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Sheet of card
Sheet of white paper or a screen
Blackout for a window (e.g. black card that can be sellotaped over a window)
Sellotape or masking tape
Aluminium foil about 3cm x 3cm
Pin or sharp point
Ruler
Scissors
Candle or other light source
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Cut a square about 2cm x 2cm in the centre of the sheet of card.
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Put the aluminum foil over the 2cm square hole and tape it to the sheet of card.
Use a pin or similar to make a small hole in the foil.
• Your sheet of card complete with pinhole is your “pinhole camera”.
1. First check that you get an image with your pinhole camera.
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Place a lit candle (or other light source) in front of the pinhole and then turn out all other lights.
Hold a sheet of white paper or a screen a few centimeters from the back of the camera (as shown in this figure).
You should be able to see an image of the candle flame (or other light source) on the screen.
Measure accurately and record the height of the image
My image was ______________ cm high.
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Measure accurately and record the distance between the image and the pinhole camera.
The distance between my image and the pinhole camera was ______________ cm.
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Measure accurately and record the distance between the candle or light source and the pinhole camera
The distance between my candle and the pinhole camera was ______________ cm.
Dr Lisa Jardine-Wright, Cavendish Laboratory December 2007

Things to find out
• What happens to the size of the image as you move the sheet of paper or screen further away from the pinhole, but keep the candle in the same place?
__ Image will get larger and fainter _______________________________________
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Try drawing a graph of image height (vertical axis) against the distance of the paper or screen from the pinhole camera (horizontal axis).
Look at your graph – what do you think it tells you?
For each pair of measurements, calculate the result when you divide the height of the image by the distance between the camera and the screen or paper.
Multiply the result of the division calculations by the distance between the candle and the pinhole camera.
cm
cm
cm
= _________________________ cm = Height of the candle flame or light source
• Measure the height of your candle flame or light source.
My light source is _____________________ cm high
How similar are the your two values for the height of the light source _________________
You should find that the graph is a straight line, because the height of the image is directly proportional to the distance between the image and the pinhole.
You should also find that all the divisions give the same result (and that this is the same as the gradient of the graph.
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Multiply the result of the division calculations by the distance between the candle or other light source and the pinhole camera.
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This should give you the height of the actual candle flame or light source – check to see how accurate the value is
2. Finding the diameter of the Sun or Moon.
It is very important that no one looks directly at the sun, as this will damage the retina in the eye. The pinhole camera is a safe way to observe the sun, since you only need to look at the image on the screen, not directly at it.
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Black out a window that faces the Sun or Moon, and cut a square about 2cm x 2cm in the centre of the blackout material.
Put the foil over the square hole and tape it to the blackout material.
Use a pin or similar to make a small hole in the foil.
Dr Lisa Jardine-Wright, Cavendish Laboratory December 2007

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Turn out all the lights in the room.
Hold a sheet of white paper or a screen as far from the pinhole as possible.
You should now be able to see an image of the Sun or Moon on the paper or screen – if it isn’t clear enough, try blacking out more windows to reduce the light in the room or experiment with the size of your pinhole.
Measure accurately and record the distance between the pinhole and the white paper or screen.
The distance between my image and the pinhole camera was ______________ cm.
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Measure accurately and record the diameter of your images of the Sun or Moon (using the same units).
The diameter of my Sun or Moon image is ______________ cm.
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Now we will calculate the real diameter of the Sun or Moon from your measurements.
Calculation
• The diameters of the moon and the cut-out semi-circle are in the same proportion as the distances between your eye and the moon and between your eye and the window.
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The distance of the Earth from the Sun is 149,600,000km
The distance of the Earth from the Moon is 384,400 km
You will need to make sure that you change all your distance measurements to the same units, e.g. metres. o Remember there are 100cm in 1m and 1000m in 1km.
Fill in the blanks in the boxes below ( example measurements )
Diameter of _ MOON_ in km
=
384,400 km
×
1.1
cm
120 cm
Diameter of _MOON_ in km
=
3523 km
OR
Diameter of _ SUN_ in km
=
149,600,00 0 km
×
1.1
cm
120 cm
km
=
km
Dr Lisa Jardine-Wright, Cavendish Laboratory December 2007

Things to think about
• How many times bigger is the Sun than the Moon?
Diameter of SUN in km
Diameter of MOON in km
=
1,371,333
3,523
≈
390
• Why do you think that they appear to be about the same size to us?
Because the Sun is about 390 times further away than the moon
Distance of SUN from Earth in km
Distance of MOON from Earth in km
=
149,600,00
384,400
0
≈
390
The calculation of relative sizes when using a pinhole camera relates to the method of similar triangles:
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In the diagram on the right the red lines show the path of the light rays from the Sun or Moon through the pinhole onto the screen.
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Note that the image is upside down, compared with the object – this won’t show up easily for the Sun or
Moon but should be evident for the candle.
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The two triangles AOB and COD are similar , although not the same size
(the same shape with a common, equal angle at the apex).
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This that:
AB
XO
CD
OY
AB
CD
XO
OY
Dr Lisa Jardine-Wright, Cavendish Laboratory December 2007