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- Take a card
the size of a post card. Punch a small hole near the centre of
the card with a paper punch. Also punch a slightly larger hole
in the card at some distance from the first hole. Hold the card
in direct sunlight, so that it casts a shadow on the ground. Increase
the height of the card and observe the shadow. You will observe
two circular images of the Sun within the shadow.
- Observe
what happens to the images as you increase or decrease the distance
between the card and the screen (ground).
a. As the distance
increases or decreases, the sizes of the circular images increase
and decrease.
b. At any given
distance, both the images are approximately the same size..
c. One image
is brighter than the other, but both are approximately the same
size..
- Measure with
a centimeter scale the diameter 'I'
of the circular image. Also measure the distance 'L'
between the card and the screen . Divide L
by L.
You will get a number which is approximately 100
( more accurately, about 110)
- You can
measure I
and L
for different distances. Then both I
and L
will change. However, the number whichyou get when you divide
L by
I will
not change. It will always be approximately 110. Look at the shade
of a tall tree. Within this shade you will see circles of light.
Measure the diameter of the largest circle, divide this number
by the height of the tree. You will get an answer of approximately
110. (Fig.4) Why is L / I always (110)? .
L
/ I= S / D = 110
In a pinhole camera the ratio
L/I
is equal to S/D
where D
is the diameter of the Sun and S
is its distance. Since S/D
is constant, therefore
L/I is always the same i.e. 110.
- This number
is the first hint of the sun's distance. It tells us that the
sun's distance is approximately 110 times the sun's diameter.
But what is the diameter of the sun? We will learn about this
in the experiments to follow.
- For now ,
we have the following result :
Distance of Sun = 110 x diameter of Sun
- What is
the diameter of the Sun? The Transit of Venus gives us an opportunity
to measure the diameter of the Sun. It tells us "How
Big is the Sun?" Here is how you can do it yourself.
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