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Focal Length

The focal length is the distance between the pole of a mirror and its principal focus.

In one line: Pole to focus gives focal length.

Understand it

For a concave mirror, parallel rays meet at the focus after reflection. For a convex mirror, the reflected rays appear to come from the focus behind the mirror. The focal length helps locate images and compare mirror strength.

Example

If the focus is 12 cm from the pole, the focal length is 12 cm.

Formula and rule

For small spherical mirrors, f = R/2.

The focus lies midway between the pole and the centre of curvature.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If the radius of curvature of a mirror is 24 cm, then its focal length is 12 cm because f = R/2.

Method you can reuse

1Identify the pole.
2Locate the focus or radius of curvature.
3Use f = R/2 when needed.
4Measure along the principal axis.

Sketch it in your answer

Mark the pole P, focus F, and centre of curvature C on the principal axis, and show the distance PF as focal length.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

A concave mirror has its focus 18 cm from the pole. What is its focal length?

Think first, then show answer

The focal length is 18 cm because focal length is the distance from the pole to the focus.

Common wrong answer

Taking focal length as the distance from the mirror surface edge to the focus. The correct reference point is the pole.

Don't mix this up

Students sometimes measure from the centre of curvature or from the mirror edge instead of from the pole.

Exam tip

If the question gives radius of curvature, remember f = R/2 for a spherical mirror.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

You may be asked to calculate f from R, identify the focus position, or explain why rays parallel to the axis meet at the focus.

1
Enough for 1 mark
The focal length is the distance between the pole of a mirror and its principal focus.
2
Add for 2 marks
Example: If the focus is 12 cm from the pole, the focal length is 12 cm.
3
Add for 3 marks
For a concave mirror, parallel rays meet at the focus after reflection.

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