Thomas Young was a true polymath.  In the early 1800s, he made many contributions to mechanics, optics and Egyptology, as one of the very first men to attempt the translation of hieroglyphics, with a deal of success. Mechanical Engineers all know of Mr Young, as one of the first things that gets drummed into us is something called Young’s Modulus.  This is, essentially, a description of how materials behave when you put any kind of load on them.  Young proved that the elasticity of something (how it behaves when you stretch it) is a property of the material you’re using and not its size, shape or geometry.  This led to a revolution in Engineering, as it is possible to predict the behaviour of a material under load very accurately.

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Young believed that light behaved as a wave and, in 1801 devised an experiment to prove it. Imagine  ripples on the surface of a pond.  If you drop a pebble into the pond, you’ll see ripples spread out from the point of impact.  If you drop two pebbles next to each other, the ripples will spread out and interact with each other.  Young thought that light behaved in the same way as these ripples and set up an experiment to show what happened using two very small holes.


What happens if you shine a light through a small hole?  It doesn’t simply pass through the hole; if the hole is very small, the light undergoes a process called diffraction.  What this essentially means is that the light spreads out as it emerges from the hole in the same way as the ripples spread out from the pebble you dropped in that pond earlier. Now, what if we shine some light through another small hole just next to the first one?  We get another bit of diffraction and the light does the spreading out thing.  However, we now have two bits of light spreading out and overlapping each other. Light, behaving as a wave, does some pretty unusual things when it meets another wave.  The interaction of overlapping waves gives us the phenomenon known as interference. 


If two waves meet and they line up perfectly, that is if their high points and low points meet up, then the two waves are added together to make a bigger wave.  Look at the picture below as an example.  If the red wave meets the blue wave and they are, as we say in my little world, in phase, then they add together and the result is the yellow wave.  You get a result that the sum of the two individual waves, hence a brighter light:



Red plus blue equals yellow.


If however, the two waves meet up where the high points meet the low points, the two waves cancel each other out, so we get darkness:


two waves cancel


When our light spreads out from the two holes, the waves overlap and interact and we get regions where these two effects are happening.  In reality, what you will see is something like this:




This may look like a nice piece of abstract science, but it’s far more than that.  Using this phenomenon of interference, we can measure very small things very accurately indeed.


For example, here’s a modern take on the interferometer currently in use in the measurement lab at JMU:


wyko RST

The Wyko RST, a Scanning Interference Microscope


And here’s a measurement I made a few years ago of the surface of a silicon chip.  The sort of thing that is at the heart of your PC:



IC surface profile: look at the measurement scale!


The coloured section is part of the surface of a processor chip that’s 0.151mm x 0.112mm square.  The scale on the side shows the height and is marked in mm (micrometres – one micrometre is one thousandth of a millimetre).  The difference between the highest and the lowest point on that chip is 7.94mm, or 0.00794mm!


How’s that for accurate?


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