# Light : Reflection and Refraction Questions and Answers Class 10

## NCERT Solutions for Class 10 Science Light : Reflection and Refraction Questions and Answers

Intext Questions

Page Number – 168

Q1. Define the principal focus of a concave mirror.

Ans. Principal Focus of a concave mirror : It is a point on the principal axis of the mirror at which the light rays coming parallel to the principal axis, after reflection, actually meet.

Q2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Ans. Focal Length, f = R/2 = 20/2 = 10cm

Q3. Name a mirror that can give an erect and enlarged image of an object.

Ans. Concave mirror.

Q4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Ans. We prefer a convex mirror as a rear-view mirror in vehicles because:

1. The field of view of a convex mirror is wider than the concave mirror.
2. A convex mirror always produces an erect image of the objects.

Page Number – 171

Q1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Ans. Given, radius of curvature, R = 32cm

Focal length, f = R/2

f = 32/2 = 16cm

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. Because the image is real, so magnification ‘m’ must be negative.

It is given that, v = 3u, m = -3

v = -30cm

Page Number – 176

Q1. 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. The light-ray bends towards the normal because water is an optically denser medium than air. so, the ray of light goes from a rarer medium to a denser medium.

Q2. 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.

Ans.

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 m/s

Refractive index of glass (ng) = 1.50

Speed of light in the glass (v) = Speed of light in vacuum/ Refractive index of glass

= c/ng

=3 × 108/1.50 = 2x 108 ms-1.

Q3. Find out, from Table, the medium having highest optical density. Also find the medium with lowest optical density.

Ans. Highest optical density = Diamond

Lowest optical density = Air

Q4. You are given kerosene, turpentine and water. In which of these does the light travel fastest?

Ans. For kerosene, n = 1.44
For turpentine, n = 1.47
For water, n = 1.33
Because water has the lowest refractive index, therefore light travels fastest in this optically rarer medium than kerosene and turpentine oil.

Q5. The refractive index of diamond is 2.42. What is the meaning of this statement?

Ans. Diamond has a refractive index of 2.42 which means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in the air.

Page Number – 184

Q1. Define 1 dioptre of power of a lens.

Ans. 1 diopter is a power of lens whose focal length is 1 metre.

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.

Q3. Find the power of a concave lens of focal length 2 m.

Exercise

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

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) Between the pole of the mirror and its principal focus.

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

(b) At Twice the focal length

(c) At infinity

(d) Between the optical centre of the lens and its principal focus.

Ans. (b) At twice the 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) Both concave

Q5. No matter how far you stand from the mirror, your image appears erect. The mirror is likely to be:

(a) plane

(b) concave

(c) convex

(d) either plane or convex.

Ans. (d) Either plane or convex.

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) A convex lens of focal length 5 cm.

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.

Range of the distance of the object = 0 to 15 cm from the pole of the mirror.

Nature of the image = virtual, erect, and larger than the object.

Q8. Name the type of mirror used in the following situations.

(b) Side/rear-view mirror of a vehicle.

(c) Solar furnace.

Ans. (a) Concave Mirror: Because concave mirrors can produce a powerful parallel beam of light.

(b) Convex Mirror: to get the largest field of view.

(c) Concave Mirror: Because it concentrates the parallel rays of the sun at a principal focus.

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.

The lens will produce a complete image but the image will have lower intensity and brightness.

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.

Height of the Object, h0 = 5 cm

Distance of the object from converging lens, u = -25 cm

Focal length of a converging lens, f = 10 cm

Using lens formula,

Thus, the image is inverted and formed at a distance of 16.7 cm behind the lens and measures 3.3 cm.

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.

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.

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.

Focal length of convex mirror (f) = +15 cm

Object distance (u) = – 10 cm

According to the mirror formula,

The image is located at a distance of 6 cm from the mirror on the other side of the mirror.

The image formed is virtual,erect and diminished.

Q13. The magnification produced by a plane mirror is +1. What does this mean ?

Ans. m = 1 indicates the size of the image is the same as that of the object. Positive sign of m indicates that an erect image is formed.

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. Object distance (u) = – 20 cm

Object height (h) = 5 cm

According to the mirror formula,

The positive value of image height indicates that the image formed is erect.

Hence, the image formed is erect, virtual, 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. Object distance (u) = – 27 cm

Object height (h) = 7 cm

Focal length (f) = – 18 cm

According to the mirror formula,

The negative value of image height indicates 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. Here, P = -2.0 D

The type of lens is concave because the focal length is negative.

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. Here, P = +1.5 D

Because the focal length is positive, this lens is converging.