Dot Physics

Light and Waves – at a basic level

Yes, this can be very complicated. But what should a middle-school student understand about light? You see stuff in textbooks that is either wrong or just a bunch of disconnected factoids (I like the word factoid). So, what do I think is important about light (not at the Maxwell’s equations level)

What is a wave

If you want to talk about light, you need to talk about waves. So what is a wave? I like to start with an example. Suppose you are in a sports stadium – maybe a football game. Some inspired fan decides to start a wave. If you look at the individual people, the wave might be represented as this:

i-34ca0fc7c5dbc79567da13845b46c1b3-peoplewave.jpg

The key thing is that in this case, the people are the medium for the wave, but the wave is really a displacement in the medium. The people move up and down, but the wave moves perpendicular to that. The great thing about this example is that it address a common misconception that the “stuff” in a wave moves in the direction the wave moves. This is not true. Do the people move all the way around the stadium? No, just the displacement moves around.

Properties of a wave

What are the features that can be assigned to a wave? Let me go back to the people-wave. Also, let me add a pole that the wave can pass. Let me make one more change. Suppose this is not a normal football wave, but a repeating wave – you know like waves in the ocean. Here is a diagram:

i-92235bad6878e842e336ee54b9b356be-waveprops.jpg

Some of the ways the wave could be characterized would be:

  • Wavelength: If the wave is repeating (we call that periodic), then I could measure the distance from one peak of displacement to the next peak. This distance is called the wavelength (typically represented with the greek letter lambda – ?) and this is measured in meters.
  • Wave speed: Suppose I were to look at one of the displacements and see how fast it was moving (the displacement, not the people). This would be the wave speed (v) in meters/second.
  • Frequency: If you look at some stationary object that the wave passes, like a pole, you could count the number of waves that pass every second. This waves per second is the frequency (f). It has units of 1/seconds or hertz.
  • Amplitude: The distance the people in this football wave travel up from the middle position would be the amplitude – or how big the displacement is (A). This is also measured in meters.

It is not too difficult to see that there is a connection between wavelength, wave speed, and frequency. That relationship is:

i-0719f7aba218630ed988b46de8f08f2f-la_te_xi_t_1_71.jpg

What about light?

Light is also a wave. However, it is different in some ways. First, light doesn’t need a medium to travel though, in a sense it is its own medium. To compare this to other waves, what would happen to the football wave in a stadium with no people? Or how would you have water waves in a pool without water? You can’t.

At first, the idea was that light did have a medium – but you couldn’t really detect it. Light is a displacement in electric and magnetic fields. You could consider the electric field to be the medium for the magnetic wave and the magnetic field as the medium for the electric fields. Yes, it is more complicated than that, but the important thing is that light is an electro-magnetic wave.

How do you make an electromagnetic wave? The key is an electric charge. An electric charge (like a proton or electron) makes an electric field. A moving electric charge also makes a magnetic field. So, if you take an electric charge and oscillate it back and forth, you get an electromagnetic wave.

i-478928e13a0b968baf227def4f62764f-light_waves.jpg

Above is a diagram of an oscillating electric charge and a representation of the electric field it creates (I did not show the magnetic field). The important thing is that this wave also has a wavelength, frequency and wave speed. Since all electromagnetic waves have the same ‘medium’, they all have the same speed. Key idea – all electromagnetic waves (radio, microwave, visible light, ultra-violet, x-rays etc…) travel at the same speed. This is the speed of light (3 x 108 m/s).

What about the electromagnetic spectrum

If you look at the different frequencies of EM-waves, it can be broken down like this (from low frequency to high):

  • Radio
  • Microwaves
  • Infrared
  • Visible light (Red, Orange, Yellow, Green, Blue, Violet)
  • Ultraviolet
  • X-rays
  • Gamma rays

The different frequencies mean that the different types of EM waves interact differently with matter. Here is another diagram:

i-cc81926a9cbfe8545701f87321a3aee6-light_waves_1.jpg

Here is some visible light and radio waves going towards some glass and some brick. I hope that you realize that the light can go through glass, but not brick. The radio waves can go through both (a radio works inside, doesn’t it?).

Here is the video version:

Light and Waves from Rhett Allain on Vimeo.

Comments

  1. #1 Becky
    September 20, 2009

    This is a really great post – clear and concise. I can’t wait to use these illustrations with students! Thank you!

  2. #2 butt mubashir
    September 25, 2009

    The post should have been more sophesticated

  3. #3 liam
    May 25, 2011

    can i be your doctor