Question

Write a story in about 100- 120 words, entitled ‘My Academic Year 20-21’
​

Answer 1

Answer:

a lens has a shorter focal length it is said to be more powerful. The power of a lens is defined as the reciprocal of the focal length. Lens power is measured in dioptres (D).

\[power \\ of \\ a \\ lens \\ (D) = \frac{1}{focal \\ length \\ (m)}\]

\[P = \frac{1}{f}\]

Converging (convex) lenses have positive focal lengths, so they also have positive power values.

Diverging (concave) lenses have negative focal lengths, so they also have negative power values.

Example 1

What is the power of a convex lens with a focal length of 25 cm?

First convert focal length into metres: f = 25 cm = 0.25 m

\[lens \\ power = \frac{1}{0.25} = 4.0 \\ D\]

\[lens \\ power = 4.0 \\ dioptres\]

Example 2

A lens has a power of -3.33 dioptres. Calculate the focal length and state what type of lens it is.

\[f = \frac{1}{D}\]

\[f = \frac{1}{-3.33} = -0.30 \\ m\]

\[focal \\ length = -30 \\ cm \\ (concave \\ lens)\]