# NCERT Class 10: Science-Intext Solutions

### Chapter 10-Light - Reflection and Refraction-Intext Solutions

NCERT Book-Page Number-168
Q.1. Define the principal focus of a concave mirror.

Ans. Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on the principal axis after reflection. This point is known as the principal focus of the concave mirror.

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

Ans. Radius of curvature, R = 20 cm
Radius of curvature of a spherical mirror = 2 × Focal length or R = 2f
f = R/2 = 20 cm/ 2 = 10 cm
Hence, the focal length of the given spherical mirror is 10 cm.

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

Ans. Concave Mirror.

Q.4. 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 it has a wider field of view, which allows the driver to see most of the traffic behind him. Convex mirrors always form a virtual, erect, and diminished image of the objects placed in front of it.

NCERT Book Page Number-171

Q.1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Ans. Radius of curvature, R = 32 cm.
Radius of curvature = 2 x Focal length (f)
R = 2f
Or f = R/2 = 32cm/2
= 16 cm.
Hence, the focal length of the given convex mirror is 16 cm.

Q.2. A concave mirror produces three times magnified (enlarged) real image of object placed at 10 cm in front of it. Where is the image located?

Ans. Magnification produced by a spherical mirror is given by the relation,
m = Height of the Image/Height of the Object
= Image Distance /Object Distance
m = hi/ho = – v/u
Let the height of the object, h0 = hi
Then, height of the image, hi = –3h (Image formed is real)
∴− 3h/h = -v/u
v/u = 3
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.
NCERT Page Number-176

Q.1. 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 bends towards the normal. When a ray of light enters from an optically rarer medium (having low refractive index) to an optically denser medium (having high refractive index), it slows
down and thus it bends towards the normal. Since water is optically denser than air, a ray of light entering from air into water will bend towards the normal.

Q.2. 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.
Ans. Refractive index of a medium, μm = Speed of light in vacuum Speed of light in the medium
Speed of light in vacuum, c = 3 × 108 ms–1
Refractive index of glass, μg = 1.50
Speed of light in the glass, v = Speed of light in vacuum/Refractive index of glass
= c/μg = 3 ×108 /1.5 = 2 × 108 ms–1.

Q.3. 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
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.

It can be observed from the above table, that diamond and air respectively have the highest and lowest refractive index. Therefore, diamond has the highest optical density and air has the lowest optical density.

Q.4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in table in above question

Ans. In water light travels fastest as compared to kerosene and turpentine because the refractive index of water is lower than that of kerosene and turpentine. The speed of light is inversely proportional to the refractive index.

Q.5. 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 be reduced by a factor of 2.42 as compared to its speed in air.
In other words, the speed of light in diamond is 1/2.42 times the speed of light in vacuum.

NCERT Book Page Number-184

Q.1. Define 1 diopter of power of a lens.

Ans. The SI unit of power of lens is diopter which is denoted by the letter D. 1 diopter is defined as the power of a lens of focal length 1 metre.

Q.2. 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.3

Ans. Since the image is real and same size. The position of image should be at 2F.
It is given that 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
According to the lens formula,
1/v – 1/u = 1/f
1/f = 1 /50 – 1/(-)50 = 1/50 + 1/50 = 1/25
f = 25 cm = 0.25 m
Power of lens, P = 1/f = 1/ f (in metres)=1/ 0.25 = + 4 D

Q.3. Find the power of a concave lens of focal length 2 m.

Ans. Focal length of concave lens, f = –2 m. Power of lens, P = 1/f = 1/(–2) = –0.5 D