Spectacular Natural Phenomenon

SOME Natural Phenomenon

Why is the sky blue?Why the sun appears red at sunrise and sunset and white at noon?Why do stars twinkle and planets do not? why do stars appear higher than they actually are?Why is space black?Why are clouds white?

Name : Sahil PahoojaClass : X-BSession : 2015-16

Why is the Sky Blue?

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air. However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

Colour Of SunAt the time of sunrise and sunset when the sun is near the horizon, the sunlight has to travel the greatest distance through the atmosphere to reach us. During this long journey, most of the shorter wavelength blue colour is scattered and what reaches us directly from setting and rising sun is actually longer wavelength red colour, due to which sun and its surroundings appear red.At noon when the sun is overhead in the sky then light coming from the sun has to travel a relatively shorter distance to reach us. During this short journey, only a little of blue is scattered .Since light coming at noon from overhead has almost all colours in right proportion , therefore ,the sun overhead at noon appears white to us.

Twinkling StarsWhen light coming from a star enters the earths atmosphere, it undergoes refraction due to the varying optical densities at various altitudes. The continuously changing atmosphere reflects the light from the stars by different amounts from one moment to another. When more light is refracted toward us the sun appears to be bright, and dim when less light is refracted, thus leading to twinkling appearance of stars.

On the other hand the planets are quite big (they are near to us), thus planet can be taken as collection on point sources of light. The dimming effect produced in one part are nullified by the brighter effect. Thus on whole the brightness of a planet remains same and does not twinkle.

STARS APPEAR HIGHER Light from a star is refracted as it leaves space and enters the earths atmosphere. Air higher up in the sky is rarer but it is denser near the surface. Snells law of refraction tells us that light is refracted towards the medium with higher index of refraction. Therefore, the light coming from any star that is not situated directly

above the observer takes a curved path bent down towards the observer, arriving at the observer at a steeper angle than had it travelled on a straight line. Due to this refraction of light , star appears to be at a higher position.

Why are clouds white?In much the same way as why skies are blue, clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths, which combine to produce white light. Clouds will appear dark or grey when either they are in another clouds shadow or the top of a cloud casts a shadow upon its own base.Why is space black?If theuniverse is full of stars, why doesn't the light from all of them add up to make the whole sky bright all the time. Many different explanations have been put forward to resolve this Olbers' Paradox. The best solution at present is that

The universe is not infinitely old. That means we can only see objects as far away as the distance light can travel in 15 billion years. The light from stars farther away than that has not yet had time to reach us and so can't contribute to making the sky bright.

6