Page 138 - Science Class 07
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close to the directly lighted area. So, the shining with a cool light like that of the moon.
beholder can see objects lying in that shady They photographed it and gave its light the
area as well. name earth shine that is cold light too.
BRIGHTNESS OF LIGHT Some fish and fire-flies also shed light from
We know that some sources of light are their bodies. This light is also called cold light as
brighter than others. it has no heat.
For example, a bulb is brighter than a candle. HOW DOES LIGHT TRAVEL?
The brightness produced by a light-source is We have already read that light travels quite
called its luminous intensity. This brightness straight through its rays. Let us check this fact
was measured in the unit called candle power through observa on and an experiment as
in the past. But these days, the unit used to well.
measure brightness is called lumen. Remember v Look at the ligh ng candle at night through
that one candle power= 1256 lumens. a small rubber pipe. The candle flame will
LUMINOUS BODIES enter the pipe to reach your eyes. Bend the
The objects both natural and man-made, that pipe a li le. Can you see the candle flame?
give out light from their bodies are called No, not at all. Because the straight path has
luminous bodies. They include stars, candles, been blocked by the bend of the pipe.
oil-lamps, etc. All such bodies shed hot light. v Close all the doors and windows of a room
The object that shine with the light of the and let light enter inside through a small
luminous objects and have no light of their hole.
own are non-luminous bodies. They include all You will see sunlight coming into the room
planets, all moons and other heavenly bodies quite straight.
too. They receive sunlight that falls on them
and most of it is reflected back into space. Due
to this light, these bodies shine themselves.
The light that they reflect into space is cold Take three pieces of cardboard of the same
size. Placing them one upon the other, make
light.
a hole in all the three pieces with something
sharp and pointed. Now, amount the card-
board pieces on similar stand of equal height
and place them on a smooth table in upright
posi ons such that their holes are in the
same straight line.
Now, light a candle and place it such that its
flame is just before the hole of the card-
board piece at one end. Look through the
Non-Luminous Body holes from the other end. The flame of the
candle shall be visible because the light
The moonlight is also cold light and that is why
travels in a straight line.
it is cool and pleasant. The astronauts saw the
earth from the space and found the earth
138 Science-7
