(a) Give reasons for the folowing : (i) Colour of the clear sky is blue. (ii) The sun be seen about two minutes before actual sunrise. (iii) We cannot see an object see an object clearly if it is placed very close to the eyes. (b) What is presbyopia ? Write two causes of this defect.

(a) Give reasons for the folowing : (i) Colour of the clear sky is bl

1.	(a) Give reasons for the folowing : (i) Colour of the clear sky is blue. (ii) The sun be seen about two minutes before actual sunrise. (iii) We cannot see an object see an object clearly if it is placed very close to the eyes. (b) What is presbyopia ? Write two causes of this defect.
Answer» Solution :(a) (i) Blue colour of sky is due to Rayleigh elastic scattering of sunlight. Light from the sun, while travelling through earth's atmosphere, gets scattered from LARGE number of air molecules in the atmosphere. As size of molecules is `~~ 10^-10 m`, which is much less than WAVELENGTH of light, therefore Rayleigh scattering occurs. As blue colour has the SMALLEST wavelength, it is scattered the most. That is why sky looks blue. (ii) N/A (iii) For a normal EYE, the MINIMUM distance