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Dear student,

You are asking if the focal length of a lens is 20 cm, what is its power in diopters?

Calculating Lens Power:

• Power of a lens is defined as the reciprocal of its focal length when expressed in meters
• The unit of power of a lens is diopter (D)

Formula:

• Power (P) = 1 / focal length (in meters)

Given:

• Focal length (f) = 20 cm = 0.2 meters

Calculation:

• Power (P) = 1 / 0.2 = 5 diopters

Therefore, the power of the lens is +5 diopters

Why is it positive?

• A positive power indicates a convex lens (converging lens)
• Convex lenses are thicker at the center and thinner at the edges
• They converge light rays to a point called the focus

convex lens

Additional Notes:

• Concave Lens: If the lens were concave (diverging lens), the power would be negative
• Units: Always ensure that the focal length is converted to meters before calculating the power in diopters

Therefore, if a lens has a focal length of 20 cm, its power is +5 diopters. This means it is a convex lens capable of converging light rays.