An empty cereal box is a simple and effective way to make a DIY pinhole viewer for safely viewing a solar eclipse. Like a pinhole projector, it creates an image of the Sun—and it may provide a clearer view by blocking out surrounding sunlight.
The cardboard box keeps unwanted light out and allows only sunlight traveling through the pinhole to reach the viewing screen.
To make your cereal box eclipse viewer, cut a piece of white paper to fit the bottom of the box. Open the top flaps and tape the paper securely to the bottom. This paper will act as the screen where the projected image of the eclipse appears.
Close the box and seal it with tape. On one side of the top, cut out a small square opening that allows you to see the paper screen with one eye.
On the opposite side of the box top, cut out a second small square of the same size. Cut a piece of aluminum foil large enough to cover this opening completely.
Tape the foil over the square opening, then use a pin or needle to make one small hole in the center. This tiny opening will project the Sun’s image onto the paper screen.
Stand with your back to the Sun and look through the open square with one eye. Never look directly at the Sun through the pinhole or any other optical device.
Adjust the box until the pinhole is aligned with the Sun. You should see a small projected image of the Sun on the paper screen inside the cereal box. During a solar eclipse, this projected image will show the eclipse safely.
Source: www.bbc.co.uk