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### Class Ten Science Chapter 10 NCERT Solutions

#### Chapter 10 : Light : Reflection and Refraction

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.

Q.5. 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 = 32 cm/ 2 = 16 cm.

Hence, the focal length of the given convex mirror is 16 cm.

Q.6. 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. 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, the 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.

Q.7. 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 a low refractive index) to an optically denser medium (having a 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 the water will bend towards the normal.

Q.8. 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 a vacuum is 3 × 108 m/s.

Ans. Refractive index of a 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.