Page 140 - Science Class 07
P. 140
The diagrams given below explain these facts
clearly.
1. How light is produced?
2. Name any three natural sources of light.
3. How brightness of light is measured?
4. Why the light reflected by the moon is
called cold light?
REFLECTION OF LIGHT a) Glass b) Unpolished c) Mirror
Wood
The word ‘reflect’ means ‘to bounce back’ or Light bounces back.
‘rebound’. In science, this word is used in REFLECTION TERMS
rela on to heat, light, sound or waves. It refers
Mirror
to the bouncing back of a ray/rays or a
The smooth and polished surface of an object
wave/waves.
that can turn back the rays of light into the
More than 2000 years ago, a Greek, named same medium is called mirror. Imagine a line
Hero of Alexandira made experiments with drawn perpendicular to a floor at a point where
mirrors. He discovered that the angle of a light- a light-ray hits its. Then-
ray striking a mirror is the same as the angle of
v This perpendicular line is called normal.
the light-ray that bounces back from the mirror.
v The ray’s path along which is strikes the
Let us test it with a torch.
floor is called incident ray.
Switch on the torch and stand it to the one side
v The point where the incident ray strikes the
of a mirror. Now, note where you have to stand
floor is called point of incidence.
in order to see the image of the torch-bulb in
v The angle made by the incident ray with
the mirror. You will see that the angle of the
the normal is the angle of incidence (Ði).
real torch bulb to the mirror is the same as
your angle to the bulb inside the mirror. This Now, we know that the incident ray reflects
phenomenon is called reflec on of light in from the point of incidence. Then—
science. v The path along which the light-ray reflects
Reflec on is the process of rebounding of a is called the reflected ray.
light-ray a er striking a surface. v The angle made by the reflected ray with
the normal is the angle of reflec on (Ðr).
Remember that, the reflec on of light-rays is
regular from smooth bright surfaces. But from TWO LAWS OF REFLECTION
rough dull surfaces, it is irregular as light-rays A er understanding the terms regarding
sca er randomly a er striking such surfaces. reflec on given above, we can easily follow the
Whereas, regular reflec on of rays cause two laws of reflec on.
images to be formed while the irregular These two laws are as under:
reflec on doesn’t lead of image forma on. v The normal, the incident ray and the
reflected ray–all are in the same plane.
140 Science-7
