaces of the prism is made a mirror inwards, by silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is
(a) 60°
(b) 45°
(c) 30°
(d) zero

Answer :  B

Question. A disc is placed on a surface of pond which has refractive index 5/3. A source of light is placed 4 m below the surface of liquid. The minimum radius of disc needed so that light is not coming out is,
(a) ∞
(b) 3 m
(c) 6 m
(d) 4 m

Answer :  B

Question. Light enters at an angle of incidence in a transparent rod of refractive index n. For what value of the refractive index of the material of the rod, the light once entered into it will not leave it through its lateral face whatsoever be the value of angle of incidence?
(a) n = 1.1
(b) n = 1
(c) n > 2
(d) n = 1.3

Answer :  C

Question. A small source of light is 4 m below the surface of water of refractive index 5/3. In order to cut off all the light, coming out of water surface, minimum diameter of the disc placed on the surface of water is
(a) 6 m
(b) ∞
(c) 3 m
(d) 4 m

Answer :  A

Question. Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F1.
When the space between the two lenses is filled with glycerin (which has the same refractive index (m = 1.5) as that of glass) then the equivalent focal length is F2. The ratio F1 : F2 will be
(a) 3 : 4
(b) 2 : 1
(c) 1 : 2
(d) 2 : 3

Answer :  C

Question. Two identical glass (mg = 3/2) equiconvex lenses of focal length f each are kept in contact. The space between the two lenses is filled with water (mw = 4/3). The focal length of the combination is
(a) f/3
(b) f
(c) 4f/3
(d) 3f/4

Answer :  D

Question. When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index
(a) equal to that of glass
(b) less than one
(c) greater than that of glass
(d) less than that of glass.

Answer :  A

Question. A concave mirror of focal length f1 is placed at a distance of d from a convex lens of focal length f2. A beam of light coming from infinity and falling on this
convex lens – concave mirror combination returns to infinity. The distance d must equal
(a) f1 + f2
(b) –f1 + f2
(c) 2f1 + f2
(d) –2f1 + f2

Answer :  C

Question. A biconvex lens has a radius of curvature of magnitude 20 cm. Which one of the following options describe best the image formed of an object of height 2 cm placed 30 cm from the lens?
(a) Virtual, upright, height = 1 cm
(b) Virtual, upright, height = 0.5 cm
(c) Real, inverted, height = 4 cm
(d) Real, inverted, height = 1 cm 

Answer :  C

Question. A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed, the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is
(a) 5 cm
(b) –10 cm
(c) 20 cm
(d) –30 cm 
Answer :  D

Question. A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is 1.39 × 109 m and its mean distance from the earth is 1.5 × 1011 m. What is the diameter of the sun’s image on the paper?
(a) 6.5 × 10^–5 m
(b) 12.4 × 10^–4 m
(c) 9.2 × 10^–4 m
(d) 6.5 × 10^–4 m

Answer :  C

Question. A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses.The power in diopters of the combination is
(a) zero
(b) 25
(c) 50
(d) infinite.

Answer :  A

Question. A ray is incident at an angle of incidence i on one surface of a small angle prism (with angle of prism A) and emerges normally from the opposite surface. If the refractive index of the material of the prism is m, then the angle of incidence is nearly equal to    
(a) A/2m
(b) 2A/m
(c) mA
(d) mA/2 

Answer   C

Question. Which colour of the light has the longest wavelength?    
(a) violet
(b) red
(c) blue
(d) green 

Answer   B

Question. In an astronomical telescope in normal adjustment a straight black line of length L is drawn on inside part of objective lens. The eye-piece forms a real image of this line. The length of this image is I. The magnification of the telescope is    
(a) L + I / L - I
(b) L / I
(c) L / I +1
(d) L / I -1

Answer   B