Apparent size

The idea

The size of an object as seen by the eye, its apparent size, depends on both its actual physical size and its distance from the observer.

Hide a head with a thumb

You can cover your friend’s whole head with your thumb, if you hold it close to your eye.

Why do you think a small thumb can hide a big head?

• The thumb is much closer to the eye
• The thumb is really bigger
• The head shrinks

Move the thumb

The lines show how much of your view each one takes. The sizes are example numbers.

Size and distance together

How big something looks does not depend on its size alone. The distance matters too.

The size of an object as seen by the eye, its apparent size, depends on both its actual physical size and its distance from the observer.

Notes

The size of an object as seen by the eye, its apparent size, depends on both its actual physical size and its distance from the observer.

Check yourself

Apparent size depends on …

A small thumb can cover a friend’s head because the thumb is …

If you move your thumb farther from your eye, it looks …

Two objects have the same apparent size. Then …

• both the actual size and the distance — correct. Yes! Both matter.
• only the colour. Colour does not decide apparent size.
• only the actual size. A small thumb can look as big as a head, so size alone is not enough.
• brighter than the head. Brightness does not decide apparent size. Distance does.
• much closer to the eye — correct. Yes! Near objects look bigger.
• heavier than the head. Weight does not change how big something looks.
• exactly the same. The apparent size changes with distance.
• bigger. Things look smaller as they go farther.
• smaller — correct. Yes! The apparent size gets smaller as the distance grows.
• they can have very different actual sizes if they are at different distances — correct. Yes! The nearer one can be much smaller.
• they must be at the same distance. They can look equal even when one is much farther.
• they must have the same actual size. Not at all. A thumb and a head look alike in size but are not.