CBSE Class 10 Science Light Reflection and Refraction Assignment Set A

Question. One light wave is incident upon a plate of refracting index m . Incident angle i , for which refractive & reflective waves are mutually perpendicular will be
(a) i = 45c
(b) i = sin^-1(μ)
(c) i = cosec^-1(μ)
(d) i = tan^-1(μ )
Answer : D

Question. A perfectly reflecting mirror has an area of 1 cm². Light energy is allowed to fall on it for an hour at the rate of 10Wcm-2 . The force that acts on the mirror is
(a) 3.35 x 10^-7 N
(b) 6.7 x 10^-7 N
(c) 3.35 x 10^-8 N
(d) 6.7 x 10^-8 N
Answer : D

Question. An object is placed 20 cm from the concave mirror of focal length 10 cm, then image is formed at
(a) behind the mirror
(b) between the mirror and focus
(c) at focus
(d) centre of curvature of mirror
Answer : D

Question. A virtual image three times the size of the object is obtained with a concave mirror of radius of curvature 36 cm. The distance of the object from the mirror is
(a) 20 cm
(b) 10 cm
(c) 12 cm
(d) 5 cm
Answer : C

Question. An object is situated at a distance of f/2 from a convex lens of focal length f . Distance of image will be
(a) +(f/2)
(b) +(f/3)
(c) +(f/4)
(d) -f
Answer : D

Question. An object is placed at the centre of curvature of a concave mirror. The distance between its image and the pole is
(a) equal to f
(b) between f and 2f
(c) equal to 2f
(d) greater than 2f
Answer : C

Question. The refractive index of dens flint glass is 1.65 and for alcohol, it is 1.36 with respect to air, then the refractive index of the dens flint glass with respect to alcohol is
(a) 1.31
(b) 1.21
(c) 1.11
(d) 1.01
Answer : B

Question. Refractive index of diamond with respect to glass is 1.6. If the absolute refractive index of glass is 1.5, then the absolute refractive index of diamond is
(a) 1.4
(b) 2.4
(c) 3.4
(d) 4.4
Answer : B

Question. Two plane mirrors are set at right angle and a flower is placed in between the mirrors. The number of images of the flower which will be seen is
(a) One
(b) Two
(c) Three
(d) Four
Answer : C

Question. Velocity of light in air is 3 x 10⁸ m/s. While its velocity in a medium is 1.5 x 10⁸ m/s. Then, refractive index of this medium is
(a) 3
(b) 5
(c) 0.5
(d) 2
Answer:  D

Question. A man is 6.0 ft tall. What is the smallest size plane mirror he can use to see his entire image
(a) 3.0 ft
(b) 6.0 ft
(c) 12 ft
(d) 24 ft
Answer : A

Question. Focal length of a plane mirror is
(a) zero
(b) infinite
(c) 25 cm
(d) -25
Answer : B

Question. Light waves
(a) Require air or another gas to travel through
(b) Require an electric field to travel through
(c) Require a magnetic field to travel through
(d) Can travel through perfect vacuum
Answer : D

Question. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm.
(b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.
(d) A concave lens of focal length 5 cm.
Answer : C

Question. Morning sun is not so hot as the mid day sun because
(a) Sun is cooler in the morning
(b) Heat rays travel slowly is the morning
(c) It is God gift
(d) The sun’s rays travel a longer distance through atmosphere in the morning
Answer : D

Question. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.
Answer : B

Question. The radius of curvature of concave mirror is 12 cm.
Then, the focal length will be
(a) 12 cm
(b) 6 cm
(c) -24 cm
(d) -6 cm
Answer : D

Question. An object is placed 60 cm in front of a concave mirror. The real image formed by the mirror is located 30 cm in front of the mirror. What is the object’s magnification?
(a) +2
(b) -2
(c) +0.5
(d) -0.5
Answer : D

Question. A spherical mirror and a thin spherical lens have each a focal length of -15 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, but the lens is concave.
Answer : A

Question. The image of an object placed in front of a convex mirror is formed at
(a) the object itself
(b) twice the distance of the object in front of the mirror
(c) half the distance of the object in front of the mirror
(d) behind the mirror
Answer : D

Question. An object is placed at a distance of 10 cm in front of a plane mirror, then the distance of image from mirror will be
(a) 5 cm
(b) 10 cm
(c) 20 cm
(d) 0
Answer : B

Question. An object is placed 40.0 cm in front of a convex mirror. The image appears 15 cm behind the mirror. What is the focal length of the mirror?
(a) +24 cm
(b) +11 cm
(c) -11 cm
(d) -24 cm
Answer : D

Answer : Object at Infinity. When object is at infinity, a real image is formed at F on the other side of the lens

 

Object at infinity. Image at F on the other side of lens.

However if the rays are parallel to themselves but not parallel to principal axis, then these rays after refraction will form image at focus F' and not at principal focus F.

 

 

Object at infinity, rays parallel to themselves but not parallel to principal axis. Image is formed at F', the focus on the other side of lens. Object beyond 2F. When the object is beyond 2F, a real, inverted, diminished image is formed between F and 2F.

Object beyond 2F, real, inverted, diminished image between F and 2F. Object at 2F.

When the object is at 2F, a real, inverted image of the same size is formed on the other side of the lens at 2F as given in Fig.

Object at 2F, image at 2F on the other side of the lens. Image is of size same as that of the object. When the object is between F and 2F, its real, inverted, magnified image is formed on the other side of the convex lens as shown in fig. 

Object between F and 2F real, inverted, magnified image is formed beyond 2F on the other side of lens.

Object at F. When object is placed principal focus, a real, inverted, very highly magnified image is formed at infinity.

 

Object at F, a very highly magnified, real, inverted image is formed at infinity.

Object between F and C. When an object is placed between principal focus and optical centre of the lens, virtual, erect, magnified image is formed on the same side of the lens.

Object between F and C; a virtual, erect, magnified image is formed on the same side.

Question. “A ray of light incident on a rectangular glass slab immersed in any medium emerges parallel to itself.” Draw a labelled ray diagram to justify the statement.
Answer :

Angle of incidence = 55°
Angle of refraction = 40°
Angle of emergence = 55°
FO Incident ray
GH emergent ray

Question. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Answer : Focal length of concave lens (OF1), f = – 15 cm
Image distance, v= – 10 cm
According to the lens formula,

The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.

Question. Draw a ray diagram to show the path of the reflected ray corresponding to an incident ray which is directed towards the principal focus of a convex mirror. Mark on it the angle of incidence and the angle of reflection.
Answer :

∠i = Angle of incidence
∠r = Angle of reflection

Question. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 108 ms-1.
Answer : Refractive index of a medium, nm = Speed of light in vacuum / Speed of light in the medium
Speed of light in vacuum, c = 3 × 10⁸ ms^-1
Refractive index of glass, n_g = 1.50
Speed of light in the glass, v = Speed of light in vacuum/ Refractive index of glass= c/n_g=3 × 10⁸/1.50 = 2x 10⁸ ms^-1.

Question. (a) It is desired to obtain an erect image of an object, using a concave mirror of focal length 20 cm.
(i) What should be the range of distance of the object from the mirror?
(ii) Will the image be bigger or smaller than the object?
(iii) Draw a ray diagram to show the image formation in this case.
(b) One half a convex lens of focal length 20 cm is covered with a black paper.
(i)Will the lens produce a complete image of the object?
(ii)Show the formation of image of an object placed at 2F1 of such covered lens with the help of a ray diagram.
(iii)How will the intensity of the image formed by half-covered lens compare with nn-covered lens?
Answer : 

• Light: The natural agent that stimulates sight and makes things visible. Light is reflected from all surfaces.

• Regular reflection takes place when light is incident on smooth, polished and regular surfaces.

• Diffused/irregular reflection takes place from rough surfaces.

• Reflection of Light: Bouncing back of light after striking the surface, in the same medium, is called reflection.

• Types of Reflection:

(i) Regular Reflection: When a narrow beam of light strikes a mirror, the light will not reach your eye unless your eye is positioned at just the right place where the law of reflection is satisfied.
(ii) Diffused or Irregular Reflection: When light is incident upon a rough surface, it is reflected in many directions.

• Two laws of reflection are

• The angle of incidence is equal to the angle of reflection.

• Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.

• Image formed in a plane mirror undergoes lateral inversion.

• Two mirrors inclined to each other give multiple images.

• Beautiful patterns are formed in a kaleidoscope because of multiple reflections.

• Sunlight, called white light, consists of seven colours.

• Splitting of light into its constituent colours is known as dispersion.

• Important parts of the eye are cornea, iris, pupil, lens, retina and optic nerve.

• A normal eye can see nearby and distant objects clearly.

• Visually challenged persons can read and write using Braille system.

• Visually challenged persons develop their other senses more sharply to improve their interaction with their environment.

• Parts of Human Eye:

(i) Cornea: Transparent bulge on the front surface of the eyeball which protects the eye and helps in refraction of light.
(ii) Iris: Coloured diaphragm behind the cornea which controls the amount of light entering the eye.
(iii) Pupil: Dark hole in the middle of iris through which light enters the eye.
(iv) Eye lens: Transparent, crystalline structure behind pupil and iris.
(v) Ciliary muscles: Hole the eye lens in position and control the focal length of the eye lens.
(vi) Retina: Surface of the rear part of the eyeball where the light entering the eye is focused.
(vii) Rods and Cones: Rod cells respond to the brightness of light while cone cells respond to colours.
(viii) Blind spot: It is the least sensitive point where no rodsd and cones are present.
(ix) The space between the cornea and the eye lens is filled with aqueous humour.
(x) The space between the eye lens and the retina is filled with vitreous humour.

Assertion and Reasoning Based Questions

Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.

Question. Assertion : The nature of the image formed by a concave mirror is real and inverted.
Reason : The image formed by a concave mirror depends on the position of the object at infinity.
Answer : (a) 

Question. Assertion : Convex mirror are used for rear view on vehicles.
Reason : The size of the image formed by a convex mirror will be same.
Answer : (b) 

Question. Assertion : A ray of light that travels obliquely from one transparent medium into another will change its direction in the second medium.
Reason : Refraction is due to change in the speed of light as it enters from one transparent medium to another.
Answer : (a) 

Question. Assertion : The refractive index of Kerosene is 1.44 which is optically denser than water.
Reason : The mass density of kerosene is lesser than water.
Answer : (b) 

Question. Assertion : The sequence of rainbow colour is represented as VIBGYOR.
Reason : Formation of VIBGYOR sequence colour is due to the dispersion of white light.
Answer : (a) 

Very Short Answers Type Questions

Question. Where is the image formed when an object is at large distance from a concave mirror ?
Answer : The image formed is at the focus of the mirror.

Question. What kind of image can be obtained on the screen ?
Answer : Real image.

Question. Which metal is the best reflector of light ?
Answer : Silver metal is the best reflector of light.

Question. What is the nature of the image formed by a concave mirror if the magnification produced by the mirror is + 2 ?
Answer : Virtual and erect.

Question. Between which two points of a concave mirror should an object be placed to obtain a magnification of – 2 ?
Answer : Between F and C.

Question. List four characteristics of the images formed by plane mirrors.
Answer :
(a) Size of the image is equal to the size of object.
(b) Formed image is erect.
(c) Image formed by plane mirror is virtual.
(d) Formed image is laterally inverted.

Question. Why is convex mirror used as a rear view mirror in vehicles ? State any one reason. 
Answer : Convex mirror is used as a rear view mirror in vehicles because it gives a wider field view of the approaching traffic.

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

Question. Define the term principal axis of a spherical mirror.
Answer : A straight line passing through the pole of spherical mirror and joining it to the centre of curvature of the mirror is known as the principal axis of the spherical mirror.

Short Answer Questions

Question. State two laws of refraction of light.
Answer : Two laws of refraction of light are :
(a) The incident ray, the refracted ray and the normal to the interface of two transparent medium at the point of incidence, all lie in the same plane.
(b) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of medium.

Question. (a) What is an optical medium ?
(b) What do you mean by :
(i) an optically dense medium ?
(ii) an optically rare medium ?
Answer :
(a) A medium through which light can travel is called an optical medium.
(b) (i) While passing from one medium to the other, if the speed of light slows down, then the second medium is said to be optically denser than the first medium.
(ii) While passing from one medium to the other, if the speed of light increases, then the second medium is said to optically rarer than the first medium.

Question. (a) State the two laws of reflection of light.
(b) A light ray is incident normally on a plane mirror. What is its angle of incidence ?
Answer :
(a) The two laws of reflection are :
(i) The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane.
(ii) The angle of incidence is equal to the angle of reflection (∠i = ∠r).
(b) The angle of incidence will be 0°.

Question. (a) Define lateral shift.
(b) State and explain the factors on which it depends.
Answer :
(a) The perpendicular distance between the path of emergent ray and the direction of incident ray is called lateral shift.
(b) Lateral shift depends on the following factors :
(i) It is directly proportional to the thickness of glass slab.
(ii) It is directly proportional to the angle of incidence.
(iii) It is directly proportional to the refractive index of the glass slab.
(iv) It is inversely proportional to the wavelength of the light used.

Question. Write the relationship between the S.I. unit of the power of a lens and S.I. unit of focal length.
Answer : Reciprocal of focal length of a lens, expressed in metre, is called the power of that lens. Its S.I. unit is 1 dioptre (1 D), where 1 D = 1 m^–1.

HOTS Questions

Question. Amit visited a fair and saw a mirror in which he got a very funny image. The above part of his body was big in size, middle part was of normal size and the lower part of the body showed very small size. What kind of mirror is this?
Answer : Upper part – Concave mirror; Middle part : Plane mirror; Lower part : Convex mirror.

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.

Question. Write the laws of reflection.
Answer : (a) The angle of incidence is equal to angle of reflection.
(b) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Question. Give uses of plane mirror.
Answer : • Looking glass
• Used in submarines
• Solar cooker
• Kaleidoscope

Question. Give uses of concave mirror.
Answer : • Used as reflectors in car headlights, searchlights, etc.
• Used as shaving mirror.
• Used in solar cooker to focus the sunlight on one point.

Question. What are the two types of refractive index? 
Answer : • Relative refractive index– It is the ratio of speed of light in one medium to the speed of light in another medium 
• Absolute refractive index– It is the ratio of light in vacuum to the speed of light in another medium

Question. Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Answer : (a) Concave mirror
(b) Convex mirror
(c) Concave mirror

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.

Case Study Based Questions

1. Read the following and answer any four questions:

Light is a form of energy that produces in us the sensation of sight. Reflection of light is the phenomenon of bouncing back of light in the same medium on striking the surface of any object. The two laws of reflection are the incident ray, the reflected ray and the normal (at the point of incidence), all lie in the same plane and the angle of reflection (r) is always equal to the angle of incidence (i). Refraction of light is the phenomenon of change in the path of light in going from one medium to another.

Question. What is mirage?
(a) Depends on the position of object
(b) Mirror is concave and the lens is convex
(c) Goes straight into the second medium
(d) Optical illusion caused due to total Internal Reflection
Answer : D

Question. What are the two types of mirage?
(a) Thin mirage and thick mirage
(b) Inferior mirage and superior mirage
(c) Intense mirage and diminished mirage
(d) Light mirage and dark mirage
Answer : B

Question. How a mirage is formed?
(a) Between focus and centre of curvature
(b) Is formed away from the normal
(c) Illusion of the presence of water and is a result of refraction of light from a nonuniform medium
(d) Is reflected along the same path
Answer : C

Question. Mirage is observed mainly during ............... days.
(a) Sunny
(b) Winter
(c) Spring
(d) Hot
Answer : A

2. Read the following and anwer any four questions:

Rear view mirror is a device that allows the driver to see rearward. It usually finds its place at the top of windscreen in side the cabin. This device is one of the most basic but essential safety devices in the vehicle. It provides assistance to the driver during overtaking, parking in reverse gear etc. Generally, vehicles also have a pair of mirrors attached to the body from outside. They are popular as ‘side mirrors’ or Outer Rear View Mirrors (ORVM) which serve the same purpose.
Almost all modern cars mount their side mirrors on the doors-normally at A-pillar rather than the wings (the portion of the body above the wheel well).

Question. For a real object, which of the following can produce a real image?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) None of these
Answer : B

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. A convex mirror is used:
(a) by a dentist
(b) for shaving
(c) as a rear view mirror in vehicles
(d) as a light reflector for obtaining a parallel beam of light
Answer : C

Question. The word ‘AMBULANCE’ is written on the vehicle as:
(a) CNALUBMA
(b) AMBULANCE
(c) ECNALUBMA
(d) None of these
Answer : B

Question. Mark the correct statement:
(a) Convex mirror conform images of objects spread over a large area.
(b) Convex mirrors are used by dentist.
(c) In convex mirror image is formed larger in size, erect and real.
(d) Convex mirror forms real image.
Answer : A