Class 10th Science Chapter 9 – Light – Reflection and Refraction – In-text Questions

Page 142 – Questions

Q1. Define the principal focus of a concave mirror.

Answer:
The principal focus of a concave mirror is the point on the principal axis at which all the light rays parallel to the principal axis converge after reflection from the mirror.

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

Answer:
We know: R = 2f
So, f = R/2 = 20/2 = 10 cm

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

Answer:
A concave mirror can give an erect and enlarged image when the object is placed between the pole (P) and the focus (F).

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

Answer:
Convex mirrors are used as rear-view mirrors because:

• They always form an erect image.

• The image is diminished, so a larger area can be seen.

• They provide a wide field of view, helping the driver see more traffic behind.

Page 145 – Questions

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

Answer:
Using R = 2f
So, f = R/2 = 32/2 = 16 cm

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Q2. A concave mirror produces three times magnified real image of an object placed at 10 cm in front of it. Where is the image located?

Answer:
Magnification m = –v/u (negative because image is real and inverted)

Given: m = 3, u = –10 cm

$$3=-v\mathrm{/}(-10)\Rightarrow v=30\text{ cm}$$

So, the image is formed 30 cm in front of the mirror.

Page 150 – Questions

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Q1. A ray of light travelling in air enters obliquely into water. Does it bend towards the normal or away from the normal? Why?

Answer:
It bends towards the normal, because light travels slower in water (denser medium) than in air (rarer medium).

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Q2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass?

Answer:

$$n=\frac{c}{v}\Rightarrow v=\frac{c}{n}=\frac{3\times {10}^8}{1.50}$$

$$v=2\times {10}^8\text{ m/s}$$

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Q3. Find out, from Table 9.3, the medium having highest optical density. Also find the medium with lowest optical density.

Answer:

• Highest optical density: Diamond (n = 2.42)

• Lowest optical density: Air (n = 1.0003)

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

Answer:
Light travels fastest in the medium with lowest refractive index.

Refractive indices:
Water = 1.33, Kerosene = 1.44, Turpentine = 1.47

So, light travels fastest in water.

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

Answer:
It means the speed of light in air is 2.42 times more than the speed of light in diamond, or light slows down by 2.42 times when entering diamond.

Page 158 – Questions

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Q1. Define 1 dioptre of power of a lens.

Answer:
1 dioptre is the power of a lens whose focal length is 1 metre.

$$1D=1m^{-1}$$

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Q2. A convex lens forms a real and inverted image of a needle at 50 cm from it. Where is the needle placed if the image is equal in size to the object? Also find the power of the lens.

Answer:
For a convex lens, an image equal in size to the object is formed when object is at 2F and image is also at 2F.

So, 2F = 50 cm → F = 25 cm

Thus, object is placed 25 cm from lens.

Power:

$$P=1\mathrm{/}f=1\mathrm{/}0.25=4D$$

Power = +4 dioptre

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

Answer:

$$P=1\mathrm{/}f=1\mathrm{/}(-2)=\text{–}0.5D$$

Power = –0.5 dioptre (concave lens)