Light & Optics - Optical Illusion Activities

Wag a pencil in front of a television picture. The screen lights up to make a fresh picture 25 times a second - producing many separate outlines of the moving pencil.

Flicker Stick

Light will not travel in straight lines through a transparent substance (water, glass, plastic, air, etc) if there is any variation in the optical density or "thickness" of the substance.

Warm air is thinner (less dense) than cold air, and so light is refracted when it passes from one into the other. You can observe this happening just above the roof of a car on a hot sunny day.

The hot roof heats the air just above it. If you look through the layer of hot rising air, you will notice that the light coming from cooler air, through the moving hot air, gets bent. You see this happening when the background seems to be shimmering.

A traveller in a hot desert may see a mirage. Distant rocks or bushes seem to be reflected by the ground, giving the impression that a pool of water is present.

A mirage can be explained by refraction and reflection inside the air. Near the ground the air is hot, but higher up the air is cooler,

On a hot summer's day, look for a mirage in the distance on a hot road. It will look as if the sky is being reflected by the ground. But you will not be fooled into seeing a pool of water there, will you?

Fix a little card in a slot cut in a stick. On one side of the card draw a bird. Draw a cage on the other side. Roll the stick between your hands. The pictures appear in succession. A picture persists or lasts in your brain for about a tenth of a second. Your brain puts the pictures together to make one.

Hold together the ends of a finger from each hand about arms length in front of you. Look under your fingers at some distant object. The fingers will now appear in three sections. Separate the fingers slightly and a finger with two fingernails will seem to be suspended between your two fingers.

This is an optical illusion caused by the path of light travelling from the distant object to the two eyes.

When light rays strike the curved surface of the cornea, they are partially bent, or refracted, toward each other through the pupil and lens to a point called the focus. If the focus is on the retina, you see a clear image. Light rays coming from an object close to the eye are divergent, or spread out. Light rays coming from a distant object are almost parallel. The lens cannot focus divergent light rays from an object close and parallel rays from a distant object on the retina at the same time.

Place the tube in front of one eye like a telescope and look at a distant object. Now bring the palm of your free hand up next to the tube. A hole will appear allowing you to see the distant object through your hand.

The optical illusion is seen because our eyes see two images that are combined by our brain. In this case, one eye sees the distant object and the other sees the hand. These two images are combined by the brain to create the illusion.

Hole in Hand