Light - Reflection and Refraction Question Answers: NCERT Class 10 Science

Welcome to the Chapter 10 - Light - Reflection and Refraction, Class 10 Science - NCERT Solutions page. Here, we provide detailed question answers for Chapter 10 - Light - Reflection and Refraction.The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.

Our solutions explain each answer in a simple and comprehensive way, making it easier for students to grasp key topics Spherical mirrors, image formation, mirror formula (without derivation), magnification, Laws of refraction, refractive index, image formation by spherical lenses, lens formula (without derivation), power of a lens and excel in their exams. By going through these Light - Reflection and Refraction question answers, you can strengthen your foundation and improve your performance in Class 10 Science. Whether you're revising or preparing for tests, this chapter-wise guide will serve as an invaluable resource.

We all are familiar with word light. Light produces us the sensation of sight. During day time, sunlight falls on the objects get reflected and we are able to see the object. But at night we cannot see the object and cannot identify the colour, while lighting the bulb we are able to see the object. You have seen shadow of our own and others because light travels along a straight path. In this chapter we will understand the characteristics of light, reflection, refraction and diffraction of light, formation of an image on a plane mirror, spherical mirror & lense and their application in real life.

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Exercise 1

Q1	Define the principal focus of a concave mirror.
Ans:	The ray of light that is parallel to the principal axis of a concave mirror converges at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

Q2	The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Ans:	Given, Radius of curvature, R = 20 cm Radius of curvature of a spherical mirror = 2 × Focal length (f) R = 2f by putting the values we get The focal length of the given spherical mirror is 10 cm.

Q3	Name a mirror that can give an erect and enlarged image of an object.
Ans:	The image formed is virtual, erect, and enlarged.When an object is placed between in the pole and the principal focus of a concave mirror.

Q4	Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Ans:	Convex mirrors are preferred as rear view mirrors because they give a virtual, erect, and diminished image of the objects when placed in front of them and cover a wider field of view, which allows the driver to see the traffic behind him.

Exercise 2

Q1	Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Ans:	Given: Radius of curvature, R = 32 cm Radius of curvature = 2 × Focal length (f) R = 2f On putting the values we get Therefore, the focal length of the given convex mirror is 16 cm.

Q2	A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?
Ans:	Magnification produced by a spherical mirror is expressed as. Suppose, the height of the object, ho = h Then, height of the image, hI = −3h (Image formed is real) Object distance, u = −10 cm v = 3 × (−10) = −30 cm Here, the negative sign indicates that an inverted image is formed at a distance of 30 cm in front of the given concave mirror.

Exercise 3

A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

Ans:

A ray of light when it travels from air to water it bends towards the normal.

When a ray of light travels from an optically rarer medium to an optically denser medium, it gets bent towards the normal.As water is more optically denser than air, a ray of light travelling from air into the water will bend towards the normal.

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 × 108 m s–1.

Ans:

Refractive index of a medium nm is expressed as

Speed of light in vacuum, c = 3 × 10⁸ m s−1

Refractive index of glass, n_g = 1.50

So , Speed of light in the glass,

Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.

Material medium	Refractive index	Material medium	Refractive index
Air	1.0003	Canada Balsam	1.53
Ice	1.31
Water	1.33	Rock salt	1.54
Alcohol	1.36
Kerosene	1.44	Carbon disulphide	1.63
Fused quartz	1.46
Turpentine oil	1.47	Ruby	1.71
Benzene	1.50	Sapphire	1.77
Crown glass	1.52	Diamond	2.42

Table 10.3 Absolute refractive index of some material media

Ans:

From the above provided information the object having, Highest optical density is Diamond and Lowest optical density is Air.Optical density of a medium is directly related with the refractive index of that medium. A medium which has the highest refractive index will have the highest optical density and vice-versa.

You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3.

Material medium	Refractive index	Material medium	Refractive index
Air	1.0003	Canada Balsam	1.53
Ice	1.31
Water	1.33	Rock salt	1.54
Alcohol	1.36
Kerosene	1.44	Carbon disulphide	1.63
Fused quartz	1.46
Turpentine oil	1.47	Ruby	1.71
Benzene	1.50	Sapphire	1.77
Crown glass	1.52	Diamond	2.42

Ans:

Speed of light in a medium is given by the relation for refractive index (n_m). The relation

It can conclude from the relation that light will travel the slowest in the material which has the highest refractive index and travel the fastest in the material which has the lowest refractive index. Therefore, light travels the fastest in water.

The refractive index of diamond is 2.42. What is the meaning of this statement?

Ans:

The refractive index of diamond is 2.42. This means that the speed of light in diamond will reduce by a factor 2.42 compared to its speed in air.

Refractive index of a medium n_m is related to the speed of light in that medium v by the relation:

Where, c is the speed of light in vacuum/air.

Exercise 4

Q1	Define 1 dioptre of power of a lens.
Ans:	1 dioptre is defined as the power of a lens of focal length 1 metre. ∴1 D = 1 m^-1 Power of lens is defined as the reciprocal of its focal length. Where P is the power of a lens of focal length f in metres, then The S.I. unit of power of a lens is Dioptre. It is denoted by D.

Q2	A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
Ans:	The image formed is inverted and of the same size as the object. When an object is placed at the centre of curvature, 2F₁, of a convex lens, its image is formed at the centre of curvature, 2F₂, on the other side of the lens. Given, The image of the needle is formed at a distance of 50 cm from the convex lens. Hence, the needle is placed in front of the lens at a distance of 50 cm. Object distance, u = −50 cm Image distance, v = 50 cm Focal length = f Putting values to the lens formula, Hence, the power of the given lens is +4 D.

Q3	Find the power of a concave lens of focal length 2 m.
Ans:	Given, Focal length of concave lens, f = - 2 m Power of a lens, a negative sign is due to the divergent nature of the concave lens. Hence, the power of the given concave lens is −0.5 D.

Exercise 5

Q1	Which one of the following materials cannot be used to make a lens? (a) Water (b) Glass (c) Plastic (d) Clay
Ans:	d) Clay cannot be used for making lenses because it is an opaque object through which light cannot pass.

Q2	The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object? (a) Between the principal focus and the centre of curvature (b) At the centre of curvature (c) Beyond the centre of curvature (d) Between the pole of the mirror and its principal focus.
Ans:	(d) The image formed is virtual, erect, and larger than the object, When an object is placed between the pole and principal focus of a concave mirror.

Q3	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.
Ans:	(b) The image formed is real, inverted, and of the same size as the object. When an object is placed at the centre of curvature in front of a convex lens, its image is formed at the centre of curvature on the other side of the lens i.e at twice of focal length.

Q4	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.
Ans:	(a) the spherical mirror and the thin spherical lens are concave in nature. By convention, the focal length of a concave mirror and a concave lens are taken as negative.

Q5	No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be (a) only plane. (b) only concave. (c) only convex. (d) either plane or convex.
Ans:	(d) The given mirror could be either plane or convex.A convex mirror always gives a virtual and erect image of a smaller size of the object when placed in front of it. Similarly, a plane mirror will always give a virtual and erect image of the same size as that of the object placed in front of it.

Q6	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.
Ans:	(c) Magnification is more for convex lenses having shorter focal length. A convex lens gives a magnified image of an object when it is placed between the radius of curvature and focal length.So, for reading small letters, a convex lens of focal length 5 cm should be used.

Q7	We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
Ans:	A concave mirror gives an erect image when an object is placed between its pole (P) and the principal focus (F). To obtain an erect image of an object from a concave mirror of focal length 15 cm, the object must be placed anywhere between the pole and the focus. The image formed will be virtual, erect, and magnified in nature:

Q8	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. Support your answer with reason.
Ans:	(a) In the headlights of a car Concave mirror is used. This is because concave mirrors converging mirrors that can produce a powerful parallel beam of light when the light source is placed at their principal focus. (b) The side/rear-view mirror of a vehicle Convex mirror is used.Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of it and they have a wide field of view therefore the driver can see most of the traffic behind. (c) In Solar furnaces Concave mirrors are used. Concave mirrors are converging mirrors. That is why they are used to construct solar furnaces. It converges the incident light on them at a single point known as principal focus. Hence, they can be used to produce a large amount of heat at that point.

Q9	One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Ans:	Yes, The convex lens will form a complete image of an object, even if its one half is covered with black paper. It can be experimentally verified:. In first Case If the upper half of the lens is covered then a ray of light coming from the object will be refracted by the lower half of the lens and these rays meet at the other side of the lens to form the image of an object, In Second Case When the lower half of the lens is covered then a ray of light coming from the object is refracted by the upper half of the lens and these rays meet at the other side of the lens to form the image of the given object.

Q10	An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
Ans:	Given Converging lens is convex lens Object distance, u = −25 cm Object height, h_o = 5 cm Focal length of convex lens, f = +10 cm According to the lens formula, The value of v positive this shows that the image formed at the other side of the lens. The negative sign shows that the image is real and formed behind the lens. The negative value of image height shows that the image formed is inverted. Ray diagram showing The position, size, and nature of image are:

Q11	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.
Ans:	Given: Focal length of concave lens, f = −15 cm Image distance, v = −10 cm According to the lens formula, The value of u is negative which shows that the object is placed 30 cm in front of the lens. The ray diagram is

Q12	An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
Ans:	Given, Focal length of convex mirror, f = +15 cm Object distance, u = −10 cm The mirror formula is The value of v is positive which shows that the image is formed behind the mirror. The positive value of magnification indicates that the image formed is virtual and erect.

Q13	The magnification produced by a plane mirror is +1. What does this mean?
Ans:	The magnification produced by a plane mirror is +1. It shows that the image formed by the plane mirror is of the same size as that of the object. Whereas the positive sign shows that the image formed is virtual and erect.

Q14	An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.
Ans:	Given, Convex mirror Radius of curvature, R = 30 cm Radius of curvature = 2 × Focal length R = 2f f = 15 cm Object distance, u = −20 cm Object height, h = 5 cm the mirror formula, The value of v is positive which indicates that the image is formed behind the mirror. The value of image height is positive which explains that the image formed is erect. Hence, the image formed is virtual, erect, and smaller in size.

Q15	An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.
Ans:	Given Object distance, u = −27 cm Object height, h = 7 cm Focal length of concave mirror , f = −18 cm the mirror formula, The screen should be placed at a distance of 54 cm in front of the given mirror. The negative value of magnification indicates that the image formed is real. The Value of image height is negative that shows that the image formed is inverted.

Q16	Find the focal length of a lens of power – 2.0 D. What type of lens is this?
Ans:	Power of lens = -2.0 D A concave lens has a negative focal length.therefore , it is a concave lens.

Q17	A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Ans:	Given, Power of lens = + 1.5 D A convex lens has a positive focal length.Therefore , it is a convex lens or a converging lens.