Tag: structure of human eye

Given,

Minimum distance for clear vision of healthy eye is  $25 \ cm$ and maximum distance is infinite.

Answer is A.

A person who is short sighted can focus clearly on near objects but cannot focus on distant objects. This is because the eyeball is too long. Light from distant objects is focused at a point in front of the retina resulting in a blurred image.
This defect can be corrected by wearing a concave (diverging) spectacle lens. The rays of light from a near object are diverged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.
Hence, the statement is true.

Here, $u = \infty$ and $ v = -200\; cm$

\begin{array}{l} According\, \, to\, question................... \ Near\, point\, of\, the\, child\, (u)=10cm \ Far\, point\, of\, the\, child\, (u)=100cm \ and,the\, \, retina\, is\, \, \, 2\, cm\, behind\, the\, eye\, lens. \ \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, V=2\, cm=0.02\, m \ Now, \ \frac { 1 }{ f } =\frac { 1 }{ { 0.02 } } -\frac { 1 }{ { (-0.1) } } \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \left| { from\, lens\, formula:\frac { 1 }{ v } -\frac { 1 }{ u } =\frac { 1 }{ f }  } \right.  \ \, \, \, \, \, \, \, \Rightarrow 50+10=60\, m \ \therefore \, \, Power\, \, of\, the\, lens:\, \, P=\frac { 1 }{ f } =60\, D \ Now,\, \, \, consider\, the\, far\, point\, is\, \, 100\, cm. \ where, \ \, u=-100cm=-1m,\, \, \, and\, \, \, \, V=2\, cm=0.02\, m \ then, \ \frac { 1 }{ f } =\frac { 1 }{ { 0.02 } } -\frac { 1 }{ { (-1) } } \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \left[ { we\, have\, the\, lens\, formla: } \right. \frac { 1 }{ v } -\frac { 1 }{ u } =\frac { 1 }{ f }  \ \frac { 1 }{ f } =50+1=51\, m \ \therefore \, \, \, Power\, \, of\, the\, lens:\, \, P=\frac { 1 }{ f } =51\, D \ so,\, that\, the\, rangeof\, the\, \, power\, of\, eye\, lens\, is\, from\, \, \, \underline { 60\, D\, \, to\, \, 51\, D }  \ and\, the\, correct\, option\, is\, B. \ \, \,  \ so\, the\, correct\, optuon\, is\, A. \end{array}

Hyperopia or Far-sightedness is the condition where person can see the far objects clearly but cannot focus on near objects. This occurs since the image of nearby objects is formed behind retina. This corrected by using a convex lens which increases the power of the eye, and image is obtained on the retina.

Near sightedness, or myopia, is the most common refractive error of the eye. Myopia occurs when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. So, to treat myopia the glasses will be preceded by a minus sign ().
Hence, to treat myopia concave lens should be used.

A person cannot see beyond 50 cm. Hence, he should use a concave lens of 50 cm focal length.Hence $Power=\dfrac{-100}{50} =-2 D$