Revision NotesExam QuestionsPast Papers
ASPhysicsEdexcelRevision Notes5. Waves & Particle Nature of LightRefraction, Reflection & PolarisationPower of a Lens

Power of a Lens (Edexcel AS Physics): Revision Note

Exam code: 8PH0

Lindsay Gilmour

Last updated

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

Calculating the power of a lens

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

P space equals space 1 over f

  • Where: 

    • P = power (dioptres, D)

    • space f = 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 a 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.

5-19-power-of-a-lens-we-qun_edexcel-al-physics-rn

Assume that the candle is very far away from the lens, such that the rays from it are effectively parallel.

Determine the power of the lens.

Answer:

Step 1: Write the known quantities in SI units

  • Since the object is assumed to be very far away, the parallel rays will converge at the focal point of the lens

  • 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

Power of the lens: P space equals space 1.7 space straight D

Examiner Tips and Tricks

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

You've read 0 of your 5 free revision notes this week

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:Using Ray DiagramsNext:Thin Lenses in Combination
    Lindsay Gilmour

    Author: Lindsay Gilmour

    Expertise: Physics Content Creator

    Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.