- A marked innate ability, as for artistic accomplishment
- Natural endowment or ability of a superior quality
- A person or group of people having such ability
- A variable unit of weight or money used in Ancient Greece, Rome and the Middle East.
Add to that a complication in my subjective assessment of my own ability – if I can think of someone better than me at something, my self-esteem won't let me think myself good at something. So I can't say “I'm a good writer” because I know people better than me. I can't allow myself talent in any artistic area at all, and I can't say I'm clever, because I'm surrounded all day everyday by people better than me at science.
What I do have, that I allow myself the indulgence of thinking as a 'talent', is an ability to explain concepts to a lay audience, be they scientific or literary ideas. I don't think I'm perfect, but I like to think I have an ability that is very nearly 'natural'.
For example, have you ever wondered what gives thin layers of oil those distinctive colours?
This is a lightwave. Light moves in transverse waves – meaning that the waves oscillate in a plane perpendicular to the direction of motion. The light wave has the following properties that are pertinent to this post:
WAVELENGTH (λ) - the length between successive peaks on the wave. Measured in nm (nanometers) because it is a unit of length. We interpret different wavelengths as colours. Red light, for instance, has a wavelength of around 400nm, and blue light around 450nm.
Most light sources produce light of a range of wavelengths. 'Natural light' contains all wavelengths in the visible spectrum, which when combined we see as white. Combinations of different wavelengths produce different colours.
AMPLITUDE (Ǘ): the change between the peak of the wave and the baseline in the centre. This is not measured in distance, because it's a change in energy and our picture is just a diagram. It does, however, relate to the intensity, or brightness of the light.
SPEED: The 'speed of light' that you've heard referenced, that super special speed which is the fastest anything can go, ever, known to scientists as c – that's more accurately named 'the speed of light in a vacuum', because the speed that light actually travels at depends on the medium through which it's travelled. Light moves faster in air than it does in water, for instance, or oil, and this change in speed results in an effect called refraction.
The difference in speed between light travelling in any one material and a vacuum is the refractive index of that material. Refraction is the change in direction of a light (well, any wave, but we're talking about light) when it moves from one medium to another at an angle. One side of the light beam enters the new material before the rest and changes speed, inducing a change in direction.
Imagine a group of people holding hands and walking abreast at the same speed. If you were to grab the person at a far end and pull them back, slowing them down, the column of people will bend towards you. This is how a light beam changes direction between, for example, air and glass or water. It is also why straws in water look bent. Depending on the angle, some or all of the light will be reflected instead of transmitted – think of your reflection in window or in water.
So what does this mean for oil and its spectrum of visible colours?
Imagine a thin film of oil on a surface of water. The refraction index of water is 1.33, and that of oil varies depending on the specific oil, but we're only interested in it being greater than that of water water, because there are two surfaces we care about – the interface between the air and the oil, and that between the oil and water.
As light moves from air to the oil, some of it is reflected, and some of it refracts according to the angle at which it hit (the angle of incidence) and the refractive index of that specific oil. The same thing happens at the interface between the oil and water: some is transmitted into the water, and some is reflected back through the oil and air.
The colours emerge, however, when the light that was reflected from the oil interferes with the light reflected from the water, and this is because the former the reflected light undergoes something called the Reflection Phase Change, which doesn't happen at the oil/water interface.
When light is reflected within a material of a lower refractive index (RI) off a material with a higher one (for example, reflecting off oil, within air), the light wave is inverted – peaks become troughs and troughs become peaks. This is called a 180° phase change, where 'phase' indicates parts of the wave cycle.
This does not occur when light is reflected within a material with a higher RI, like at the oil/water interface. Oil has a higher RI than water, Reflection Phase Change does not occur, and the peaks and troughs continue like nothing happened. So, coming out of the oil towards your eye are two different beams of light
It's also important to note that light moves slower through oil that it does through air, and the wavelength is slightly altered as well. Depending on the thickness of the oil and the wavelength of the light therefore, the light will emerge into the air at a varying point along it's wave path.
Why is this important? Well, because when you've got two (or more, we'll stick with two) lightwaves, interference occurs. If for example, peaks and troughs occur at the same point:
They are said to be in phase, and add together, making extra big peaks and troughs. The amplitude is increased. Remember: Amplitude=intensity, so the light is brighter. This is called constructive interference
Conversely, if the peaks are aligned with troughs and vice versa:
Then the light waves are out of phase and cancel out completely. This is destructive interference. I'm not going to draw a straight line here. You'll just have to imagine it.
White light, remember, contains light of all different wavelengths, and when reflected through a thin film, like oil or the surface of bubbles, some of that light is slowed relative to the rest of that light by a a part of a wave path. For some wavelengths, this will result in constructive interference, and for some destructive. The result is that some wavelengths will be completely cancelled out of that original white light, and some emphasised. Wavelength=colour. So the wavelengths that do reach your eye give you a spectrum, depending on the angle at which you're viewing the film.
And now you know.
And I hope you're grateful that I wrote that up instead of catching up on one of the THREE of my programmes I'm behind on.
Basic Electromagnetic Wave Properties at Molecular Expressions hosted by the Florida State University
Thin Film Interference at PhysicsLAB, © Catharine H. Colwell
Oil Film Interference at HyperPhysics (created by Carl R. Nave, hosted at Georgia State University)
List of Refractive Indices at Wikipedia