Page 139 - Science Class 07

Time for sunlight to reach the earth’ is 8
minutes 20 seconds

= 8 × 60 + 20 seconds = 500 seconds
Speed of light = (150,000,000 ÷ (500) km/sec.

Light travels in a straight line. = 300000 km per second (approx).
Now, remove the middle card-board a li le Again, the moon is about 400,000 kilometres
to one side and again look at the flame of away from the earth. Let us calculate how long
the candle. The flame will not be visible does it take its light to reach the earth.
because the holes are not in the same
Distance from moon to earth
straight line now. If you place the middle
card-board in its posi on once again, the = 400000 km.
flame shall be again visible. Speed of light = 300000 km./sec
So, it is proved that light travels in straight Time = 400000 ÷ 300000
lines. This property of light is called
rec linear propaga on of light. = 1.33 sec.
It is because of this fast speed of light that as
This fact is used in a simple device to take soon as we switch on a bulb, it begins to glow
photograph–like images on a screen. This instantly without any me-gap in between.
device is called a pin-hole camera.
Rays and Beams
A Pinhole Camera
We know that light travels in the form of rays. A
Take a cubical box of cardboard and paint its
ray of light is depicted as a straight line with an
inside roof and the inside walls black. Cut of
arrow head at one of its ends.
one of its walls and cover it with a tracing
So, we can define a ray of light as under:
paper that will act as its screen.
A very narrow path of light represented by a
Make a fine pin-hole in the middle of the wall
thin straight line with an arrow-head to one
that is opposite to the tracing paper. Place a
end called a ray of light.
vase in front of the pinhole and then look at
the tracing paper. You will see an image of the When several rays of light run parallel and close
vase on the screen. It will be inverted (upside to each another, they form a beam of light,
down) also smaller in size than the vase. So, we can define a beam of light as under:

SPEED OF LIGHT
We have studied clearly that light travels in a
straight line. Let us now try to know how fast it
A Ray of Light Parallel Beam
travels. We know that the sun is at a distance
of 150,000,000 kilometres from the earth. Its

light reaches the earth in 8 minutes only.
So, we can calculate the speed of light as under:
Distance of the sun from the earth =
Convergent Beam Divergent Beam
150,000,000 km.

Science-7 139