After working through this chapter, you should be able to:
Explain ‘thin film interference’
Explain why the interference pattern is colourful
Produce a thin film demonstration of your own
Before we delve deep into the wonderful world of thin film interference , let’s take a moment to remind ourselves what interference is in the first place (you can also review chapter 10 for additional revision on this). Interference (in optics) describes the variation in wave amplitude that occurs when multiple waves (e.g. of light) interact with one another. The type of interference (and amplitude of the resultant, combined wave) is determined by the relative phase difference or path difference of the individual waves. Fig. 23.1 A shows an example of light interfering constructively, with two in-phase blue waves shown (phase difference 0°) and a path difference that equates to a whole wavelength (nλ). This leads to an increase in amplitude in the resultant wave (shown in orange). Contrarily, Fig. 23.1 B shows an example of light interfering destructively, with the two out-of-phase blue waves (phase difference 180°) and a path difference that equates to half a wavelength (n + 0.5λ). This leads to a decrease in amplitude in the resultant wave (shown in orange; the example in Fig. 23.1 B is complete destructive interference, meaning the resultant amplitude is zero). The important part of this is that the difference in path length of the waves will determine the interference.
A great example of interference in the real world is thin film interference , where light will partially reflect off the front and back surfaces of a very thin film – which essentially reflects two versions of the light back towards you. This allows the light to produce interference which will either produce a reflection (constructive) or not (destructive). Interestingly, if white light (comprising lots of wavelengths) is incident upon one of these films, the individual wavelengths of light will interfere differently with one another, which produces a nice rainbow pattern of interference. Again, the key factor determining this is the path difference, so the angle of the approaching light plays a role, but crucially the thickness of the film does too (see Fig. 23.2 ).
This chapter aims to allow you to see this for yourself whilst also demonstrating that the thickness of the film plays a key role in the observed interference pattern. This means we’re going to make our own thin films!
The goal of this experiment is to show that the path differences in light can be introduced when it reflects off the front and back surfaces of a thin film and to show that the thickness determines the shape of the pattern. To that end we need to make our own thin film that varies in thickness, and we need to shine some white light through it.
Before starting the experiment, please make sure you have all the equipment you need (and a mobile device to record your results!).
Some cold water in a dish or tub of some kind
Some liquid soap for washing dishes (or bubble-blowing mixture, if you have it)
A light source (torch or mobile phone light)
Something with an aperture (e.g. a metal bangle that you wouldn’t mind getting soapy and wet) or some craft wire (the thinner the wire the better) and wire cutters (as shown in Fig. 23.3 )