Give scientific reasons :

*1. The sky appears dark instead of blue to a person located in space.
Ans. 1. The blue colour of the sky is due to scattering of light by the atmosphere. 2. At higher altitudes there is no atmosphere hence, the scattering of light does not take place at all. Hence in space the sky appears dark
instead of blue.
*2. Stars twinkle at night.
Ans. 1. Stars are point sources of light as they are very far away.
2. The refractive index of air in the given region in atmosphere goes on changing continuously and randomly.
3. When the atmosphere refracts more light towards us, the star is seen bright.
4. When the atmosphere refracts less light towards us, the star is seen dim.
5. Thus, due to change in refractive index of atmosphere, stars appear twinkling at night.
*3. The sun appears reddish early in morning.
Ans. 1. At the time of sunrise or sunset, the sun is very close to horizon.
2. Sunlight has to travel a longer path through the atmosphere to reach the observer.
3. The blue and violet colours are scattered in a greater amount than red colour.
4. The light that reaches to the observer is mostly red and yellow. Hence the sun appears reddish early in the morning.
*4. It is possible to enjoy a rainbow at fountains in any season. Ans. 1. The rainbow appears in the sky after a rain shower.
2. The water droplets act as small prism.
3. When sunlight enters the water droplets present in the atmosphere,
they refract and disperse the incident sunlight, then they reflect it
internally and finally again refract resulting in a rainbow.
4. The water droplets of a fountain can do the same thing if sunlight passes through them. Hence it is possible to enjoy a rainbow at fountains
in any season.
5. Planets do not twinkle.
Ans. 1. As compared to stars, planets are nearer to the earth.
2. So a planet can be considered as a collection of a large number of point sources of light.
3. If the intensity of light from one point source decreases, it increases from the other source. Hence average intensity remains the same. Hence planets don’t twinkle.
6. During holi, we see flickering of an object through a turbulent stream of hot air rising above the fire of holi.
Ans. 1. The air just above the fire become hotter than the air further up.
2. The hotter air is lighter (rarer) than the cooler air (denser) above it.
Hence the refractive index of hotter air is less than cooler air.
3. As the physical conditions of air are not stationary, when we observe, the apparent position of object fluctuates. Hence during holi, we see
flickering on an object behind the fire.
*7. Red colour is used in danger signals.

Ans. 1.

In the visible range of light, maximum scattering of blue light and least

scattering of red light takes place.

2. As red light is scattered the least by atmosphere. It can travel larger
   distance.
   
3. Hence danger signals are red coloured. 
   

8. A coin immersed in water or bottom of a swimming pool appears raised.

Ans. 1.

When light travels from denser (water) to rarer (air) medium it bends

away from the normal.

2. Due to this, it appears to come from a point above the actual point.
   
3. Hence, a coin immersed in water or bottom of a swimming pool appears
   raised.