Ncert class 10th chapter Light Reflection and Refraction

                    Ncert Class 10th Science

               Light Reflection and Refraction

                       Shiva Valley School 

Question and Answers

Question 1: Define the principal focus of a concave mirror.

Answer:

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

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

Answer:

Radius of curvature, R= 20 cm

Radius of curvature of a spherical mirror = 2 x Focal length (R = 2f R 20 1=22 =10 cm

Hence, the focal length of the given spherical mirror is 10 cm.

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

Answer:

When an object is placed between the pole and the principal focus of a concave mirror, the image formed is virtual, erect, and enlarged. 

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

Answer:

Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of them. They are preferred as a rear-view mirror in vehicles because they give a wider field of view, which allows the driver to see most of the traffic behind him.

Question 5: 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?

Answer:

The light ray bends towards the normal. When a ray of light travels from an optically rarer medium to an optically denser medium, it gets bent towards the normal. Since water is optically denser than air, a ray of light travelling from air into the water will bend towards the normal.

Question 6: Which one of the following materials cannot be used to make a lens?

(a) Water

(b) Glass

(c) Plastic

(d) Clay

Answer:

(d) A lens allows light to pass through it. Since clay does not show such property, it cannot be used to make a lens.

Question 7: The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?

(a) Between the principal focus and the centre of curvature

(b) At the centre of curvature

(c) Beyond the centre of curvature

(d) Between the pole of the mirror and its principal focus.

Answer:

(d) When an object is placed between the pole and principal focus of a concave mirror, the image formed is virtual, erect, and larger than the object.

Question 8: Where should an object be placed in front of a convex lens to get a real image of the size of the object?

(a) At the principal focus of the lens

(b) At twice the focal length

(c) At infinity

(d) Between the optical centre of the lens and its principal focus.

Answer:

(b) When an object is placed at the centre of curvature in front of a convex lens, its image is formed at the centre of curvature on the other side of the lens. The image. formed is real, inverted, and of the same size as the object.

Question 9: A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the lens are likely to be

(a) both concave

(b) both convex

(c) the mirror is concave and the lens is convex

(d) the mirror is convex, but the lens is concave

Answer:

(a) By convention, the focal length of a concave mirror and a concave lens are taken as negative. Hence, both the spherical mirror and the thin spherical lens are concave in nature.

Question 10: No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

(a) plane

(b) concave

(c) convex

(d) either plane or convex

Answer:

(d) A convex mirror always gives a virtual and erect image of smaller size of the object placed in front of it. Similarly, a plane mirror will always give a virtual and erect image of same size as that of the object placed in front of it. Therefore, the given mirror could beveither plane or convex.