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Focal Length

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

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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

Example

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

Rule to remember

For small spherical mirrors, f = R/2. The focus lies midway between the pole and the centre of curvature.

Memory hook

Pole to focus gives focal length.

Examples and method

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 to apply

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

Diagram support

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

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.

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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

Answer writing and exam use

1-mark answer

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

2-mark answer

The focal length is the distance between the pole of a mirror and its principal focus. For small spherical mirrors, f = R/2. The focus lies midway between the pole and the centre of curvature. If the focus is 12 cm from the pole, the focal length is 12 cm.

3-mark answer

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. For small spherical mirrors, f = R/2. The focus lies midway between the pole and the centre of curvature. If the radius of curvature of a mirror is 24 cm, then its focal length is 12 cm because f = R/2. 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. Taking focal length as the distance from the mirror surface edge to the focus. The correct reference point is the pole.
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