NCERT QUESTIONS WITH THEIR ANSWERS

IN-TEXT QUESTIONS

Page 168

1. Define the principal focus of a concave mirror.

Ans. A point in front of a concave mirror, on its principal axis, such that parallel rays travelling parallel to the principal axis, after reflection actually converge at this point.

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

Ans. Focal length = Radius of curvature / 2 = 20 cm / 2 = 10 cm

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

Ans. The mirror is called concave mirror.

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

Ans. It is because, it covers a wide rear field and forms small, erect and virtual image, close to the eye of the driver of the vehicle.

Page 171

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

Ans. 16 cm [ f= R/2 ]

2. 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?

Ans.: Using Mirror Formula You Will Get V= -30 cm. Thus the real and inverted image is formed at a distance of 30 cm from the pole and in front of it.

Page 176

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

Ans. The ray bends towards normal. It is because the velocity of light decreases in water as compared to the air.

2. Light enters from air to glass having refractive index 1.5. What is the speed of light in the glass? Speed of light in vacuum is 3×10^8 ms^–1.

Ans. Speed of light in glass = Speed of light in vacuum
Refractive index = (3 x 10^8 ms^-1) / 1.5 = 2 x 10^8 ms^-1

3. Name a medium which has : (i) highest optical density (ii) lowest optical density.

Ans. (i) Highest optical density of material medium is 2.42 for diamond.
(ii) Lowest optical density of material medium is 1.0003 for air.

4. You are given kerosene oil, turpentine oil and water having optical densities 1.44, 1.47 and 1.33 respectively. In which of these medium the light travels fastest and why?

Ans. Light travels fastest in water, as lesser the optical density, more is the speed of light.

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

Ans. From the statement it implies that the ratio of speed of light in vacuum to the speed of light in diamond is 2.42.

Page 184

1. Define 1 dioptre power of lens.

Ans. A lens having a focal length of 1 m is said to have a power of 1 dioptre.

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

Ans.
Power of lens = 1 / Focal length in metres = 1 / -2 m = – 0.5 D

3. 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. As the convex lens forms a real and inverted image of the needle equal to the size of the object, therefore, we can say that the needle is located at 2F which is 50 cm from the convex lens.