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The physics of light – introduction

Light is something we tend to take for granted. But what actually is it?

Properties

  • Light travels at the speed of light, 299 792 458 m/s, or about 300 000 kilometres per second. This is true no matter how fast you are travelling – even if you chase a beam of light at half the speed of light it will still get further away from you at the speed of light. The speed of light is denoted by the letter c.
  • Acts like a wave in some situations. Light refracts, reflects, diffracts and has a wavelength and frequency — just like any other wave (except when you wave at someone).
  • Acts like a particle in other situations. Light is made from little particles callled photons.
  • Has a ‘colour’. This is dependant on the wavelength of the light, which is something we will come onto.

What constitutes light? Some terminology

Light usually means the visible part of the ‘electromagnetic spectrum‘. The colour of light depends on its wavelengths — there are some wavelengths we cannot see. All the wavelengths make up the electromagnetic spectrum.

Electromagnetic radiation includes everything from radio waves, microwaves (not the kitchen appliance), ultraviolet light, visible light, infra-red, x-rays and gamma rays. Every time I say the word ‘light’ in this article, I am really talking about all forms of electromagnetic radiation, of which visible light is one type.

What is it made of?

Light is made of little packets of energy (for those who know quantum theory, each photon represents a quantum of light energy). As a result, light has properties that suggest it is lots of little ball bearings whizzing around. Isaac Newton himself favoured this idea (before the true nature of light was discovered), he called them corpuscles.

However, there is a snag. Light refracts and diffracts, and can be split into different wavelengths. The corpuscle model does not allow this.

So what’s going on? is light a wave, like a wave on the sea or like sound, or is it made up of little ‘ball bearings’?

And the answer is…

Both. Light is what’s called a wave-particle duality. All fundamental particles exhibit this behaviour, particles such as electrons or quarks. To really understand it requires some heady quantum theory, so I won’t go into it here. Below is a diagram of a photon in motion (in fact, special relativity says it is impossible for a photon to be anything but in motion).

Light

It’s not a great diagram, I know. From it you can see that a photon consists of magnetic and electric fields vibrating at right angles to each other. This is why light exhibits both wave and particle behaviour. It is also why it is called electromagnetic radiation.

Wavelength, frequency and energy

The wavelength of a wave is the distance between wave peaks (or any equivalent point on the wave — wherever you measure it it makes no difference). The amplitude of a wave is how big the wave peak is. This diagram explains it all.

Wave labels

(Click to enlarge)

The frequency of a wave is how many waves pass a single point in one second. For an ocean wave, it’s how many times the water would go up and down in one second at one particular point.

Wavelength, frequency and speed are related in one simple equation:

V equals f lambda

(The dot just means multiplied by). Because the speed of light (V in the above equation) will always be constant, knowing the frequency of a ray of light means you also know its wavelength. A bigger wavelength means a smaller frequency.
Wavelength (and frequency, because the two are related) is connected to the energy of a photon by another simple equation:

Energy of a photon

Planck’s constant, h, is just a number, and is very important in quantum theory.

Because light has is a wave (it has a wavelength, it is moving energy etc.), it has properties just like other waves. Below are a few of them.

Colour

Colour is how we see different wavelgths of light. Light with a long wavelength, low frequency and low energy is seen as red light. Light with a short wavelength, high frequency and energy is senn as violet light. Everything between is on the spectrum, for example green light is about halfway between red and violet light.

There are wavelengths of light we can’t see. Ultra-violet light is higher in energy and frequency and has a shorter wavelength than anything we can see, for example. Some animals can see wavlengths that we can’t. Bees, for example, can see ultra-violet.
Below is a diagram of the electromagnetic spectrum. It should give you an idea of how little we can actually see.

Electromagnetic spectrum

You’ll notcie there is no white light on the spectrum. White light (like light from the sun) is made up of a mixture all wavelengths of visible light.

The phenomona of light part I

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Comments

  1. * viagelon says:

    Nun main Säiten da. Ech um wäit frou, weisen d’Lëtzebuerger blo un, um wee geet Dall Hemecht.

    | Reply Posted 9 years, 9 months ago
  2. * Amit says:

    Sensation of color is not just related to the wavelength of the wave.
    It varies from person to person (Color blind for example don’t differentiate between green and red)
    It also depends even on our experience!!

    | Reply Posted 9 years, 5 months ago
  3. * sara says:

    nice …….helped me a lot in my project

    | Reply Posted 1 year, 10 months ago
  4. * urja chhabra says:

    I got clariffied about my problm

    | Reply Posted 1 year, 8 months ago


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