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To draw a ray diagram:

1. Draw a ray from the object to the lens that is parallel to the principal axis. Once through the lens, the ray passes through the principal focus F.
2. Draw a ray which passes from the object through the optical centre of the lens. It passes straight through without being bent.
3. A third ray can be drawn from the object through F on the same side of the lens, and then to the lens. Once through the lens, the ray passes parallel to the principal axis.

Convex lenses

The type of image formed by a convex lens depends on the lens used and the distance from the object to the lens.

A camera or human eye

Cameras and eyes contain convex lenses.

For a distant object that is placed more than twice the focal length from the lens, the image is:

• between F and 2F on the opposite side of the lens to the object;
• inverted;
• diminished;
• real.

Projector

Projectors contain convex lenses. For an object placed between one and two focal lengths from the lens, the image is:

• further away than 2F on the opposite side of the lens to the object;
• inverted;
• enlarged;
• real.

In a film or data projector, this image is formed on a screen.

Film must be loaded into the projector upside down so the projected image is the right way up.