Describe the physical shape of a converging lens and what happens to parallel rays passing through it.

A converging lens is thicker in the middle than at the edges. Parallel rays passing through it converge to a point after refraction.

What is the principal focus of a converging lens?

The principal focus of a converging lens is the point where rays parallel to the principal axis converge after refraction through the lens.

How is the principal focus of a diverging lens defined?

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

Define the focal length of a lens.

Focal length is the distance from the optical centre of a lens to its principal focus.

State the formula for the power of a lens and give its unit.

Power P = 1/f, where f is the focal length in metres. The unit of lens power is the dioptre (D).

What does each symbol represent in the lens power equation P = 1/f?

P represents the power of the lens in dioptres, and f represents the focal length in metres.

AQA A-Level Physics 7408

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Medical Physics

Defects of vision and their correction using lenses

6 things to know
Paper 3, section B

Overview

Lens power in dioptres, the lens equation, and correcting myopia and hypermetropia.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

How is the power of a lens calculated, and what is focal length?

Numbers

What do the symbols in P = 1/f mean, and what sign do converging and diverging lenses take?

Numbers

What is linear magnification, and what units must focal length be in for dioptres?

Numbers

What is myopia, and how does the lens equation rearrange for image distance?

Numbers

How is the corrective power found, and what image does a diverging lens form?

Key term

In an astigmatism prescription, what does the cylindrical power (cyl) correct for?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the physical shape of a converging lens and what happens to parallel rays passing through it.
Model answer
A converging lens is thicker in the middle than at the edges. Parallel rays passing through it converge to a point after refraction.
2
What is the principal focus of a converging lens?
Model answer
The principal focus of a converging lens is the point where rays parallel to the principal axis converge after refraction through the lens.
3
How is the principal focus of a diverging lens defined?
Model answer
The principal focus of a diverging lens is the point from which rays parallel to the principal axis appear to diverge after refraction through the lens.
4
Define the focal length of a lens.
Model answer
Focal length is the distance from the optical centre of a lens to its principal focus.
5
State the formula for the power of a lens and give its unit.
Model answer
Power P = 1/f, where f is the focal length in metres. The unit of lens power is the dioptre (D).
6
What does each symbol represent in the lens power equation P = 1/f?
Model answer
P represents the power of the lens in dioptres, and f represents the focal length in metres.

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