Objectives
After working through this chapter, you should be able to:
Create a demonstration of polarisation by scattering
Explain how polarisation by scattering works
Introduction
As a brief recap, in chapter 14 we discussed how light can be made to only have one (or at least very few) orientations of electric field if it undergoes a process of polarisation . This is important because light from the sun is unpolarised , meaning that the light is vibrating in all orientations, and therefore the orientation of the electric field varies randomly over time. One way in which this natural sunlight can become polarised is through polarisation by scattering in which the light from the sun will be scattered (and polarised) when it comes into contact with molecules in the atmosphere. In particular, shorter wavelengths (within the sunlight) are scattered more easily than the longer wavelengths, which means that as light travels through the atmosphere, it scatters shorter (blue) wavelengths into the atmosphere (making the sky look blue), which subsequently makes the light from the sun appear to be shifted slightly towards the longer wavelengths (making the sun look more yellow).
In this experiment we are going to investigate this principle by making our very own atmosphere in a glass and using a torch as our ‘sun’.
The experiment
For this experiment we are going to attempt to prove that when light travels through a busy medium (like the atmosphere), it will scatter shorter wavelengths more than longer wavelengths. This will be achieved by creating an atmosphere (milky water in a glass) and seeing what the sun (a torch light) looks like as it passes through the atmosphere.
Before starting the experiment, please make sure you have all the equipment you need (and a mobile device to record your results!).
Equipment required
- •
A clear glass (a reasonable size glass, no pattern if possible)
- •
A white-light torch (mobile phone lights work OK, but torches work better)
- •
Some cold tap water
- •
A small amount of milk
- •
Something to stir the mixture with
- •
Optional (but advised): a towel or kitchen roll to mop up water/milk spillages
Method
- 1.
Fill the glass with cold water.
- 2.
Shine the torch from behind the glass, directly towards yourself.
- 3.
Make a note of the colour of the torch light – hopefully it looks quite white (unaffected by travelling through the water).
- 4.
Make a note of the colour of the water – hopefully it looks quite clear.
- 5.
Make a note of the colour of the milk (before we put it into the water) – hopefully it looks white.
- 6.
Now add a very tiny amount of milk into the water (if you add too much then the experiment won’t work, so it’s better to add too little than too much!). You may need to stir the mixture to make it work appropriately.
- 7.
Shine the torch from behind the glass, directly towards yourself.
- 8.
Make a note of the colour of the torch light – hopefully it looks more yellow.
- 9.
Keep adding milk until the torch light begins to become convincingly yellow in appearance.
- 10.
Make a note of the colour of the milky water – hopefully it looks to be bluer than without the light.
- 11.
Now keep the torch still but view the milky water from above the glass; take notice of what’s happened to the colour – does it look bluer with the torch than without?
Results
Hopefully you could see that the torch light magically (or scientifically and predictably) changed colour to become more yellow-orange when the milk was added to the water, and that the milky water became blue. The reason this works is because the milk in the water scatters the light from the torch through polarisation by scattering ( Fig. 19.1 ). As we learned in chapter 14 , shorter wavelength (blue) light scatters more easily than longer wavelength light, so as a result of this the blue light from the torch ends up diffusing through the milk, making it look bluer, and the light source will look more yellow (due to the missing blue). However, if you find that the experiment isn’t working very well, you can troubleshoot the issue using Table 19.1 as a guide.
