What does a converging lens do to a beam of light travelling parallel to its principal axis?

It refracts the beam so that all the rays meet at a single point on the axis, the principal focus, on the far side of the lens.

What does a diverging lens do to a beam of light travelling parallel to its principal axis?

It spreads the beam out so that the emerging rays appear to come from a single point on the axis on the same side as the incoming light.

Define the principal focus of a converging lens.

The principal focus is the point on the principal axis at which rays arriving parallel to the axis are brought together after refraction by the lens.

Define the principal focus of a diverging lens.

The principal focus of a diverging lens is the point on the principal axis from which rays arriving parallel to the axis appear to diverge after refraction.

CCEA A-Level Physics 1210

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Lenses

Converging and diverging lenses

3 things to know
Unit AS 2

Overview

Shape of a converging lens and of a diverging lens, principal axis of a lens and principal focus of a converging lens.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe the shape of a converging lens and of a diverging lens.

Key term

Define the principal axis of a lens.

Key term

Define the focal length of a lens.

Questions from this subtopic

Each one with the model answer underneath.

1
What does a converging lens do to a beam of light travelling parallel to its principal axis?
Model answer
It refracts the beam so that all the rays meet at a single point on the axis, the principal focus, on the far side of the lens.
2
What does a diverging lens do to a beam of light travelling parallel to its principal axis?
Model answer
It spreads the beam out so that the emerging rays appear to come from a single point on the axis on the same side as the incoming light.
3
Describe the shape of a converging lens and of a diverging lens.
Model answer
A converging lens is convex, thicker at its middle than at its edges, and a diverging lens is concave, thinner at its middle than at its edges.
4
Define the principal axis of a lens.
Model answer
The principal axis is the straight line through the optical centre of the lens at right angles to both of its faces.
5
Define the principal focus of a converging lens.
Model answer
The principal focus is the point on the principal axis at which rays arriving parallel to the axis are brought together after refraction by the lens.
6
Define the principal focus of a diverging lens.
Model answer
The principal focus of a diverging lens is the point on the principal axis from which rays arriving parallel to the axis appear to diverge after refraction.

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