NCERT Exemplar Solutions Class 10 Science Light Reflection and Refraction

Multiple Choice Questions...............................

Question 1:  Which of the following can make a parallel beam of light when light from a point source is incident on it?

(a) Concave mirror as well as convex lens

(b) Convex mirror as well as a concave lens

(c) Two plane mirrors placed at 90° to each other

(d) Concave mirror as well as a concave lens 

Solution 1: (d) Concave mirror as well as a concave lens 

Emergent rays form a parallel beam of light when a point source of light is centred on a convex or concave mirror.

Question 2:  A 10 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 

Solution 2:  (b) – 20 cm 

Size of the object (O) is + 10.0 mm i.e. + 1.0 cm (because, 1 cm = 10 mm)

Image dimension (I) is 5.0 mm i.e. 0.5 cm

Image distance (v) is − 30 cm (as image is real)

Let's assume that the object distance is u.

Focal length is f.

Question 3:  Under which of the following conditions a concave mirror can form an image larger than the actual object?

(a) When the object is kept at a distance equal to its radius of curvature

(b) When an object is kept at a distance less than its focal length

(c) When an object is placed between the focus and centre of curvature

(d) When an object is kept at a distance greater than its radius of curvature 

Solution 3:  (c) When an object is placed between the focus and centre of curvature 

An expanded image beyond C is created when an object is positioned between F and C.

Question 4: Below figure shows a ray of light as it travels from medium A to medium B. Refractive index of the medium B relative to medium A is

(a) 3/( 2)

(b) 2/( 3)

(c)  1/( 2)

(d) 2

Question 5:  A light ray enters from medium A to medium B as shown in below 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

Solution 5:  (a) greater than unity 

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 greater than unity.

Question 6:  Beams of light are incident through the holes A and B and emerge out of the box through the holes C and D respectively as shown in below figure. Which of the following could be inside the box?

(a) A rectangular glass slab

(b) A convex lens

(c) A concave lens

(d) A prism

Solution 6:  (a) A rectangular glass slab 

As incident rays strike the point of incidence perpendicularly. It will refract and then refract again in a rectangular glass slab.

Question 7:  A beam of light is incident through the holes on side A and emerges out of the holes on the other face of the box as shown below given figure. Which of the following could be inside the box?

(a) Concave lens

(b) Rectangular glass slab

(c) Prism

(d) Convex lens

Solution 7:  (a) Concave lens 

Since incident rays are parallel and emergent rays are diverging, the response is concave lens.

Question 8:  Which of the following statements is true?

(a) A convex lens has 4 dioptre power having a focal length 0.25 m

(b) A convex lens has –4 dioptre power having a focal length 0.25 m

(c) A concave lens has 4 dioptre power having a focal length 0.25 m

(d) A concave lens has –4 dioptre power having a focal length 0.25 m 

Solution 8:  (a) A convex lens has 4 dioptre power having a focal length 0.25 m 

A convex lens has a positive focal length value.

 
Question 9:  Magnification produced by a rear view mirror fitted in vehicles

(a) is less than one

(b) is more than one

(c) is equal to one

(d) can be more than or less than one depending upon the position of the object in front of it 

Solution 9:  (a) is less than one 

In the rear view mirror, a convex mirror is used. The image produced by a convex mirror is often smaller. As a result, the magnification provided by the rear view mirror is often less than one.

Question 10: Rays from Sun converge at a point 15 cm in front of a concave mirror. Where an object should be placed so that size of its image is equal to the size of the object?

(a) 15 cm in front of the mirror

(b) 30 cm in front of the mirror

(c) between 15 cm and 30 cm in front of the mirror

(d) more than 30 cm in front of the mirror 

Solution 10: (d) more than 30 cm in front of the mirror

Focal length = 15

The focal length is doubled by the radius of curvature.

Hence, radius of curvature =30

When an object is put on c, its image is the same size as the original, but reversed, and shaped on c.

Question 11:  A full-length image of a distant tall building can definitely be seen by using

(a) a concave mirror

(b) a convex mirror

(c) a plane mirror

(d) both concave as well as plane mirror 

Solution 11:  (b) a convex mirror

The field of a convex mirror is larger than that of every other kind of mirror. As a result, a convex mirror can be used to see the full length of a structure.

Question 12:  In torches, searchlights and headlights of vehicles the bulb is placed

(a) between the pole and the focus of the reflector

(b) very near to the focus of the reflector

(c) between the focus and centre of curvature of the reflector

(d) at the centre of curvature of the reflector 

Solution 12: (b) very near to the focus of the reflector 

Concave mirror reflectors and searchlights are used in headlights and searchlights. Reflected light appears as a beam when the source of light is positioned at the focus.

Question 13:  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 

Solution 13:  (d) all mirrors irrespective of their shape

Regardless of their form, the laws of reflection apply to all mirrors.

Question 14:  The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as A, B, C and D in the given figure. Which one of them is correct?

(a) A

(b) B

(c) C

(d) D 

Solution 14:  (b) B

As light travels from air to glass, it bends back to its original shape. As light moves from glass to air, it bends away from the usual route. Figure 1 depicts this correctly (b).

Question 15: You are given water, mustard oil, glycerine and kerosene. In which of these media a ray of light incident obliquely at same angle would bend the most?

(a) Kerosene

(b) Water

(c) Mustard oil

(d) Glycerine 

Solution 15:  (d) Glycerine

In water a ray of light bends 1.33, in kerosene ray of light bends 1.44, in mustard oil ray of light bends 1.46, and in glycerine ray of light bends 1.47.

Question 16: Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in below figure?

(a) Fig. A

(b) Fig. B

(c) Fig. C

(d) Fig. D

Solution 16:  (d) Fig. D

After reflection, an incident ray parallel to the principal axis moves through F in the case of a concave mirror.

Question 17:  Which of the following ray diagrams is correct for the ray of light incident on a lens shown in the figure given below?

(a) Fig. A

(b) Fig. B

(c) Fig. C

(d) Fig. D

Solution 17:  (a) Fig. A

After refraction, the incident ray passing through F is parallel to the principal axis in a convex lens.

Question 18:  A child is standing in front of a magic mirror. She finds the image of her head bigger, the middle portion of her body of the same size and that of the legs smaller. The following is the order of combinations for the magic mirror from the top.

(a) Plane, convex and concave

(b) Convex, concave and plane

(c) Concave, plane and convex

(d) Convex, plane and concave 

Solution 18:  (c) Concave, plane and convex 

An expanded image is created behind the mirror when the object is between F and P of the concave mirror. As a result, a concave mirror allows the child to see her head larger. Since the plane mirror reflects the original dimension, she can see her body in the same size. Convex mirrors distort images, making babies' legs look shorter.

Question 19:  In which of the following, the image of an object placed at infinity will be highly diminished and point sized?

(a) Concave mirror only

(b) Convex mirror only

(c) Convex lens only

(d) Concave mirror, convex mirror, concave lens and convex lens 

Solution 19:  (d) Concave mirror, convex mirror, concave lens and convex lens 

If the mirror is concave or convex, lens is concave or convex; then the image of the object placed at infinity will be highly diminished and point sized.

Short Answer Questions........................

Question 20:  Identify the device used as a spherical mirror or lens in the following cases, when the image formed is virtual and erect in each case.

(a) Object is placed between the device and its focus, an image formed is enlarged and behind it.

(b) Object is placed between the focus and device, an image formed is enlarged and on the same side as that of the object.

(c) Object is placed between infinity and device, an image formed is diminished and between focus and optical centre on the same side as that of the object.

(d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it. 

Solution 20:  (a) If object is placed between the concave mirror and its focus, an image formed is enlarged and behind it.

(b) If the object is placed between the focus and convex lens, an image formed is enlarged and on the same side as that of the object.

(c) If object is placed between infinity and concave lens, an image formed is diminished and between focus and optical centre on the same side as that of the object.

(d) If object is placed between infinity and convex mirror, image formed is diminished and between pole and focus, behind it. 

Question 21:  Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.

Solution 21:  As light from a rarer medium reaches a denser medium, it bends towards the regular. The ray's bending at the opposite parallel is the same in this situation. As a result, the emergent ray is perpendicular to the incident ray.

Question 22:  A pencil, when dipped in water in a glass tumbler, appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine? Support your answer with reason. 

Solution 22:   The bending of light is a result of refraction in this case. The refractive indices affect refraction. Kerosene or turpentine has different refractive indices than water. As a result, the degree of bend can vary depending on the medium.

Question 23:  How is the refractive index of a medium related to the speed of light? Obtain an expression for the refractive index of a medium with respect to another in terms of speed of light in these two media? 

Solution 23:   The element by which the speed and wavelength of radiation are decreased in comparison to their vacuum values is known as the refractive index.

w=c/v (where n: refractive index, c: speed of light, v: velocity of light in that medium)

The ratio of the speed of light in the second medium to the speed of light in the first medium determines the refractive index of one medium in comparison to another.

Question 24:  Refractive index of diamond with respect to glass is 1.6 and the absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond. 

Solution 24:

Absolute Refractive index of diamond is 1.6

Absolute Refractive index of glass is 1.5

Multiplying absolute refractive index of diamond with glass, we get 2.4.

Question 25:  A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images? 

Solution 25: When an object is positioned in the focal planes F and F2 of a convex lens, an inverted, enlarged, and true image is created beyond 2F2, which is on the opposite side of the lens. As a result, the object must be placed between 20 and 40 cm from the lens. When an object is positioned between the focal planes F and 0 of a convex lens, its expanded, erect, and virtual image is created beyond FL, on the same side of the lens. As a result, the object must be placed at a distance of less than 20 cm from the lens.

Question 26:  Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens? 

Solution 26:   When an object is positioned between the focal planes F and 0 of a convex lens, its expanded, erect, and virtual image is created beyond FL, on the same side of the lens. As a result, the object must be placed at a distance of less than 20 cm from the lens. As parallel rays of light strike a convex lens, the rays converge at focus on the opposite side of the lens after refraction.

Question 27:  How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm respectively. Which lens will you use to obtain more convergent light? 

Solution 27:   The focal length of a lens is inversely proportional to its power. The strength of a lens with a focal length of 20 cm is greater than that of a lens with a focal length of 40 cm. To get more convergent light, use a lens with a higher power.

Question 28:  Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be the angle of incidence. Show the same with the help of a diagram. 

Solution 28:   The incident and reflected rays will always be parallel if two plane mirrors are positioned perpendicular to each other.

Question 29:  Draw a ray diagram showing the path of rays of light when it enters with oblique incidence

(i) from air into water;

(ii) from water into air. 

Solution 29:  As light travels from a rarer medium to a denser medium, its speed decreases and light rays bend back towards normal.

Light rays bend away from the average when passing from a denser medium to a rarer medium.

Long Answer Questions........................

Question 30: Draw ray diagrams showing the image formation by a concave mirror when an object is placed

(a) between pole and focus of the mirror

(b) between focus and centre of curvature of the mirror

(c) at the centre of curvature of the mirror

(d) a little beyond the centre of curvature of the mirror

(e) at infinity 

Solution 30:  

Question 31:  Draw ray diagrams showing the image formation by a convex lens when an object is placed

(a) between optical centre and focus of the lens

(b) between focus and twice the focal length of the lens

(c) at twice the focal length of the lens

(d) at infinity

(e) at the focus of the lens 

Solution 31:

Question 32:  Write laws of refraction. Explain the same with the help of a ray diagram, when a ray of light passes through a rectangular glass slab. 

Solution 32:   Refraction laws are a set of rules that govern how light bends:

1. At the point of incidence, the incident light, refracted ray, and regular are all in the same plane.

2. For a given colour and pair of media, the ratio of the sine of incidence and sine of refraction is constant.

• ABCD is a piece of glass. EF is an incident ray that hits the air-glass interface at point 0.

• NO stands for natural, and _LEON = I stands for angle of incidence.

• NV is usually extended onto the glass slab, and the angle of refraction is..LV '00 = Zri.

• The refracted ray from surface AB is 00′. It behaves in the same way as Incident rayon surface CD.

• As the ray EF reaches the slab, it bends to become 00′.

• On a surface CD, MO' and O'M' are common.

• The emergent ray is GH.

• ZOO' Al = Li, which is the incidence angle at the surface CD.

• The angle of refraction at surface CD is Z.110′ H = Zr.

• EF. NO and 00′ are found to be in the same plane, which is consistent with the first law of refraction.

• EF II GH is also observed, indicating that the emergent ray is parallel to the incident ray. This occurs since the degree of bend on the glass slab's opposite surfaces is the same.

Question 33:  Draw ray diagrams showing the image formation by a concave lens when an object is placed

(a) at the focus of the lens

(b) between focus and twice the focal length of the lens

(c) beyond twice the focal length of the lens 

Solution 33:  

Question 34:  Draw ray diagrams showing the image formation by a convex mirror when an object is placed

(a) at infinity

(b) at a finite distance from the mirror 

Solution 34:

Question 35:  The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between the lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens? 

Solution 35:

It is true that the picture on the screen is genuine.

So,

Magnification, m = –3

v = 80 cm

u = ?

 

Question 37: Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?

 

Solution 37:    

The power of a lens is the degree of convergence and divergence it produces. Diopter D is the unit of power of lens.

The first student's lens has a positive focal length, indicating that it is a convex lens. The second student's lens is a concave lens.

 

P = 1/f =  1/0.5 = 2

Lens power (first student) = +2

Lens power (second student) = 0.2

 

   

Question 38: A student-focussed the image of a candle flames on a white screen using a convex lens. He noted down the position of the candle screen and the lens as under 

Position of candle = 12.0 cm

Position of convex lens = 50.0 cm

Position of the screen = 88.0 cm

1. What is the focal length of the convex lens?
2. Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?
3. What will be the nature of the image formed if he further shifts the candle towards the lens?
4. Draw a ray diagram to show the formation of the image in case (iii) as said above. 

Solution 38:

Position of the candle flame = 12.0cm

Position of the convex lens = 50.0 cm

Position of the screen = 88.0 cm

1. u is 50-12 i.e. 38 cm.

Image distance, 

v = 88-50 is equals to 38cm.

 

Focal length =  1/v - 1/u = 1/f

F is 19cm.

 

2. Object distance,

u is 50-31 i.e. 19 cm

Here,

Object distance = focal length

As a result, the picture is generated at infinity.

 

3. If he moves the candle closer to the lens. Between F and 0 is the object. In this instance, on the same side of the lens, a virtual, expanded, and erect image is created.