Give reasons-
(a) A concave lens is also known as a diverging lens.
(b) An object seen through a prism appears coloured.
(c) In old age eyesight becomes foggy.
(d) It is not possible to see an object clearly if it is kept too close to the eye.
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Answers
a) A concave lens is called a diverging lens because -
It is thinned out in the middle. When parallel beam of light is passed through concave lens, light rays spread out to different directions. Hence, a concave lens is called a diverging lens.
b) We see that when light falls on a prism, it splits up into many colours.The process of splitting up of white light in different colours is called dispersion. The band of colours produced when white light splits up is called a spectrum.
c) Cataract is caused in old age when eyesight becomes foggy due to the eye lens becoming cloudy.
d) A normal eye can see objects kept at various distances clearly due to the ability of the ciliary muscles to increase or decrease its focal length of the eye lens.
We cannot see the objects closer than 25 cm because the ciliary muscles are unable to contract after a certain limit. Hence, the object closer than 25 cm appears blurred because light rays coming from the object meet beyond retina.
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