Name: Scattering of Light
Uploaded: 2019-07-19
Duration: 5 min 47 s
Description: When a beam of sunlight enters a dark room through a small hole, its path becomes visible. Wonder why this happens? Let’s find out in this video.

Question

An eye has a near point distance of

0.75 m

. What sort of lens in spectacles would be needed to reduce the near point distance to

0.25 m

? Also, calculate the power of the lens required. Is this eye long-sighted or short-sighted?

Accepted Answer

A person suffering from hypermetropia can correct the defect by wearing spectacles made of convex lenses.

To find the power of the convex lens, first, we need to calculate its focal length.
Given that,
The near point of the hypermetropic person is $0.75 m$ from the eye. It means the person can see objects kept at the normal near point of $0.25 m$ in front of the eye if the image of the objects forming at the person's own near point of $0.75 m$ from the eye.
The distance of the object at the normal near point, $u = - 0 .25 m$
The near point of the defective eye in front of the lens, $v = - 0.75 m$
From the lens formula, $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
Substituting the values in the formula, we get
$\frac{1}{f} = \frac{1}{- 0.75} - \frac{1}{- 0.25}$
$\frac{1}{f} = (\frac{- 1 + 3}{0.75})$
$∴ f = 2.67 m$

The relation between focal length and power of the lens is,
$P = \frac{1}{f (in meters)}$
$∴ P = \frac{1}{2.67 m} = 0.37 D$
Hence, the power of the convex lens to correct the defect is $+ 0.37 D$ .
This eye is long-sighted.

An eye has a near point distance of $0.75 m$ . What sort of lens in spectacles would be needed to reduce the near point distance to $0.25 m$ ? Also, calculate the power of the lens required. Is this eye long-sighted or short-sighted?

Important Questions on The Human Eye and The Colourful World

Learn and Prepare for any exam you want !

An eye has a near point distance of 0.75 m. What sort of lens in spectacles would be needed to reduce the near point distance to 0.25 m? Also, calculate the power of the lens required. Is this eye long-sighted or short-sighted?

Important Questions on The Human Eye and The Colourful World

Learn and Prepare for any exam you want !

An eye has a near point distance of $0.75 m$ . What sort of lens in spectacles would be needed to reduce the near point distance to $0.25 m$ ? Also, calculate the power of the lens required. Is this eye long-sighted or short-sighted?