# 5.19 Power of a Lens

## 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

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 =

• 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, = 60 cm = 0.6 m

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

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.

### Get unlimited access

to absolutely everything:

• Unlimited Revision Notes
• Topic Questions
• Past Papers