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    Make a diagram to show how hypermetropia is corrected. The near point of a
    hypermetropic eye is 1 m. What is the power of the lens required to correct this
    defect? Assume that the near point of the normal eye is 25 cm.

    Fig. Hypermetropia and its correctionThe object placed at 25 cm from
    Fig. Hypermetropia and its correction
    The object placed at 25 cm from the correcting lens must produce a virtual image at 1 m or 100 cm.
    ∴            u = – 25 cm, v = –100 cm
    By lens formula,      1 over straight f space equals space 1 over straight v minus 1 over straight u space equals space fraction numerator 1 over denominator negative 100 end fraction minus fraction numerator 1 over denominator negative 25 end fraction space equals negative 1 over 100 plus 1 over 25 space equals space plus 3 over 100 

    or                       straight f space equals space plus 100 over 3 cm space equals space plus 1 third straight m
            Power, straight P space equals space 1 over straight f space equals plus 3 over 1 space equals space plus space 3 space straight D.                                   Ans. 





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    Explain why the planets do not twinkle.

    The planets do not show twinkling effect: As the planets are much closer to the earth, the amount of light received from them is much greater and the fluctuations caused in the amount of light due to atmospheric refraction are negligible as compared to the amount of light received from them.
    The planets do not show twinkling effect: As the planets are much clo
    Fig. Apparent star position due to atmospheric refraction
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    Why do stars twinkle?

    The twinkling effect of star is due to the phenomenon of atmospheric refraction. On entering the surface of the earth, the starlight undergoes continuous refraction in medium which undergoes gradual change in the refractive index. The atmosphere bends starlight towards the normal so, the apparent position of the star is slightly differnt from it's actual position. When viewed near the horizon, the position of the star appears slightly raised. Since, the physical conditions of the earth's atmosphere are not stationary, the apparent position of the star keeps on changing slightly. The stars are very distant and they appear as point-sized sources of light. As the path of rays of light coming from the star is continuosly varying, the apparent position of the star fluctuates and the amount of starlight entring our eye flickers. The star sometimes appear bright and sometimes faint. This is known as the twinkling effect of stars. 

     
    Fig. Apparent position of star due to atmospheric refraction.
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    What happens to the image distance in the eye when we increase the distance of an object from the eye?

    When the object distance is changed, the focal length of the eyelens accomodates itself in a manner so that, a clear image is formed on the retina. This is done by the action of ciliary muscles. Therefore, the image distance remains the same even when, the object distance is varied. 
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    Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
    At distance less than 25 cm, the ciliary muscles cannot bulge the eyelens any more, the object cannot be focussed on the retina and it appears blurred to the eye, as shown in Fig.
    At distance less than 25 cm, the ciliary muscles cannot bulge the eye

    Fig. Object O within 25 cm from the eye is not focussed on retina and seen blurred.
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