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Physics Week – Light

Light controls what the world looks like

Light controls what the world looks like.  Plants use its energy to help them grow.  The future’s bright—it’s blindingly obvious!

Light is made of billions of tiny particles called photons. These photons travel from one place to another in waves.

Visible light is the subset of photons that move at a wavelength that we can see. Among the different photons that are in visible light, the ones that have the longest wavelength look red to us, and the ones that have the shortest wavelength look blue to us.

All colours come from different wavelengths of light.

The big bright Sun!

Most of the light on Earth comes to us from the Sun. The Sun shoots out billions of photons every second in all directions, and the ones that happen to be pointed toward the Earth come here.

When these photons get to the Earth, they first run into Earth’s atmosphere. Some of the photons get absorbed by the atmosphere itself, especially the ultraviolet ones whose wavelength is shorter.

This is a good thing for us, because too much ultraviolet light would kill us, and all other living things on the planet. (In fact, doctors sometimes use ultraviolet light to kill germs!).

Ever wondered why the sky is blue?

Because blue light has the shortest wavelength of the visible light photons, some of the blue light is also absorbed by the atmosphere.

That’s why the sky looks blue from the ground in the daytime, because you can see all that blue light up there. But when the light hits clouds instead, the clouds reflect all of the light down to Earth, so the clouds look white to us.

Most of the visible light does get through the atmosphere and comes down to the surface of the Earth.

In addition, a lot of infrared light also gets through the atmosphere. This is light that has a longer wavelength, so we can’t see it.

Fun Facts

  1. A rainbow does not actually exist at a particular location in the sky. Its apparent position depends on the observer’s location and the position of the Sun.
  2. Newton originally (1672) named only five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven colours by analogy to the number of notes in a musical scale.
  3. Early physicists who experimented with Prisms thought the prism created the colours, later it was realised that the colours were in the light and the prism just unlocked them.
  4. It is normal for sundials to have a motto: “I am a sundial and I make a botch / of what is done far better by a watch”.

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