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PART - 3 CH - 10 The Human Eye and The Colourful World — Science STD 10 — Question

Gujarat BoardEnglish MediumSTD 10SciencePART - 3 CH - 10 The Human Eye and The Colourful World2 Marks
Question
State causes of hypermetropia. How can this defect be corrected? Draw suitable diagram to show it.
✓

Answer

Causes of hypermetropia:
( 1 ) The focal length of the eye lens is increased due to least curvature of the eye lens.
(2) The length of eyeball is shortened (shorter than normal), i.e., the distance between the retina and the eye lens is decreased.
$\rightarrow$ Due to either or both the causes, the eye lens is not able to focus rays coming from an object at 25 cm from the eye on the retina.

Image

Correction of the defect: To correct a hypermetropic eye, the person has to wear spectacles with a convex lens of suitable focal length or power.
So, that rays of light from the normal near point on passing through the convex lens converge and appear to come from $N$, the near point of the hypermetropic eye. (figure 10.6)
[Suppose,
$x^{\prime}=$ distance of near point N of defective eye
$d=$ normal near point, i.e., 25 cm
$f=$ focal length of convex lens to be used
For correcting lens, object is at normal near point $N^{\prime}$, i.e., $u=-d=-25 cm$ and image is at N , i.e., $v=-x^{\prime}$
So, from the lens formula
$\begin{array}{l}\frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\ \frac{1}{f}=\frac{1}{-x^{\prime}}-\frac{1}{-d} \\ \therefore \frac{1}{f}=\frac{-d+x^{\prime}}{x^{\prime} d} \\ \left.\therefore f=\frac{x^{\prime} d}{x^{\prime}-d}\right]\end{array}$

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