Give reasons for the following:-

a) Colour of clear sky is blue.

b)Sun can be seen about two minutes before actual sunrise.

c)Traffic light signals are of red colour.

d)Stars appears to twinkle.

e)Planets do not twinkle.

Here is your answer

a.The sky appears blue, when viewed from Earth, because of the scattering of the light. Scattering is the phenomenon by which a beam of light is redirected in many different directions when it interacts with a particle of matter. When sunlight strikes molecules in our atmosphere, the light is redirected in many directions. The blue light is scattered more than the red light because blue color has smallest wave length so it got scattered most and causing the sky to be blue.

b.The Sun is visible to us two minutes before the sunrise and two minutes after the Sunset, because of the bending of the light due to atmospheric refraction. The light from the Sun got refracted by the atmospheric layers and reaches the observer. This causes the observer to think that the light is coming straight from the Sun.

c.Traffic signals are red in colour due to the following reasons-

1.It has the highest wavelenght in all colours and due to its property it is visible from far distance

2.It is least scattered by fog,cloudy weather and translucent situations

d.Stars twinkle because our atmosphere is constantly moving around or is turbulent. The light travelling from the stars gets bent in different direction making them twinkle. It is quite similar to the effect when viewing the reflection of the moon on a lake's surface when its (lake) moving.

e.Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to earth. Planets can be considered as a collection of a large number of point-size sources of light. The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling effects of the planets are nullified and they do not twinkle.

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(a)The molecules of air and other fine particles present in the atmosphere hace size smaller than the wavelength of visible light .These fine particles are moreeffective in scattering of light of shorter wavelenght at the blue end of the spectrum .Thus when sunlight passes through the atmosphere , the fine particles in air scatter blue colour more strongly than red colour .The scattered blue light enters out eyes and the sky appears blue in colour.

(b) The danger signals are of red colour because red colour has the longer wavelength which are not scattered away by the fine particles present in the air and can travel long distance .

(b) During sunrise and sunset the light rays coming from the sun near the horizon passes larger distance through the atmosphere before reaching our eyes .During this time most of the blue light and shorter wavelength are scattered away by the particles .Therefore , the light rays thatf reach our eyes is of longer wavelenght .This gives rise to the reddish appearance of the sun during sunrise and sunset.

c)The red colour are least least scattered by fog and smoke . therefore, it can be seen in the same colour at a distance.That is why red colour of danger sign is decided.

d)On a clear night, if one watches the stars, one can observe that the stars twinkle. The reason is that even in clear weather, there are small changes in the density of air, causing the refractive index to change and hence also the path taken by the light ray. The intensity of light received by the human eye thus tends to vary slightly and randomly with time, leading to the twinkling effect.

e)As the light from a star enters the atmosphere, it hits gas molecules and scatters(reflection and interference).

Since the star is so far away, we only see a tiny beam of light from it.This beam gets scattered away from our eyes and then back into them almost like it is blinking on and off.It happens so fast that it just looks like it is twinkling.Planetsare closer to us and send more light; if some of the light beams are scattered away, others still get through to us, so planets don 't usually twinkle.


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A)The molecules of air and other fine particles in the atmosphere have size smaller thn the wavelength of visible light.These are more efffective in scattering light of shorter wavelengths at the blue end than the light if longer wavelengths at the red end.The red light has wavelength about 1.8 times greater than blue light.Thus when sunlight passes through the atmosphere the fine particles in air scatter the blue colour(shorter wavelengths) more stongly than red.THe scattering blue light entres ours eye. Thus the sky appers blue.

b)The rays of light frm the sun blow the horizon reaches our eyes bcuz of refraction of light of light.These crays appear to cum frm the apparent position of the sun which is above the horizon. Hence we can see the the sun for few minutes(about 2min.) even after it has actually set. Similarly , the sun can be seen abotu 2min. before it actually rises.Thus ,we gain about 4 min. of additonal daylight each day.

c)This is bcuz the red is the lest scattered by fog or smoke.Therefore it can be seen in the same colour at a distance.

d)The star light undergoes refrection continuosly before it reaches the earth the earth.As the path of the rays of light coming from the stars goes on varying slightly, the amount of light entering the eye changes and the star appear twinkling.

e)This is bcuz planets are much closer and acts as extended source.



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A)Blue colour has a shorter wavelength than red , so according to Rayleigh scattering law, blue colour of sunlight scattere much more strongly by the large number of molecules present in the earth's atmosphere. hence the sky appears blue.

C) It has the highest wavelenght in all colours and due to its property it is visible from far distance.

D) This is due to refraction of light coming from the stars by atmosphere. light from stars passing through the atmosphere , light bends towards the normal. so, the stars appear slightly higher than they actually appear .

E) Planets being larger size can be taken as a collection of large number of point sized objects of light, which nullify the twinkling effect of each other. Hope this helps u ☺ If u like give me thums up

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