The least distance of distinct vision of a long sighted man is 50 cm. He reduces this distance to 10 cm by wearing spectacles. What is the power and type of lenses used by him?

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Solution

A person is suffering from long-sightedness.

Object is at, u = –10 cm and
Least distance of distict vision, v = –50 cm.

Now, using the lens formula we have,

\frac{1}{f} = \frac{1}{v} - \frac{1}{u}

\frac{1}{f} = \frac{1}{- 50} + \frac{1}{10} = + \frac{4}{50}

\Rightarrow

f = + \frac{50}{4} = + 12.5 cm

Therefore, power of the lens,

P = \frac{100}{f}

\Rightarrow

P = \frac{100}{12.5} = 8 D

∴ The type of lenses used is convex lens.

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Wave Optics

The least distance of distinct vision of a long sighted man is 50 cm. He reduces this distance to 10 cm by wearing spectacles. What is the power and type of lenses used by him?

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