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The images formed by a lens can be:

• upright or inverted (upside down compared to the object)
• magnified or diminished (smaller than the object)
• real or virtual

A real image is an image that can be projected onto a screen. A virtual image appears to come from behind the lens.

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 should pass through the principal focus.
2. Draw a ray which passes from the object through the centre of the lens.

Some ray diagrams may also show a third ray.

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:

• inverted
• diminished
• real

Projectors

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

• inverted
• magnified
• 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.

Magnifying glasses

A magnifying glass is a convex lens used to make an object appear much larger than it actually is. This works when the object is placed at a distance less than the focal length. The image is:

• upright
• magnified
• virtual

Concave lenses

Peep holes contain concave lenses. They are set into doors so the occupant can identify a visitor before opening the door.

For an object viewed through a concave lens, light rays from the top of the object will be refracted and will diverge on the other side of the lens. These rays will appear to come from a point:

• the same side of the principal axis, meaning the image will be upright
• closer to the principal axis, so the image will be smaller than the object