A ray diagram can be used to find

• the position
• the type (real or virtual)
• the size

of the image formed by a lens.

At least two rays must be drawn.

A ray passing through the optical centre of the lens

This ray is not deviated (this is true for both convex and concave lenses)

A ray parallel to the principal axis

For a convex lens (also called a converging lens), this ray is deviated to pass through the principal focus, F.

For a concave lens (also called a diverging lens), this ray is deviated so as to appear to be coming from the principal focus, F.

When drawing ray diagram for lenses, for simplicity we imagine that the refraction takes place at the centre of the lens. In reality, of course, the light changes direction at the surfaces.

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