Objectives
After working through this chapter, you should be able to:
Explain why light can disperse through water if a mirror is introduced
Explain dispersion
Introduction
Looking back over the book so far, in chapter 1 we learned that white light (e.g. sunlight) contains all wavelengths of visible light, and in chapter 8 we discussed that these wavelengths of light can be dispersed to be independently visible (e.g. in the case of a rainbow where white light disperses through a raindrop, or when light travels through a prism ).
Dispersion is something that we’ll have experienced ourselves in real life if we’ve ever seen light pass through a crystal ornament in a window – it makes a rainbow pattern appear. This rainbow pattern is simply the white (incident) light from the sun dispersing (splitting out into its constituent wavelengths), which makes it appear to be lots of colours instead of just white. This phenomenon occurs because the wavelength of the light is directly related to how much refraction it undergoes as it travels from one material to another. There is an old adage ‘ blue bends best ’ which is supposed to help us remember that shorter wavelength light (e.g. blue) will refract to a greater degree than longer wavelength light (e.g. red) when dispersion takes place.
As any science teacher will probably tell you, the best way to understand this process is to get your hands on a prism and see the effect first-hand. Unfortunately, optical prisms aren’t that common around the home, so instead I’d like to encourage you to build your own prism.
For this experiment we are going to create our own prism at home in order to split white light into the colours of the rainbow – which will also allow us to see first-hand how changing the apical angle will affect the deviation (and the amount of dispersion).
The experiment
Our ‘at-home’ prism experiment works best (and is most impressive) when constructed outside on a sunny day, but it can also work very effectively if you have a torch that produces white light. The only thing to note is that the torch light might make a slightly less-convincing rainbow, depending on its particular spectrum, so please bear this in mind when completing the experiment outlined here.
Before starting the experiment, please make sure you have all the equipment you need (and a mobile device to record your results!).
Equipment required
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A tray or a bowl that can be filled with water
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A plane (flat) mirror that can be placed in the tray/bowl
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Something to see the dispersed light on (e.g. piece of paper or a wall)
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Optional: something to secure the mirror in place, for example, a stone to rest it on or some tape (just in case it makes setting up the experiment easier)
Method
- 1.
Fill the tray/bowl with water – make sure there is enough water that the mirror would be at least partially submerged if placed in the tray/bowl at an angle. I’ve found that this works best with at least 2 inches of water, so please bear this in mind when selecting your tray.
- 2.
Place the mirror into the tray/ bowl – it must be at an oblique angle (e.g. 45°) and must be at least partially submerged. This is made easiest by taping the edge of the mirror to the edge of the tray or by resting it on something (or if you have someone helping you, one of you can hold the mirror in place).
- 3.
Place the tray/bowl in a location where the sun will shine directly onto the water, or angle the torch so that it is shining into the water, towards the mirror.
- 4.
Hold the paper parallel to the tray/bowl above the water and look to see if you’ve produced dispersion (see Figs. 20.1 and 20.2 for a demonstration of this).
