(a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
(b) Why do stars twinkle ? Explain.
OR
(a) Write the function of each of the following parts of human eye :
(i) Cornea
(ii) Iris
(iii) Crystalline lens
(iv) Ciliary muscles
(b) Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon ? Give reason to justify your answer
Answers
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Stars do not really twinkle, they just appear to twinkle when seen from the surface of Earth. The stars twinkle in the night sky because of the effects of our atmosphere. When starlight enters our atmosphere it is affected by winds in the atmosphere and by areas with different temperatures and densities.
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OR
a)...
1 ) cornea :- when light enters the eye most of the refraction occurs at the surface of cornea.
2) iris :- it is a dark muscular diaphragm that controls the size of pupil .
3) crystalline lens :- Helps in the finer adjustment of the focal length required to focus object at different distance of retina .
4) ciliary muscle :- contraction and relaxation of lens is done with the help of ciliary muscle .
b)
light from the sun near the horizon passes through a thicker layer of air and a large distance in the earth's atmosphere before reaching our eyes.
However light from the sun overhead has to travel a lesser distance .at noon the sun appears whitish or yellowish as only little blue and violet light are scattered . thus the sun appears reddish during sunrise and sunset .
a)...
1 ) cornea :- when light enters the eye most of the refraction occurs at the surface of cornea.
2) iris :- it is a dark muscular diaphragm that controls the size of pupil .
3) crystalline lens :- Helps in the finer adjustment of the focal length required to focus object at different distance of retina .
4) ciliary muscle :- contraction and relaxation of lens is done with the help of ciliary muscle .
b)
light from the sun near the horizon passes through a thicker layer of air and a large distance in the earth's atmosphere before reaching our eyes.
However light from the sun overhead has to travel a lesser distance .at noon the sun appears whitish or yellowish as only little blue and violet light are scattered . thus the sun appears reddish during sunrise and sunset .
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