Edexcel A Level Physics

Revision Notes

5.19 Power of a Lens

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Power of a Lens

  • The power of a lens is a measure of its ability to refract light
    • The more refraction a lens causes, the higher its power

  • The power of a lens measures how strongly it focuses the light
    • The more curved the lens, the shorter the focal length
    • The shorter the focal length, the greater the power of the lens

  • The power of a lens is related to:
    • The focal length of the lens
    • The shape of the lens

Lens Power, downloadable IGCSE & GCSE Physics revision notes

The power of a lens depends on its focal length

Calculate the power of a lens

  • The power of a lens, P is calculated using the following equation:

P = 1 over f

  • Where: 
    • = power (dioptres, D)
    • = focal length of the lens (m)

  • Power is inversely proportional to focal length
  • For a concave mirror, where the focal length is negative, power has a negative value

Worked example

A lens is set up between an object candle and a screen. All the equipment is arranged perpendicular to the desk. The image of the candle is in focus when the screen is positioned 60 cm from the lens.


Determine the power of the lens.

Step 1: Write the known quantities in S.I. units

    • Image is in focus, so the screen is at the focal point
    • Focal length, f = 60 cm = 0.6 m

Step 2: Write the equation for power and substitute the values

P space equals space 1 over f space equals fraction numerator 1 over denominator 0.6 end fraction equals 1.67

Step 3: Give the full answer, to correct significant figures and with units

      The power of the lens, P = 1.7 D

Exam Tip

The explanations above relate to life in a common sense way. Stronger, more powerful reading glasses are used by people who have the weakest eye sight. They need lenses to do more of the focusing for them. 

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