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# Class 10 Light-Reflection and Refraction Short answers

#### Short Answer Type Questions – I [2 Marks]

Q.1. What makes things visible?

Ans. In a lighted room, when light falls on an object, it gets reflected. This reflection, when received by our eyes, enables us to see things.

Q.2. What do you mean by lateral inversion of the images in a plane mirror?

Ans. If we look into the plane mirror to see the image, we see that the image is inverted sideways, i.e. if we move our left hand the image appears to move its right hand. This is known as lateral inversion.

Q.3. What are the values of angle of incidence i and angle of reflection r for a normal incidence?

Ans. Normal incidence implies i = 0°. Hence from the second law of reflection r = 0°.

Q.4. An object of width 10 cm is placed 40 cm away from the plane mirror. Where is the image formed and what will be its width?

Ans. The image will be formed at a distance of 40 cm. The width of the image will be 10 cm.

Q.5. List the four characteristics of the image formed by the plane mirror.

Ans. Four characteristics of the image formed by the plane mirror. (i) It is always erect and virtual. (ii) The size of the image is equal to that of the object. (iii) Image is formed at the same distance behind the mirror as the object is in front of the mirror. (iv) Image is laterally inverted.

Q.6. State the laws of reflection of light.

Ans. (i) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane. (ii) The angle of incidence is equal to the angle of reflection.

Q.7. Draw a labelled diagram to show the formation of an image on a plane mirror.

Ans:

Q.9. AB and CD, two spherical mirrors from parts of a hollow spherical ball with its centre at ‘O’ as shown in the diagram. If Arc AB = 1/2 arc CD, what is the ratio of their focal length? Ans. The focal length of both the mirrors will be the same 1:1

#### Short Answer Type Questions – [3 Marks]

Q.1. Distinguish between the real image and virtual image

Q.2. Explain the reflection of light.

Ans. When light falls on an object or any surface, it may pass through it, it may get absorbed by it or it may bounce back after striking the object (surface). The bouncing back of light rays from the surface of an object is called reflection. The ray of light which falls on an object is called an incident ray of light and the ray of light which gets bounced back from the surface of an object is called reflected ray of light. The angle between the incident ray and the normal at the point of incidence is called the angle of incidence (∠ i). The angle between the reflected rays and the normal one is called the angle of reflection (∠ r).

Q.3. Draw a neat ray diagram to illustrate the formation of images due to a beam of light incident parallel to the principal axis of an (i) concave mirror, (ii) convex mirror.

Ans. Ray diagrams have been shown in following [Fig. (a) and (b)].

Q.4. Show geometrically that for a spherical mirror R = 2f.

Ans. Consider a light ray PQ incident parallel to the principal axis of a concave spherical mirror as shown in the figure.

After reflection, the ray is reflected along with QF. Q ∠PQC, ∠i = ∠CQF, ∠r [By law of reflection]

and ∠QCF = ∠PQC [Alternate angle]

∴ ∠CQF = ∠QCF Hence, the triangle QCF is an isosceles triangle.

i.e. CF = FQ. For a mirror of a small aperture FQ = FP.

Hence, CF = FP or PF = PC/2

Q.5. Complete the path of a ray of light after reflection in the mirror in the given diagram.

Q.6. Differentiate a real image from a virtual image giving two points of difference.

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