Wearing the wrong prescription glasses may be bad for your eyes, but looking at the Sun is much worse. Next week, on December 4, there will be a solar eclipse visible from many places in the southern hemisphere. Throughout Australia, there will be a partial eclipse, whereby the Moon covers part of the Sun. The Royal Australian and New Zealand College of Ophthalmologists advises that there is no safe way to directly observe an eclipse, including sunglasses, welder's masks, or any other device not specifically designed to observe the sun. This week we will look at some methods for indirectly observing the eclipse.
Safety: Neither of the methods described here involve looking directly at the Sun, you will be looking at an image of the Sun projected onto paper. Do not look directly at the Sun, during an eclipse or at any other time. It can permanently damage your retina (the light sensors on the back of your eye).
You will need
The simple version
Two pieces of thin white cardboard or paper.
A pin
The fancy version
A cardboard box, roughly shoebox or tissue box size, open on the top.
A piece of white paper.
Glue
A pin
Scissors
What to do
The simple version
Using the pin, make a hole in one piece of cardboard.
Stand with your back to the Sun.
Hold the white piece of cardboard in one hand, so the Sun is shining on it.
Using the other hand, hold the piece of cardboard with the hole in it between the white cardboard and the Sun.
You should find a small image of the Sun is projected onto the white cardboard. It may take a little experimenting to get the pieces of cardboard lined up correctly.
The fancy version
Stick the paper to the inside of one end of the box. (If the inside of the box is white already, don't bother.)
Make a pinhole in the other end of the box.
Take the box outside.
Stand with your back to the Sun.
Hold the box so the end with the pinhole pointing towards the Sun.
You should find a small image of the Sun is projected onto the white paper. It may take a little bit of experimenting to get the box pointing the right way.
To be sure that what you are seeing is an image of the Sun, try standing near a tree so there are some leaves between your viewer and the Sun. You should see the shadow of the leaves in the projected image.
What's happening?
Both of these eclipse viewers use the pinhole method for producing an image. The main difference between them is that when you use the box, you don't get as much ambient light on the paper, so the image is easier to see.
Normally light from a point on the Sun shines onto all of the paper.
With a pinhole projector, light from a point on the Sun only reaches one point on the paper, so an image is produced.
If you hold a piece of paper so it is facing the Sun, then the light from a point on the Sun can shine on the entire piece of paper, so there is no image formed. When you hold a sheet of paper with a pinhole between the Sun and the paper, the light from a point on the Sun has to travel through the hole to get to the paper, so it can only illuminate a small area of the paper. If the hole is small enough, the light from different points on the Sun will land on different parts of the paper and an image will be formed.
The size, clarity and brightness of the projected image depend on the size of the hole and the distance from the hole to the image.
A smaller hole makes a clearer image, but it is fainter, because less light gets through.
Increasing the distance between the hole and the image will make the image larger, but will appear fainter, because the light is spread out over a larger area.
If the eclipse is visible from your area, try looking at the shadows cast by trees and bushes during the eclipse. During a partial eclipse in Kalgoorlie a few years ago, I noticed the gaps between the leaves in the trees were projecting images of the eclipse on the ground. (There is an image of this on the web site)
Sunlight shining through the leaves of a tree caused this natural pinhole projection.
The pinhole projection technique has been used for centuries for safely observing the Sun. The idea was also used in a device called a Camera Obscura, an ancestor of modern cameras. A Camera Obscura was originally a room with a small hole in one wall, which would project an upside-down image of the outside world onto the opposite wall. Over time, portable versions of the Camera Obscura were made, their images were improved using lenses and eventually it became possible to record their images directly using light-sensitive chemicals, which was the start of photography.
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