The object distance is the distance between an object and a lens.

The image distance is the distance between a lens and an image formed by that lens.

The principal focus is the point to which parallel rays (from distant objects) are focused.

The focal length is the distance between the centre of a lens and the principal focus.

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

Scale diagrams for convex 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

Rays from a nearby object are converging.

Rays from a nearby object are very nearly parallel.

Rays from a very distant object are converging.

Rays from a very distant object are very nearly parallel.

Which of these statements are correct about rays of light that hit a lens?

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

It is the distance between the image and the lens.

It is the distance between the image and the object.

It is the distance needed between the screen and lens for the image to be sharp.

When a lens forms an image on a screen, which of these statements correctly describe the image distance?

No change is needed because the back of the eye is at the principal focus.

The lens becomes fatter so that it has higher power.

The lens becomes thinner so that it has lower power.

The eye becomes longer so that the screen is further from the lens.

The eye becomes shorter so that the screen is closer to the lens.

The diagram shows rays of light from an object entering an eye through its lens. The rays are focused onto the back of the eye which acts as a screen. 
How does the eye focus on a more distant object?

parallel to principal axis, then bends to pass through principal focus

passes through centre of lens without being refracted

passes through principal focus, then bends parallel to principal axis

In this ray diagram, the principal focus on each side of the convex lens is shown as a cross. Three rays from the top of the object (an arrow) are shown. 
Match each ray with its description.

The rays are converging when they leave the lens.

The lens cannot refract the rays enough to make them converge.

The student has placed the screen between the lens and the object.

A student tries to use a convex lens to make an image of a bright object. He varies the distance between the screen and lens but finds no image. Which of the following are possible explanations?

A lens that bends light rays towards each other so that they meet or cross is called a {{}} lens.

A student wants to use a lens to create an image on a screen. 
At which position should the student place the screen to see a sharp image?

The diagram shows light rays being focused by a lens. 
What are the correct names for the distance labelled A, the point labelled B and the line labelled C?

Which of these words can be used to describe images made on screens by lenses?

Lens P refracts rays through the largest angles.

Lens P and lens Q are made of different materials.

The diagrams show parallel light rays being brought to a focus by three different lenses, labelled P, Q and R. Lenses P and Q have the same curvature. 
Which of these statements must be correct?

The light rays hitting the lens are parallel or very nearly parallel.

By moving the screen, the student can see sharp images of different sizes.

The lens brings all rays from one point on the object to one point on the image.

The distance between the lens and the image equals the focal length of the lens.

A student uses a lens to make a sharp (clear) image of an object on a screen. 
Which of these statements must be correct?

This lesson will be removed by end of Summer Term 2025.

Scale diagrams for convex lenses

Lesson outcome

This lesson will be removed by end of Summer Term 2025.