CBSE Class 10 Science HOTs Reflection and Refraction

Chapter Light : Reflaction And Refraction

MCQ Questions for NCERT Class 10 Science Light Reflection and Refraction

Question. An object at a distance of + 15 cm is slowly moved towards the pole of a convex mirror. The image will get
(a) shortened and real
(b) enlarged and real
(c) enlarge and virtual
(d) diminished and virtual

Answer : D

Question. Focal length of plane mirror is
(a) at infinity
(b) zero
(c) negative
(d) none of these

Answer : A

Question. The object distance in both concave as well as convex mirror is
(a) negative
(b) positive
(c) zero
(d) none of these

Answer : A

Question. The unit of power of lens is
(a) metre
(b) centimeter
(c) diopter
(d) m–1

Answer : C

Question. The power of a lens is + 1.6 D. The nature of lens is
(a) Convex lens
(b) Concave lens
(c) both concave an convex
(d) none of these

Answer : A

Question. If the object is placed at focus of a concave mirror, the image is formed at
(a) infinity
(b) focus
(c) centre of curvature
(d) between F and O.

Answer : A

Question. The radius of curvature and focal length of a concave mirror are
(a) positive
(b) negative
(c) both
(d) none of these

Answer : B

Question. The refractive indices of some media are given below:
Medium       Refractive index
X                           1.51
Y                           1.72
Z                           1.83
W                          2.42
In which of these is the speed of light minimum and maximum, respectively.
(a) X-minimum, W-maximum
(b) Z-minimum, W-maximum
(c) W-minimum, X-maximum
(d) X-minimum, Z-maximum

Answer : C

Question. A student does the experiment on tracing the path of a ray of light passing through a rectangular glass slab for different angles of incidence. He can get a correct measure of the angle of incidence and the angle of emergence by following the labelling indicated in figure:
(a) I
(b) II
(c) III
(d) IV

Answer : D

Question. The angle of reflection in the given figure is

(a) 90°
(b) 180°
(c) 0°
(d) 30°

Answer : C

Question. When the object is placed between f and 2f of a convex lens, the image formed is
(a) at f
(b) at 2f
(c) beyond 2f
(d) between O and f

Answer : C

Question. Light from the Sun falling on a convex lens will converge at a point called So, the light will travel faster in a medium having lower refractive index.
(a) centre of curvature
(b) focus
(c) radius of curvature
(d) optical centre

Answer : B

Question. The nature of the image formed by concave mirror when the object is placed between the focus (F) and centre of curvature (C) of the mirror observed by us is
(a) real, inverted and diminished
(b) virtual, erect and smaller in size
(c) real, inverted and enlarged
(d) virtual, upright and enlarged

Answer : C

Question. As light travels from a rarer to a denser medium it will have Hence, focal length of concave mirror is 75 cm.
(a) increased velocity
(b) decreased velocity
(c) decreased wavelength
(d) both (b) and (c)

Answer : D

Question. A spherical mirror and a spherical lens each have a focal length of -10 cm. The mirror and the lens are likely to be
(a) both concave
(b) both convex
(c) the mirror is concave and the lens is convex
(d) the mirror is convex and the lens is concave

Answer : A

Question. A10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is 
(a) – 30 cm
(b) – 20 cm
(c) – 40 cm
(d) – 60 cm

Answer : B

Question. For a real object, which of the following can produce a real image?
(a) Plane mirror
(b) Concave mirror
(c) Concave lens
(d) Convex mirror

Answer : B

Question. The angle of incidence i and refraction r are equal in a transparent slab when the value of i is
(a) 0°
(b) 45°
(c) 90°
(d) depend on the material of the slab

Answer : A

Question. If an incident ray passes through the focus, the reflected ray will
(a) pass through the pole
(b) be parallel to the principal axis
(c) retrace its path
(d) pass through the centre of curvature

Answer : B

Question. An object is placed at a distance of 0.25 m in front of a plane mirror. The distance between the object and image will be
(a) 0.25 m
(b) 1.0 m
(c) 0.5 m
(d) 0.125 m

Answer : C

Question. The laws of reflection hold good for 
(a) plane mirror only
(b) concave mirror only
(c) convex mirror only
(d) all mirrors irrespective of their shape

Answer : D

Question. A concave mirror of radius 30 cm is placed in water. It’s focal length in air and water differ by
(a) 15
(b) 20
(c) 30
(d) 0

Answer : D

Question. When object moves closer to convex lens, the image formed by it shift
(a) away from the lens
(b) towards the lens
(c) first towards and then away from the lens
(d) first away and then towards the lens

Answer : A

Question. When object moves closer to a concave lens the image by it shift
(a) away from the lens on the same side of object
(b) toward the lens
(c) away from the lens on the other side of lens
(d) first towards and then away from the lens

Answer : B

Question. If the power of a lens is – 2 D, what is its focal length?
(a) +50 cm
(b) -100 cm
(c) -50 cm
(d) +100 cm

Answer : C

Question. In which of the following, the image of an object placed at infinity will be highly diminished and point sized? [NCERT Exemplar Problems]
(a) Concave mirror only
(b) Convex mirror only
(c) Convex lens only
(d) Concave mirror, convex mirror, concave lens and convex lens.

Answer : D

Question. A light ray enters from medium A to medium B as shown in figure. The refractive index of medium B relative to A will be
(a) greater than unity
(b) less than unity
(c) equal to unity
(d) zero

Answer : B

Question. Which of the following mirror is used by a dentist to examine a small cavity?
(a) Convex mirror
(b) Plane mirror
(c) Concave mirror
(d) Combination of convex and concave mirror

Answer : C

True and False :

Question. Lateral displacement increases with the thickness of the given slab.
Answer: True

Question. Lateral displacement is directly proportional to thickness of the glass slab.
Answer: True

Question. Nature of image formed by mirror gives an ideal about nature of mirror. 
Answer: True

Question. The power of a concave lens is positive.
Answer: False

Question. When incident angle i satisfies n = \(\frac{1}{\sin i}\), the refracted light will pass along the surface. 
Answer: True

Fill in the Blanks :

Question. The SI unit of power is ………. .
Answer: dioptre

Question. A ………. mirror is used as a head mirror by the doctors to concentrate light on the body parts to be examined.
Answer: concave

Question. Light shows the phenomena of reflection, refraction and ………. .
Answer: dispersion

Question. The speed of light in vacuum is ………. .
Answer: 3 x 108 m/s

Question. No matter how far you stand from a spherical mirror, your image appears erect. The mirror may be ………. .
Answer: plane or convex mirror .

Question. ____________ mirror is used as a security mirror in shops and on roads at sharp bends and concealed entrances.
Answer: Convex

Question. A positive sign in the value of magnification indicates that the image is ………. .
Answer: virtual

Important Questions for NCERT Class 10 Science Light Reflection and Refraction

Very Short Answer type Questions :

(1 Mark Questions)

Question. Is light a ray or a wave?
Answer: It is considered in both the forms.

Question. Give the mirror image of “AMBULANCE”
Answer: AMBULANCE

Question. In what condition, the image formed by a concave mirror is virtual?
Answer: When the object is placed between the focus and the pole of a concave mirror, a virtual image is obtained.

Question. Specified the size of image formed by a concave mirror when m > 1.
Answer: The image is enlarged.

Question. An incident ray makes an angle of 60° with the mirror. What is the angle of reflection?
Answer: 60°.

Question. Write any one observation from everyday life which show us that light travels in a straight line.
Answer: A small source of light costs a sharp shadow of an opaque object tells us that light travels in a straight line.

Question. Size of the image formed on a concave mirror is highly diminished, state the position of object and image.
Answer: Position of object: at infinity Position of image : at the focus F

Question. Name the mirror that can be used to check theft in shops.
Answer: Convex mirror.

Question. What is the position of the object placed on the side of reflecting surface of a concave mirror of focal length 15 cm if the image is formed at the distance of 30 cm from the mirror?
Answer: 30 cm

Question. Which mirror, concave or convex always converges the light rays?
Answer: Concave mirror.

Question. What are the characters associated with light as a wave?
Answer: Frequency and wavelength.

Question. What is a ray?
Answer: Ray is a line defining the path of light.

Question. What is the relation connecting focal length and radius of a spherical mirror?
Answer: F = \(\frac{R}{2}\) 

Question. How does image changes when the face is slowly moved away from inner face of a shining spoon?
Answer :  As the face is moved away than after a particular time image becomes inverted.

Question. Due to which property of light, sharp shadow of an object is obtained?
Answer : straight line property of the light.

Question. Identify the type of lens or mirror placed at XY where O is object and I is image.

Answer : Convex lens (when object is placed between pole and focus)

Question. What type of lens must be placed at XY so that image I shifts to I’

Answer : concave lens

Question. A ray AFB is incident on a spherical mirror whose centre of curvature is 2 F. In which direction will it reflect?
Answer : It will reflect towards the object side parallel to principal axis.

Question. A ray of light is incident at angle of 350 to a plane surface. What will the angle of reflection?
Answer :  550

Question. A fish under water is viewing obliquely a fisherman standing on the bank of lake. Does the man look taller or shorter?
Answer : As light travels from rarer to denser medium, it bends towards normal and appears to come from greater height.. Therefore to fish under water man looks taller.

(Two marks Questions)

Question. An Object is placed 15 cm in front of a lens ‘A’ and lens gives real, inverted , magnified image and formed at large distance. Lens ‘A’ is replaced by Lens ‘B’ and a real, inverted image of the same size as of object is formed.
i) What is the nature of Lens A&B?
ii) What is the focal length of A&B?
Answer : i) A=Convex lens ,B=Convex Lens
ii) fA=15cm, fB=7.5cm

Question. Two lenses 1&2 are placed in contact .Focal length of lens 1 is 20 cm and of 2 is -10 cm. Calculate
i) Total Power of combination
ii) What is the nature of combination..
Answer : (i) P= -5 D ,(ii)Concave Lens

Question. For the same angle of incidence the angle of refraction in three different media A, B and C are 300 , 450 and 600 respectively. In which medium will the velocity of light be minimum?
Answer : µA=sin i/sin 30 , µB=sin i/sin45, µC=sin i/sin 60
µA >µB >µC , µ=c/v as µA= max. ,vA=min.

Question. You are given three lenses.
i) a concave lens of focal length 25 cm.
ii) a convex lens of focal length ¼ m and
iii) a convex lens of focal length 100 cm.
Which combination out of these three lenses will form a lens of zero power?
Answer : Combination of concave lens of focal length of 25 cm and a convex lens of focal length of 1/4m

(Three Marks Questions)

Question. A rod of length 10 cm lies along the principal axis of a concave mirror of f= 10 cm in such a way that the end closer to the pole is 20cm away from it. Find the length of image?
Answer : R =2f=20cm. Thus the nearer end B of the rod AB is at C and hence its image will be formed at B itself
For end A u = -30 cm, f = -10 cm, v = -15cm
Length of image will be at 5 cm

Question. Absolute refractive Index of some of material is tabulated below

i) In which of these does light travel fastest and why? 
ii) arrange these materials in ascending order of their optical densities. 
Answer : i) Water due to least refractive index.
ii) Water ,Kerosene, Rock salt ,diamond

Very Short Answer Questions

Question. Define absolute refractive index.
Answer : When medium 1 is vacuum, then refractive index of medium 2 is considered with respect to vacuum. This is called absolute refractive index.

Question. Define optical density.
Answer : The ability of the medium to refract light is called optical density.

Question. State lens formula and write it mathematically.
Answer : The relationship between object distance (u), image distance (v), and focal length of lens is known as lens formula. It is given by
1/v – 1/u = 1/f

Question. Define a lens.
Answer : A transparent material bounded by two surfaces of which one or both surfaces are spherical forms a lens.

Question. Define centre of curvature, principal axis, optical centre, aperture, focus and focal length for a lens.
Answer : (a) Centre of curvature: It is the centre of the spheres of which the each surface of the lens forms a part. Represented by C or 2f.
(b) Principal axis: An imaginary straight line passing through the two centres of curvatures.
(c) Optical centre: It is the central point of the lens. Represented by O.
(d) Aperture: It is the diameter of circular outline of a spherical lens.
(e) Focus: The point at which rays of light parallel to principal axis converges (convex lens) or appears to diverge (concave lens) after refraction. Represented by F.
(f) Focal length: The distance between focus and optical centre is called focal length. It is represented by f.

Question. Define power of a lens.
Answer : The degree of convergence or divergence of light rays achieved by lens is expressed in terms of power. It is given by
P = 1 / f 

f = focal length in metre

Question. What is the magnification of a plane mirror?
Answer : m = +1

Question. Which lens bends a light ray more or less with a shorter or with longer focal length?
Answer : The lens with the shorter focal length bends the light more.

Question. What happens if a light falls on a glass slab making 90° at its surface?
Answer : It undergoes normal refraction that is there is no deviation in the light.

Question. What is silvering of mirror?
Answer : Silvering of mirror means coating the surface of mirror with a thin layer of silver, aluminium or some other shiny, opaque material.

Short Answer Questions

Question. Which kind of mirrors are used in the headlights of a motor-car and why? 
Answer : Concave mirror, to get the parallel beam of light.

Question. Which type of mirrors are used to give an erect and enlarged image of an object?
Answer : Concave mirror.

HOTS Questions and Answers

Question. Where is the image formed in a convex mirror, when the object is anywhere in front of it ?
Answer : Between pole and focus, behind the convex mirror.

Question. A person uses concave mirror for shaving, where should he position his face in front of it ?
Answer : Between pole and principal focus.

Question. A ray of light is incident on a concave mirror along its principal axis. What will be the angle of reflection?
Answer : Angle of reflection = 0

Question. What will happen to ray of light when it travels from rarer medium to a denser medium ?
Answer : Bends towards the normal .

Question. What does negative sign in the value of magnification of a mirror indicate?
Answer : Image is real.

Question. Name the point inside the lens through which a ray of light goes undeviated?
Answer : Optical centre.

Question. Which of the two has a great power? A lens of short focal length or a lens of large focal length?
Answer : Lens of short focal length.

Question. Name the lens which always gives an erect and diminished image?
Answer : Concave lens.

Question. Find the focal length of a lens of power –2.0 D. What type of lens is this?
Answer : Given, Power = –2.0
The given lens is concave (power of concave lens is –ve)
P = 1/f (in metre)
-2.0 = 1/f
f = 1/-2 m
f = - 50 cm
∴ The focal length is – 50 cm.

Question. Give characteristics of image formed by plane mirror
Answer : • Image is virtual and erect.
• Size is same as of the object.
• It is formed at same distance.

Question. Which mirror is used as rear view mirror in vehicles and why ?
Answer : Convex mirror, wider field of view.

Question. Define one dioptre?
Answer : One dioptre is the power of a lens of focal length one meter.

Question. The size of an object is 2cm.The magnification produced by a mirror is +1. What is the size of the image?
Answer : +2cm, because m=I/O , +1=I/2 =+2

Question. When a ray of light passes from a denser medium to a rarer medium which angle is greater: angle of incidence or angle of refraction?
Answer : Angle of refractions.

Question. An image formed in a spherical mirror has magnification -2.Is the image real or virtual?
Answer : Real.

Question. The power of a lens is -2D. Is the lens convex or concave?
Answer : Concave lens.

Question. Focal length of a convex mirror is 10cm.Find the radius of curvature of the mirror?
Answer : 20cm.

Question. An object is placed at a distance of 50cm from a convex mirror. State two characteristics of the image formed.
Answer : (1) Image is virtual and erect. 
(2) Image is diminished.

Question. Write two uses of concave mirror.
Answer : (1) Used as reflectors for automobile headlights. 
(2) Used as shaving mirror.

Question. An object 1cm high produces a real image 1.5 cm high, when placed at a distance of 15 cm from concave mirror. Calculate the position of the image.
Answer : – v/u = h’/h , -v/-15 = -1.5/1 
v = 15x 1.5 = -22.5cm.

Question. Find the power of a concave lens of focal length 2m.
Answer : – p = 1/f 
=1/-2 = -0.5D.

Question. Which phenomenon occurs when light falls on(a) highly polished surface (b) a transparent medium ?
Answer : (a) Reflection of light.
(b) Refraction of light.

Question. What will happen to a ray of light when it falls normally on a surface ?
Answer : No bending of light ray occurs. It means light rays goes straight from one medium to another.

Question. What is absolute refractive index ?
Answer : When first medium is taken as vaccum, the refractive index of second medium is called as absolute refractive index.

Question. The magnification “ m “ for a mirror is +1 what does this signify ?
Answer : (a) Image is of same size as the object. 
(b) Image is virtual and erect .

More Questions for Practice

1. What is angle of incidence?

2. A ray of light passing through centre of curvature of a concave mirror retraces  its path on reflection, Why?

3. An object is placed at the focus of a concave mirror, Where is the imageformed?

4. What is meant by refraction of light?

5. Define principal focus of a concave mirror?

6. State Snell’s law of refraction?

7. Will the lateral displacement increase/decrease if glass block is made more thicker?

8. Why convex lens is called conversing lens?

9. Printed letters appears diminished, when viewed through a lens. What is the nature of lens.?

10. At What angle a ray of light should strike the surface of glass, So that it does not suffer any refraction?

11. Does the value of speed of light change with medium?

12. What is the cause of refraction of light?

13. Which lens is used as a magnifying glass?

14. What is an optically denser medium of light?

15. What is the difference between reflection and refraction?

16. If a ray of light traveling in air is incident on the water surface obliquely, Draw a ray diagram and show the change in its path in water?

17. Define refractive index in terms of a speed of light in two media. What is the unit of refractive index?

18. A ray of light strikes the mirror at 15o , What is the angle of reflection?

19. What is refractive index of air? Why the refractive index of other medium is taken with respected to air?

20. Distinguish between real and virtual images?

21. For what position of an object, a virtual image is formed by a convex lens?
Give ray diagram?

22. Find the position and nature of image formed in a concave mirror for the following position of an object. (a) At infinity (b) Beyond C.

23. An object is placed at a distance of 10cm from convex mirror of focal length 15cm; find the position and nature of image?

24. A thin lens has a focal length of -25cm. What is the power of the lens? Is it convex or concave?

25. Calculate the distance at which an object should be placed in front of convex lens of focal length 10cm to obtain an image double its size?

26. Why a mirror does has one principal focus while a lens has two principal foci?

27. Focal length of the lens in a photographic camera is 5cm.What is the power and nature of the lens?

28. Define linear magnification. Does it have any unit?

29. Why a concave mirror has a real principal focus, while convex mirror has a virtual principal focus?

30. Which of the following lenses would you prefer to use while reading the small letters found in dictionary.
a. A convex lens of focal length 30 cm.
b. A concave lens of focal length 30 cm.
c. A concave lens of focal length 5 cm.
d. A convex lens of focal length 5 cm.

31. Show that the refractive index of a medium 1 with respect to medium 2 is reciprocal to the refractive index of medium 2 with respect to 1 i.e. n₁₂ = 1 / n₂₁

32. From the diagram given below calculate 
a) angle of incidence
b) angle of refraction .
c) the refractive index of the substance X. 30X

33. A man standing in front of special mirror finds his image having a small face,big tummy and legs of normal size .what are the shapes three parts of mirror?

34. A diverging lens of focal length 15cm forms an image of 10cm from the lens.
Draw a scale
diagram for the formation of image.

Very Short Answer Questions

Answer : Convex lens (when object is placed between pole and focus)

 

Question. What type of lens must be placed at XY so that image I shifts to I’ 

Answer : concave lens

 

Question. A ray AFB is incident on a spherical mirror whose centre of curvature is 2 F. In which direction will it reflect?

Answer : It will reflect towards the object side parallel to principal axis.

 

Question. A ray of light is incident at angle of 35⁰ to a plane surface. What will the angle of reflection?

Answer : 55⁰

 

Question. A fish under water is viewing obliquely a fisherman standing on the bank of lake. Does the man look taller or shorter?

Answer : As light travels from rarer to denser medium, it bends towards normal and appears to come from greater height.. Therefore to fish under water man looks taller.

 

Question. The magnification produced by a plane mirror is +1. What does this mean?
Answer : This means that the size of the image is equal to the size of the object.

Question. Name some phenomenon associated with light during image formation by mirrors.
Answer : Reflection.

Question. Define incident ray, reflected ray, normal ray, angle of incidence and reflection.
Answer : Incident ray – light which falls on the mirror/ polished surface is called incident ray.
Reflected ray – ray of light which goes back in the same medium after striking the surface is called reflected ray.
Normal – the perpendicular drawn to the reflecting surface is called normal at that point.
Angle of incidence – the angle between the incident ray and the normal is known angle of incidence.
Angle of reflection – the angle between reflected ray and the normal is known angle of reflection.

Question. What are the properties of image formed by a plane mirror?
Answer : Image is virtual and erect.
• Size of the image is equal to that of object
• Image is laterally inverted.
• The image formed by a plane mirror is always at the same distance as the object is in front of it.

Question. Define pole, centre of curvature, radius of curvature, principal axis, aperture, focus and focal length of a spherical mirror.
Answer : Pole: the centre of reflecting surface. It is represented by letter P.
Centre of Curvature: The centre of the sphere of which the mirror forms the part.
Represented by “C”.
Radius of Curvature: The radius of the sphere of which the mirror forms the part.
Represented by “R”.
Principal axis: The straight line joining the pole (P) and the centre of curvature. It is normal to the mirror at its pole.
Aperture: The diameter of the spherical mirror is called its aperture. The reflecting surface of the mirror. 
Focus: The point of the principal axis at which the rays parallel to principal axis meet (concave mirror) or appear to meet (convex mirror) after reflection. Represented by F.
Focal Length: The distance between the pole and the principal focus of a spherical mirror is called focal length. Represented by f.

Question. Give some uses of concave mirror.
Answer : (a) Used in torches, search lights and vehicle headlights.
(b) Used as shaving mirror.
(c) Used by dentist.
(d) Used in solar furnance.

Question. Give the relation between focal length and radius of curvature.
Answer : f = R/2

Question. Define refraction of light.
Answer : The change in direction of light, when it travels from one medium to another medium is called refraction of light.

Question. What do you observe when light ray passes through rectangular slab?
Answer : (a) Angle of incidence is equal to angle of emergence.
(b) Incident ray is parallel to the emergent ray.
(c) Lateral displacement is proportional to the thickness of glass slab.
(d) Lateral displacement is proportional to the angle of incidence.

Question. Define refractive index.
Answer : Refractive index is defined as the ratio of speed of light in medium 1 to the speed of light in medium 2 and is represented as n₂₁ and is read as refractive index of medium 2 with respect to medium 1.
n₂₁ = speed of light in medium 1/speed of light in medium 2.

Short Answer Questions

Question. An Object is placed 15 cm in front of a lens ‘A’ and lens gives real, inverted magnified image and formed at large distance. Lens ‘A’ is replaced by Lens ‘B’ and a real, inverted image of the same size as of object is formed.

i) What is the nature of Lens A&B?

ii) What is the focal length of A&B?

Answer : i) A=Convex lens ,B=Convex Lens

ii) f_A=15cm, f_B=7.5cm

 

Question. Two lenses 1&2 are placed in contact .Focal length of lens 1 is 20 cm and of 2 is -10 cm. Calculate

i) Total Power of combination

ii) What is the nature of combination..

Answer : (i) P= -5 D ,(ii)Concave Lens

 

Question. For the same angle of incidence the angle of refraction in three different media A, B and C are 30⁰ , 45⁰ and 60⁰ respectively. In which medium will the velocity of light be minimum?

Answer : μ_A=sin i/sin 30 , μ_B=sin i/sin45, μ_C=sin i/sin 60

μ_A >μ_B >μC , μ=c/v as μ_A= max. ,v_A=min.

 

Question. You are given three lenses.

i) a concave lens of focal length 25 cm.

ii) a convex lens of focal length ¼ m and

iii) a convex lens of focal length 100 cm.

Which combination out of these three lenses will form a lens of zero power?

Answer : Combination of concave lens of focal length of 25 cm and a convex lens of focal length of 1/4m

Question. A rod of length 10 cm lies along the principal axis of a concave mirror of f= 10 cm in such a way that the end closer to the pole is 20cm away from it. Find the length of image?

Answer : R =2f=20cm. Thus the nearer end B of the rod AB is at C and hence its image will be formed at B itself

For end A u = -30 cm, f = -10 cm, v = -15cm

Length of image will be at 5 cm

 

Question. Absolute refractive Index of some of material is tabulated below

i) In which of these does light travel fastest and why?

ii) arrange these materials in ascending order of their optical densities. 

Answer : i) Water due to least refractive index.

     ii) Water ,Kerosene, Rock salt ,diamond

Question. What are the properties of the image formed by plane mirror?
Answer : Image formed by plane mirror is always virtual and erect. The size of the image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it and the image is laterally inverted.

Question. Give the uses of concave mirrors.
Answer : (i) Concave mirror are used in torches, search-lights and vehicle headlights to get powerful parallel beams of light.
(ii) They are also used as shaving mirrors to see a large image of the face.
(iii) Dentists use concave mirrors to see large images of the teeth of patients.
(iv) Large concave mirrors are used in making solar furnaces, solar cookers etc.

Question. Give the laws of refraction of light.
Answer : The laws of refraction of light are
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

Question. What is absolute refractive index of the medium?
Answer : When the refractive index of medium 2 is considered with respect to vacuum. This is called the absolute refractive index of the medium.
n/2 = Speed of light in 2 / Speed of light in 1

Question. When a ray of light entering from air is incident on the surface of a glass slab at an angle of 90°, what will be the measure of angle of refraction. Why does a ray change its path when it passes from one medium to another medium?
Answer : The angle of refraction will be zero. A light ray changes its path when it passes from one medium to another medium.

Question. Which is optically denser out of the two medium M₁ = 1.71 (refractive index) and M₂ = 1.36 (refractive index). How does speed of light change when it travels from optically rarer to denser medium.
Answer : Medium M₁ with refractive index 1.71 is optically denser than the other medium M₂. Speed of light decreases when it travels from rarer to denser medium.

Question. Comment on the size, position of the image formed by a concave mirror of focal length 18 cm when an object is placed:
(i) at 22 cm (ii) 14 cm (ii) 40 cm.
in front of mirror without calculations.
Answer : (i) When the object is placed at 22 cm, the image is formed beyond 36 cm, real, inverted image is magnified.
(ii) When the object is at 14 cm then the image formed is virtual, behind the mirror and magnified.
(iii) When the object is placed beyond 40 cm, then the image is formed between 18 cm and 36 cm, it is real inverted and diminished image.

Question. Define power of lens. What is the S. I. unit of power of a lens? If power of lens is +2D what is the nature and focal length of the lens?
Answer : Power of lens: The degree of convergence or divergence of light rays obtained by a lens is expressed in terms of its power.
Power of a lens is defined as the reciprocal of its focal length.
P = 1 / f
S.I. unit of power of a lens is ‘diopter’
P = +2 D.
Lens is convex and the focal length of the lens is +0.50 m.
(P = 1/f  , 2 D = 1/f , ∴ f = 1/2 = 0.05)

Question. The refractive index of water is 1.33 and kerosene is 1.44. Calculate the refractive index of kerosene with respect to water.
Answer : Refractive index of water = n_w = 1.33
Refractive index of kerosene = n_k = 1.44
∴Refractive index of kerosene with respect to water is n_kw = n_k / n_w = 1.44 / 1.33
= 1.082

Very Short Answer Questions

Question. What is light?
Answer : Light is a form of electromagnetic radiation that causes the sensation of sight. It doesn’t require any material medium to travel.

Question. Define reflection of light.
Answer : The phenomenon of coming back of light in the same medium after striking a plane and polished surface is called reflection of light.

Question. State laws of reflection.
Answer : Incident ray, reflected ray and normal at the point of incidence all lie in the same plane. The angle of incidence is equal to the angle of reflection.

Question. What are spherical mirrors?
Answer : Mirrors whose reflecting surface are part of a sphere are called spherical mirrors.

Question. Give uses of convex mirror.
Answer : (a) Used as rear view mirror in vehicles.
(b) Used to see full length image of a tall building.

Question. State mirror formula and write it mathematically.
Answer : The relation between focal length of mirror, distance of the object and distance of the image is known as mirror formula. It is given by
1 / u + 1 / ν = 1 /f 
u = Image distance
ν = Object distance
f = Focal length

Question. Define magnification of mirror. 
Answer : The ratio of height of the image to the height of the object is called magnification. It is represented by ‘m’.
m = Height of image (h′)/Height of object (h) = –v/u
Magnification of real image is negative and of virtual image is positive.

Question. State laws of refraction.
Answer : The ratio of sin of angle of incidence to the sin of angle of refraction for a light of given colour and for a given pair of media is constant. This is called Snell’s law. 
i.e.,
sin i /sin r = Constant
The incident ray, refracted ray and the normal at the point of incidence lie on the same plane.

Question. Define lateral displacement.
Answer : Lateral displacement is the perpendicular distance between the incident ray and the emergent ray.

Short Answer Questions

Question. State the laws of reflection of light.
Answer : Laws of reflection of light are:
(i) The angle of incidence is equal to the angle of reflection and
(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Question. Define pole and centre of curvature of spherical mirrors.
Answer : Pole: The centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror. It is represented by “P”. Centre of Curvature: The reflecting surface of a spherical mirror forms a part of a sphere this sphere has a centre and this point is called the centre of curvature of the spherical mirror.

Question. Give the uses of convex mirrors.
Answer : Convex mirrors are commonly used as rear-view mirrors in vehicles. As these mirrors can give an erect image, wider field of view, these mirrors are used in vehicles to see the traffic behind.

Question. What is refractive index?
Answer : If ‘i’ is the angle of incidence and ‘r’ is the angle of refraction then
sin i/ sin r = Constant
This constant value is called the refractive index of the second medium with respect to the first. 

Question. Two medium with refractive index 1.31 and 1.50 are given. In which case
(i) bending of light is more?
(ii) speed of light is more?
Answer : (i) Bending of light is more in the medium where refractive index is 1.50.
(ii) Speed of light is more in the medium with refractive index 1.31

Question. Refractive index of kerosene oil is 1.44 and that of water is 1.33. A ray of light enters from kerosene oil to water. Where would light ray bend and why?
Answer : A ray of light enters from kerosene oil to water i.e., refractive index 1.44 to 1.33 i.e., from denser to rarer medium. Hence the ray of light bends away from the normal.

Question. State the mirror formula, lens formula and power of lens.
Answer : Mirror formula
1 / v + 1 /u = 1/f
(ν = image distance, u = object distance, f = focal length)
Lens formula
1 /v – 1/u = 1/f
Power of lens
P = 1 / f
f = focal length in metres

Question. If the speed of light in water is 2.25 × 10⁸ m/s and the speed in vacuum is 3×10⁸m/s.
Calculate the refractive index of water.
Answer : Refractive index of water = Speed of light in 1 medium (air) / Speed of light in 2 medium (water)
.
∴ n_m = c / v
 n_m = 3 × 10⁸ / 2.25 × 10⁸
n_m  = 1.33
∴The refractive index of water = 1.33.

Question. Define the following.
(a) What is ray?
(b) What is beam?
(c) What is reflection of light?
(d) What is reflector?
(e) What is focal length?
(f) What is principal focus?
(g) What is refraction?
(h) What is optically rare medium?
(i) What is optically denser medium?
(j) What is power?
(k) What is 1 dioptre?
Answer : (a) It is the path of light.
(b) Group of parallel light rays emitted by the source of light.
(c) Bouncing back of light after striking any surface.
(d) The surface which reflects the light.
(e) The distance between the pole and the principal focus of the spherical mirror.
(f) A point of the principal axis where the rays of light parallel to principal axis meet.
(g) Bending of light ray when it travels from one medium to another.
(h) When the speed of light is more as compared to other medium.
(i) When the speed of light is less as compared to another medium.
(j) The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power
(k) It is the power of lens whose focal length is 1 m.

1. You are given kerosene, turpentine oil and water. In which of these does the light ray travel fastest? Given refractive index of kerosene = 1. 44, Refractive index of turpentine 1· 47, refractive index of water = 1· 33.

2. Under what condition a convex lens when placed in a medium behaves as an ordinary glass plate?

Light having a free space wavelength λ= 500 nm passes from vacuum into diamond having refractive index 2.4. What would be the wavelength of light in the diamond?

4. A ray of light is travelling in water medium falls on the water-air interface at an angle of 45° with the vertical. Will it be possible by the ray of light to come out of the water
surface?

5. A virtual image cannot be focused on a screen.” this statement appears to be contradictory as we focus a virtual image onto a screen, i. e retina explain it.

6. In what way is the word AMBULANCE painted in front of the hospital vans? Why is it painted this way?

7. A concave mirror and convex lens are held in water. What change, if any, do you expect to find in the focal length of either?

8. ‘ Vehicles in this mirror are closer than they appear’. This warning is printed on the convex mirror outside the driver’s side of a vehicle. What is the reason for this warning?

9. A glass tumbler is placed over a coin and filled with water. Coin disappears after sometime when seen from the sides of the tumbler. Give suitable name for the phenomena observed.

10. If a lens has been blackened in strips, what will be the difference in the nature of the image? 

11. How many images are seen when two mirrors are placed in parallel?

12 . Is it possible for a lens to act as a convergent in one medium and divergent in another?