Light

Ollie Pye

January 30, 2018

Light is fundamental to our existence. It is what allows us to see the world and the universe, and with it our eyes can distinguish all the colours of the rainbow. There is more to light than just these colours, however.

“Light” is the word we use to describe what we see. We can see the light from a TV, a light bulb and the sun. As previously stated though, it is much more than this. Light is better described as electromagnetic radiation (EM radiation). This is because all forms of light are made from electric and magnetic fields that interact with each other. Visible light makes up just a tiny fraction of the electromagnetic spectrum. Along this electromagnetic spectrum are radio waves (used by radios in our car), microwaves (what we use to quickly heat up food), ultra-violet waves (UV rays that cause sun burn) and X-rays (used to take images of your bones). Below is the full make-up of this spectrum.

 The electromagnetic spectrum. As you can see, visible light is only makes up a small percentage of the light that exists in the universe. Image credit: NASA

The electromagnetic spectrum. As you can see, visible light is only makes up a small percentage of the light that exists in the universe. Image credit: NASA

Note the use of the word wave when describing light. This is because light travels as a wave (and also as a particle, but this is for another article). This is important to understanding why there are different types of light, why we see so many different colours in a rainbow. The difference between red light and blue light, or X-rays and microwaves is the lights wavelength. That is, the distance between the peaks of the waves. This is comparable to waves in the ocean. There is a distance between each wave. This is the same with light waves. This is the fundamental difference between the different types of light. For example, radio waves can have wavelengths greater than one metre whereas the light we see around us (visible light) has wavelengths measured in nano meters (0.0000005 m).

Speed

Light travels at approximately 300,000 kilometres per second (km/s), and is the speed limit of the universe. That is, in space, nothing can travel faster than the speed of light. When we look at the sun (or anything), we are looking into the past. This is because the light from the sun takes approximately 8 minutes to reach Earth. The Sun could disappear without a trace, and we would not realise for these 8 minutes (even the effects of gravity are restricted to the speed of light). Although the speed of light is well known, the idea that light even had a speed was heavily debated for many years.

Galileo attempted to measure the speed of light by timing how long it took his assistant to uncover a lamp after doing so himself from an adjacent hill. When Galileo uncovered his lamp, however, his assistant almost instantaneously did the same. It was not possible to measure the speed of light over this distance. Either light travelled instantaneously, or it travelled too fast to measure. The speed of light was eventually measured by Danish astronomer Ole Rømer in 1675-76. He discovered that the moon Io of Jupiter did not orbit in regular intervals. Instead, the moon would appear consistently later than predicted throughout the year. He reasoned that as the distance between Earth and Jupiter increased (due to the planets orbits), the light took longer to reach him. To visualise this, imagine you are on a field. A friend of yours hits a metal bin with a stick right in front of you. You hear the noise as the bin is hit. Now your friend moves 100 metres away. Again, you see them hit the bin. You expect to hear the noise at the same time, however there is a delay. The sound took time to reach you. Light does the same thing, it just travels much faster and so is harder to detect.

How does light move

Another great mystery of light was whether it travelled through a medium, and if so, what was this medium? At the time, it was believed that light travelled as a wave, and so would therefore need a medium to travel through: waves in the ocean need water; sound waves need particles in the air; etc. to travel. The medium posited by scientist to address this issue was called the Luminiferous Ether.

In 1887, Michelson and Morley attempted to detect this ether in what is now known as the Michelson-Morley experiment. They used light from a laser that travelled perpendicular (at right angles) to each other. They reasoned that if there was an ether, then as the Earth rotated and orbited around the Sun, it would move through this ether. Light travelling in one direction would travel with the flow of the ether, and therefore reach its detector first.

Imagine you are in a whirl pool. You can swim much faster when you swim with the current than against it. The idea was the same with the ether. Michelson and Morley conducted their experiment numerous times and changed the direction of their laser. The result was always the same. There was no difference in the speed of the light. This means that light does not travel through a medium like sound, so the ether does not exist.

Final Remarks

Light is so much more than what we see in our daily lives. What we see is only a tiny fraction of the light, or EM radiation, around us. It is the speed limit of the universe and allows us to communicate over thousands of kilometres.


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