10. Light-Reflection and Refraction Science class 10 in English Medium ncert book solutions Text-book Questions

Text-book Questions 

Page no. 168 

Q1. Define the principal focus of a concave mirror.

Ans: The incident rays parallel to the principal axis after reflection intersect at a point F between the pole and the center of curvature of the mirror. This point is called the principal focus of a concave mirror.

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

Ans: Radius of curvature = 20 cm

Focus distance = radius of curvature/2

          = 20/2

          = 10 cm

Thus, 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: A concave mirror, when the object is placed between the pole and the principal focus of the concave mirror, it forms an erect and magnified image.

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

Ans: Convex mirrors are also preferred because they always produce an erect image though it is smaller. Their field of vision is also very large because they are curved outwards. Hence, convex mirrors enable the driver to see a much larger area behind him as compared to plane mirrors.

Page no. 171 

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

Ans: In a convex mirror,

radius of curvature R = 32 cm

radius of curvature = 2 × focus distance

Focus distance = radius of curvature/2

           = 32/2

           = 16 cm

Thus, the focal length of that convex mirror = 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:  In a concave mirror,

Object distance (u) = – 10 cm

Magnification (m) = – 3 [Since the image is real]

Page no. 178

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 ray of light will bend towards the normal, because the ray of light enters from air which is a rarer medium to water which is a denser medium than air, then in such a case the light will bend towards the normal.

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 × 108 m s–1.

Answer: 

Q3. Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.

Ans: The medium of maximum optical density is diamond whose refractive index is 2.42.

The medium of minimum optical density is air whose refractive index is 1.0003.

Q4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3.

Ans: From Table 10.3

Refractive index of kerosene = 1.44

Refractive index of turpentine = 1.47

Refractive index of water = 1.33

In this, the speed of light is highest in water and the speed of light is the least in turpentine oil because the higher the refractive index, the less will be the speed of light in that medium and the lower the refractive index of which is the speed of light in that medium. will be more

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

Ans: The refractive index of diamond is 2.42. The meaning of this statement is that the optical density of diamond is high, due to which it is a hard material, it has the lowest speed of light.

Page no. 148 

Q1. Define 1 dioptre of power of a lens.

Ans: If the focal length of a lens is 1 meter, then it is called 1 diopter power of the lens.

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.