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Physics · Class 12

Lens shape and optical power

Ray Optics & Optical Instruments

Lens power comes from index contrast and both surface curvatures. Increasing curvature or index increases focusing power. Dioptres use focal length in metres.

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Lens shape and optical power

லென்சின் ஒளித்திறன் ஒளிவிலகல் எண் வேறுபாட்டையும் இரு முகங்களின் வளைவையும் சாரும்.

Key notes, relationships and model limits

Key relationship

1/f = (n−1)(1/R₁−1/R₂); P(D)=100/f(cm).

Remember

Lens power comes from index contrast and both surface curvatures. Increasing curvature or index increases focusing power. Dioptres use focal length in metres.

Common trap

Using centimetres directly in P=1/f gives an incorrect dioptre value.

Model limits

Thin lens in air; signed radii R₁>0,R₂<0 here; paraxial rays. Index matched to air gives zero optical power. Image infinity handled separately from focal infinity.

What you will learn

Link lens geometry, focal length and dioptres.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

n=1.5,R₁=20 cm,R₂=−20 cm gives f=20 cm.

Why this matters

The model connects lens shape and optical power with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

A20 cm converging lens has power?

Read the explanation

Convert20 cm to0.2 m, then P=1/0.2=5 D.

Course connection: Class XII Chapter 9. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.