The principal axis is a line through the centre of a lens, 90° to the optical axis of the lens.

The principal focus for a convex lens is the point to which rays parallel to the principal axis are focused. For a concave lens it is the point from which such rays appear to be diverged.

When rays of light are brought to a focus a real image forms. A real image can be projected onto a screen.

Virtual images occur at points where rays appear to (but do not actually) originate from. They cannot form on a screen.

Virtual rays are dashed lines drawn to represent where rays of light appear to have come from.

Virtual images from convex and concave lenses

Physics

In A-level physics, some pupils will build on this knowledge to learn about the photon model of electromagnetic radiation and the use of electromagnetic radiation for collecting evidence about the universe or for generating images of the inside of a human body.

In the Measuring waves unit, pupils learnt about the measurable features of a wave, the relationship between them and the wave equation.

Electromagnetic waves

An image that light rays appear to come from, but do not actually come from, is called a {{}} image.

the point to which rays parallel to the principal axis converge

the point that rays parallel to the principal axis appear to have diverged from

What is the principal focus for a concave lens?

The lens in the person’s eye does not have enough power.

Images of distant objects appear in front of the retina (back of the eye).

A short-sighted person can focus on nearby objects but not distant objects.

Which of these statements are true about short-sightedness?

This scale ray diagram shows an object (an arrow), a concave lens and an image. Which of these words describe the image?

A convex lens makes a magnified image (Q) of an object (P). Which of these images correctly shows real rays (solid black lines) and virtual rays (dashed black lines)?

Converging rays are rays that are coming together. {{}} rays are rays that are spreading out.

The imaginary line through the centre of a lens, at right angles to the lens diameter, is called the {{}}.

changes direction to pass through the principal focus.

changes direction to become parallel to the principal axis.

Any ray that passes through the centre of a lens …

These terms can be used to describe images made by lenses. Match each term with its meaning.

Which of these convex lens ray diagrams is correct?

the focal point is further from the lens.

the material it is made from refracts light through greater angles.

Starter Quiz