5.19 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: 
  • P = power (dioptres, D)
  • 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

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