Explain why the magnification is equal to the ratio of the image distance to the object distance.

The ray from the top of the object through the optical centre travels in a straight line, so it makes two similar triangles whose bases are u and v and whose heights are the object and image heights, and the ratios of corresponding sides must be equal.

What unit is a magnification given in?

None, because it is the ratio of two lengths and the units cancel.

An object 3.0 cm tall is imaged with a magnification of 2.0. Calculate the height of the image.

The image height is 2.0 × 3.0 = 6.0 cm, and 6.0 cm ÷ 3.0 cm returns the magnification of 2.0 given.

An object is 0.30 m from a converging lens of focal length 0.20 m, giving an image 0.60 m from the lens. Calculate the magnification.

m = v/u = 0.60/0.30 = 2.0, and an object distance of 0.30 m with m = 2.0 requires v = 0.60 m as stated.

CCEA A-Level Physics 1210

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Lenses

Magnification

3 things to know
Unit AS 2

Overview

Magnification produced by a lens in terms of heights and equation for magnification in terms of distances, defining each symbol.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe an experiment to verify that the magnification of a real image equals the ratio of the image distance to the object distance.

Numbers

A diverging lens forms an image 0.12 m from the lens of an object 0.30 m away. Calculate the magnification.

Key term

Define the magnification produced by a lens in terms of heights.

Questions from this subtopic

Each one with the model answer underneath.

1
Define the magnification produced by a lens in terms of heights.
Model answer
The magnification m is the ratio of the height of the image to the height of the object, and being a ratio of two lengths it has no unit.
2
State the equation for magnification in terms of distances, defining each symbol.
Model answer
m = v/u, where m is the magnification, v is the image distance from the lens in metres and u is the object distance from the lens in metres.
3
Explain why the magnification is equal to the ratio of the image distance to the object distance.
Model answer
The ray from the top of the object through the optical centre travels in a straight line, so it makes two similar triangles whose bases are u and v and whose heights are the object and image heights, and the ratios of corresponding sides must be equal.
4
What unit is a magnification given in?
Model answer
None, because it is the ratio of two lengths and the units cancel.
5
An object 3.0 cm tall is imaged with a magnification of 2.0. Calculate the height of the image.
Model answer
The image height is 2.0 × 3.0 = 6.0 cm, and 6.0 cm ÷ 3.0 cm returns the magnification of 2.0 given.
6
An object is 0.30 m from a converging lens of focal length 0.20 m, giving an image 0.60 m from the lens. Calculate the magnification.
Model answer
m = v/u = 0.60/0.30 = 2.0, and an object distance of 0.30 m with m = 2.0 requires v = 0.60 m as stated.

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Magnification

Overview

What you need to know

Describe an experiment to verify that the magnification of a real image equals the ratio of the image distance to the object distance.

A diverging lens forms an image 0.12 m from the lens of an object 0.30 m away. Calculate the magnification.

Define the magnification produced by a lens in terms of heights.

Questions from this subtopic

Define the magnification produced by a lens in terms of heights.

State the equation for magnification in terms of distances, defining each symbol.

Explain why the magnification is equal to the ratio of the image distance to the object distance.

What unit is a magnification given in?

An object 3.0 cm tall is imaged with a magnification of 2.0. Calculate the height of the image.

An object is 0.30 m from a converging lens of focal length 0.20 m, giving an image 0.60 m from the lens. Calculate the magnification.

Other ways to work this

What is inside Levo

The 3-step method, run for you

Cram a whole paper in one sitting

Blank Blitz makes facts automatic

Build It gets the exam shape right

Test It catches misconceptions

Brain Breaks between sessions

Every question in Lenses, on a schedule.