Honestly, it is quite special that you can look into a mirror and see reflected in the glass an image of you. It is so incredibly clear – depending on the quality of the mirror, obviously – with all our features and details so well defined.
Of course, although we are one of the few animals on this planet of ours that can actually recognise ourselves in that reflected image in a mirror, we take this thing a little for granted.
Thinking of all this, have you seen the famous images of Eilean Donan, the castle in the Scottish Highlands? Usually, it is photographed from across a loch – as, from here, you can see the castle doubled in the surface of the water.
Have you ever shouted down a long tunnel and heard your voice return a hundred times? Have you ever had an x-ray? Have you ever seen a rainbow?
The point of all this is that reflection is something that is happening all around us at all times. Even as you are looking at this screen, you are probably seeing a faint silhouette reflecting back at you.
But what is reflection? Do you know? What makes that image of yourself return to you from the mirror? Or how come you can see Eilean Donan both on land and in the water?
It’s quite a simple scientific phenomenon, really – but, given that there is quite a bit to learn, it is what we are going to talk about here.
So, let’s get ready and dive in. Reflection all begins with the science of waves.
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Reflection All Starts with Waves.
The science of reflection all begins with waves – light waves, sound waves, seismic waves, whatever type of wave you fancy. But what are waves?
Waves are disturbances or variations in space-time which, through their propagation, transmit energy from one point to another – or indeed from one point to many others. This energy travels in straight lines from the wave’s source and disturbs the medium through which it travels as it travels.
No doubt you’ll have seen diagrams of waves in your science classes. Usually you see images of transversal waves, the waves that produce ripples, peaks and troughs, disturbances of the medium that are perpendicular to the direction of the energy’s travel. Yet, there are longitudinal waves too, in which the disturbance is parallel to the travel of energy.
Depending on the type of wave – longitudinal or transverse, mechanical or electromagnetic – and its wavelength (essentially the size of the wave), different waves can travel through different media. So, transverse waves, such as the movement of a guitar string, can only travel through solids. Meanwhile, sound waves – which are longitudinal – can travel through solids, liquids, and gases.
It is important to note that electromagnetic waves are transversal too. However, these guys are in a different ball game.
Because they don’t actually need a material medium through which to propagate – as they create a magnetic field that allows them to ‘self-propagate’. Thus, they can travel through a vacuum.
As we said, however, depending on their wavelengths, they may not be able to travel through some solids or gases. Think about it. You can listen to your radio in your bedroom – and radio waves are electromagnetic – however light waves (also electromagnetic) cannot travel through walls.
The wavelength of light is much smaller than the wavelength of radio waves. And this is the key to their fate: absorption, reflection, or transmission.
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So, What is Reflection?
Let’s turn to reflection now. Do you know a good definition of reflection?
Reflection is the change in direction of a wave on meeting an interface between two different media – so that it returns into the medium from which it came.
If light, travelling through air, hits a reflective surface, it will bounce back. However, this change in direction cannot occur apart from at a surface – an interface – between two materials.
The Case of Light.
Light is usually the type of wave that is discussed in relation to reflection – if only because, as outlined above, it is one of the types of reflection that we see so often.
But light doesn’t reflect off every single surface, does it? When you look at a brick wall, it doesn’t reflect. Nor if you look at a transparent pane of glass. This is because the type of material that a light wave encounters will determine the effect of made upon the wave of light.
Light, when it encounters an interface, will follow four different paths:
- Transmission – When light passes through a material, such as a transparent material. This includes, refraction, when light passes into a different medium through which it slows down.
- Absorption – When the light passes into a different medium which absorbs its energy – and transforms it into a different kind of energy (such as thermal energy).
- Specular reflection – When light is reflected in such a way that it produces a mirror-like effect. Light here is reflected from a smooth surface at a definite angle.
- Diffuse reflection – When light is reflected from a rough surface and its waves are scattered. In these cases, the mirror-like image is lost.
Whilst specular reflection is what we conventionally understand to be reflection, actually all surfaces that don’t absorb light reflect it. Your skin, the computer keyboard, houses and animals – literally everything that you can see reflects light. Otherwise, you wouldn’t be able to see it.
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The Law of Reflection.
One of the most important things to learn when you are studying the reflection of light is the so-called law of reflection.
Simply put, the law of reflection states that the angle of incidence equals the angle of reflection. To translate these two terms, this means that the angle from which the wave strikes the interface will be equalled by the angle at which the reflection of light bounces back.
If you draw a line at ninety degrees (a right angle) from the reflective surface – a line which we call the ‘normal’ – the angles of incidence and reflection are measured between the incident wave and that normal.
So, if light enters at forty-five degrees, it will reflect at forty-five degrees too.
By the way, this only applies to surfaces that are ‘smooth’.
What Makes a Surface Reflective?
You know a reflective surface when you see it. It is sort of shiny, again smooth, and you can see your face in it.
Yet, this doesn’t really explain why that surface is shiny. Nor does it tell us what it is in that surface that is shiny.
The thing that determines the reflective potential of a surface is not really fully explained by a reference to ‘smoothness’. Because, as we know, waves are absolutely tiny – so a surface that is visibly smooth may well not be at a level appropriate to light waves.
Rather, the reflectiveness of materials and their surfaces is all about electrons. These subatomic particles vibrate at different frequencies depending on the material.
But light waves have differing frequencies too. ‘Light’ as we know it is a selection of a whole load of different frequencies and wavelengths.
When a particular light wave encounters a material whose electrons have the same vibrational frequency, this light wave is absorbed into this vibrational energy. However, when the frequency of the electrons’ vibration is not equal to the frequency of the light waves, the light is reflected.
Remember that every surface you see is reflective. Yet, the different light waves responsible for each colour have different frequencies. This means that some light waves might be absorbed by some materials, whilst others are reflected – giving you different colour materials.
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What is Refraction?
Do you remember that one of the things that happens to light as it reaches an interface is known as transmission? This is what happens when the wave just keeps on going.
However, with light, this is only possible with transparent glass – and the clearest of water.
When a medium is not completely transparent – or if the medium is quite large – the transmission can still happen. However, the light wave will slow down and, as a result, it will change direction. This is the process known as refraction.
One of the most common examples of refraction is that of the glass prism. These triangular objects have an effect on light that means that the waves scatter into the colours of the rainbow. This is because of the different wavelengths of the different light waves responsible for each colour. They all slow down at their own rate.