Wh-Questions concerning light

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What is light?
The Term "light" is difficult to define, because it has various meanings. We use the term "light" when talking about sunbeams, lamps or about daytime. It is also used in different phrases, with various meanings. For instance "To give the green light" means to give permission for something; to "shed light on something" means to explain a circumstance. However, the term particularly serves to describe luminosity, the phenomenon of illumination of surroundings or space which makes us able to see things. Light can come from a natural source, for instance the sun. But it can also be produced artificially by man, which is how lamps work, for example. Light is not a material, because we cannot touch it, we only perceive it. This happens through the eye with its receptor cells and through the brain.

What sources of light are there?
Sources of light are "light bodies" emitting light themselves. On the one hand, natural sources of light like the sun, on the other hand, artificial sources of light like street lights or torches.

What is light pollution?
Light pollution means that the sky is brightened at night through artificial lights like street lights or advertising billboards. In other words, the natural light provided by the moon at night is polluted by the artificial light. It may even happen that we cannot see the stars in the sky anymore, because the night is simply too bright.
If you want to know more just open the simple and attractive explanations of Life at Night.

Why do we only see the stars at night?
Stars are faraway suns, far farther away from us than our sun. That is why we only recognise them as tiny points of light in the sky. They always glow, even at daytime. However, we cannot see them during the day, because our sun is shining much brighter than the stars and therefore outshines their light. Once our sun goes down in the evening and it gets dark, the stars can stand out against the sky's background.

Here is an example:

This writing is as bright as the stars at night and in front of a bright background just like a bright sky, hardly recognisable!

This writing is just as bright as the other one, but in front of a dark background or dark sky much easier to recognise!

Actually, did you know that the star light which we can see is as old as the hills? It is send from unbelievably far away, it can travel faster than everything we know, but it still takes millions of years to reach our planet. The star you see at the sky might not exist anymore.

Why are we tired at night?
Sleep is essential for humans. That is why our brain regularly ensures that we will get tired in order to get to sleep. In turn, the brain is influenced by the shift from day to night, and therefore light and dark. Once it gets dark, the brain knows it has to produce a hormone, which makes us tired, called melatonin. When it becomes light in the morning, the brain stops producing the hormone and instead produces a stress hormone which makes us wake up. This process of tiredness happens within a day and a night, or within 24 hours, it is therefore called the circadian rhythm.