7th Class Science Light NCERT Summary - Mirrors and Reflection of Light

Mirrors and Reflection of Light

• An object acts in three ways to the light that falls on it. It may allow the light to pass through, it may completely cut off the light from passing by absorbing the light, or, the object may scatter the light incident on it.

REFLECTION OF LIGHT

• The light beam (1) above the mirror consists of unstopped light rays. Light beam (2) defines the length of the shadow. Rays (3), (4), (5) and (6) are stopped by the mirror.
• Instead of passing through, the mirror surface bounces light rays like a smooth wall bounces a ball.
• The light rays falling on a mirror are scattered back. This scattering back is called reflection.
• The reflection of light from a smooth and an uneven surface is different, as the smooth surface gives a clear image due to regular reflection. On the other hand, the reflected rays from an uneven surface are scattered in all directions. This is why a mirror that has lost its smoothness does not give a clear image.

IMAGE FORMED BY A PLANE MIRROR

• Our image in a plane mirror looks different from us. The left appears right, and the right appears left in the mirror. This phenomenon is called "lateral inversion". We, therefore, say that the image formed by a plane mirror is laterally inverted.
• On looking at the image of all the letters from A to Z in a plane mirror, the images of some letters appear to be the same as the original letters. H and 0 are two examples.
• The image formed by a plane mirror is of the same size as the object.
• The image is also erect. It means the top of the object does not become the bottom part of the image and bottom vice-versa.
• Another feature of the image formed in a plane mirror is that if we move close to the mirror, our image also moves closer. Similarly, if we move away from the mirror, our image also moves away. Thus, the distance of the image behind the mirror is equal to the distance of the object in front of the mirror.

The image formed of an object by a plane mirror is "virtual". It means that we cannot obtain the image formed in a mirror on a screen placed anywhere behind the mirror. An image which can be obtained on a screen is called "real image".

SPHERICAL MIRRORS

• The plane, convex and concave mirrors act very differently from each other. We use all three mirrors in our daily life. Plane mirrors form virtual images. Convex and concave surfaces are segments of a hollow sphere like a tennis ball. The outside surface is convex while the surface facing inside is concave.
• Concave mirrors are used in designing headlights of cars, train engines searchlights and even torchlights. On the other hand, convex mirror is used in rear-view mirrors in cars, scooters, buses and trucks. A driver looks at rear-view mirror to see the traffic behind his vehicle.