1. Why do stars twinkle and hence why do planets do not twinkle?

2. Give reason for the following: (a) Blue colour of the sky (b) Red colour of danger signals.

3. Give reason for the following (a) White colour of clouds (b) Sun appears red at the time of sun set and sun rise.

4. Explain Tyndal Effect.

5. What is rainbow? Give the conditions for observing it. State the principle involved in the formation of rainbow.

As the light from a star entres 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 is 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 is just look like is twinkling..

Planets are closer to us and send more light. If some of the light beams are scattered away, others still get through to us, so planets dont usually twinkle..

 1.Continuosly changing atmosphererefract light from the star by different amounts fromone moment to the other,when atmospere refract more starlight towarda us the star appears to be bright and when the atmosphere refract less starlight then the star appear to be dim

  however the planets are nearer to us than the star ,they appear to be comparitively bigger to us so they cannot be considered as point source ,hence no twinkling

1.The Twinkling of stars are due the atmospheric refraction of star light.Starsare very distant sources of light ,onentering the earth's atmosphere undergoes  continously refraction before it reaches the earth .The atmosphere refraction occurs in a medium of gradually changing the refractive index.As the path of rays of light coming from the star goes on varying slightly the apparent position of the star fluctuates and the amount of starlight entering the eye flivkers .tyhe star sometime become brighter and at some  other time fainter.

The planet are much more closer to the earth and are ths seen as extended sources.If we consider a planet as a collection of a large number of point sized sorces of light ,the total varition variations in the  amount of kight enteriing our eye forms all the individual point sized sources will average to be zero .So the planet do not twinkle.

 2.Light from the Sun overhead would travel relatively shorter distance. Only a little of the blue and violet colours are scattered .(B)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.

3.Because there is no apppearanse of light clouds do not have their own light At the of the rainy day sun disappears in between the clouds.(b)ZNear the horizon most of the blue light and shorter wavelength  are scattered by the particles therefore the light that reaches our eyes are longer wavelenth .this gives us to the reddish appearence of the Sun.

4.Tyndall effect – Scattering of the light beam by suspended particles in the solution.Atmospheric particles, smoke, tiny water droplets, suspended particles of dust, and air molecules scatter sunlight. Therefore, the path of light becomes visible.

 5.The rainbow is a natural spectrum appearing in the sky after a rain shower.It is caused by the dispesion of sunlight by tiny water droplets present in the atmosphere .A raibow is always form in a direction of the opposite of the sun .the tiny water droplets works as a small prisms they refract and disperse the sunlight then reflect internally and finally refract when it again when comes out of a raindrop.Due to the dispersion of light and internal reflection , different colours reach the observer's eye.

ANS 1: The planets are much rearer to the earth and they are seen as the extended sources of light .If we consider planet as a large collection of large number of point sized sources , the total variation in the amount of light entering our eye will average out to zero .So the planets will not twinkle.

ANS 2:(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 .

ANS 3:(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.

ANS 4:When light rays passes through a medium , the fine particles present in the medium causes partial and irregular reflection of light in different directions so that the path of light became visible .This phenomenon is known as ''Scattering of light'' or ''Tyndall Effect ''.

 ANS 4:A rainbow is a natural spectrum appearing in the sky after a rain shower .It is caused by the dispersion of sunlight by tiny water droplets present in the atmoshphere .A rainbow is always formed in a direction opposite to that of the sun .The water droplets act like small prisms .They refract and disperse the incident sun light , then reflect internally and finally refract it again when it comes out of the drop .Due to the dispersion of light and internal reflection , the rainbow is formed.

 Conditions to observe rainbow~~~~~~~ SUN MUST BE BEHIND THE OBSERVER!