Why is the sky blue?
When we walk out of the home and see the sky a question arises in our mind. Why is the sky blue? We will think we have studied that in the sky there are sun, planets and stars. But there are no other elements to give light. But when we look sun and stars they are not blue in colour. Then why does the sky look blue in colour? Here is the shortest answer and longest answer to know deeply.
Sun emits white light and there is no doubt in it. When seven colours (VIBGYOR) are mixed equally the mixture will look like white colour. So now we can understand that there are seven colours mixed and appears like white light. This white light is emitted by the sun. But when this white light travels through the atmosphere it scatters. That means the seven colours scatter. If a colour has long wavelength it scatters after traveling a long distance. If a colour has short wavelength it scatters after traveling a short distance.
Here, the seven colours are Violet, Indigo, Blue, Green, Yellow, Orange and Red. Among these colours, Violet colour has the shortest wavelength and so it will scatter first after travelling a short distance. Red colour has the longest wavelength and so it will scatter last after travelling a long distance.
When we see this white light from a short distance where it starts to scatter it will look like violet in colour. When we see this white light from a long distance where it starts to scatter it will look like red in colour.
So, when we see from a long distance, we will look the sun and the sky nearby as Red in colour. now, you will remember the sunset. Because the distance from the sun and the atmosphere from which we look is very long. So, in that distance (atmosphere) Red light starts to scatter. And so, sun looks like red in colour. But when we take blue colour it has medium wavelength. When the sun is above our head (afternoon) the distance is medium. So blue colour starts to scatter. And so, we see the sky as blue in colour.
When we walk along the road and see the sky, we may see the beauty of the sky and wonder. But there may be times, we see the sky and ponder why the sky is blue in colour. Have your thought gave you an answer for that question! If your answer is no, don't worry here you will find the answer.
To understand why the sky is blue first we must understand some basic concepts.
Sun emits white light. White light has seven colours. The seven colours (white light contains) are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). For example, if you send a light to pass through a prism which will divide the light into seven colours. This is called spectrum. See the fig. below.
Another one example is that if you paint that seven colours equally in a disc like card and rotate the card fast using a pencil like object fixed in the centre of the disc then we can see the disc as white colour. See the figure below.
Now coming to the point, every colour has different wavelength (wavelength means the length of a single wave. For instance, if you throw a stone in a pond the water travels in a wave motion and also when the wind is blowing strongly over the water in a lake it forms a wave motion). In the same way the light does not travel in the straight line but in the wave form.
If the wavelength is long, then it will travel more distance and if it is short then the travel distance also is short. This is because it will scatter very quickly.
Wavelength Order: Red>Orange> Yellow>Green>Blue>Indigo>Violet
Because red light has higher wavelength it travels very long (See the fig. above). When the light comes from the sun after a certain distance every colour starts to scatter. First violet and then indigo and so on. Violet and Indigo scatters very early in the space and then Blue starts to scatter in the atmosphere which we see. So that the sky appears for our eye as blue. But actually, the sky is not blue in colour.
Then a question comes to our mind that what about yellow, orange and red. While the sky appears blue in colour at the same time the sun (longer than the sky) appears in yellow or orange in colour in the daytime. In the same manner while the sky appears blue in colour the same time the sun (very longer than sky) and the nearby area appears red in the evening at sunset.
Only for this reason we use red colour or red light in dangerous, serious and very important situations (In traffic light for Stop light and reflectors, Ambulance light, Dangerous symbols, etc.). If we use red colour at dangerous situations, then it will visible to anyone who is in the long distance.
There are seven colours in white light which is emitted by the sun. When the light travels through the atmosphere it scatters. That means the seven colours scatter. If a colour (Red) has long wavelength it scatters after traveling a long distance. If a colour (Violet) has short wavelength it scatters after traveling a short distance. Blue colour has medium wavelength. When the sun is above our head (afternoon) the distance in which the blue colour scatter is in our atmosphere. So, we see the sky as blue in colour. When the sun is far away (sunset) the distance in which the red colour scatter is in our atmosphere. So we see the sky near the sun as red in colour.