Seven. Simple. That’s easy. The number of colours in a rainbow? Seven. We all know that! But we couldn’t be more wrong than that. If we do have the patience and stamina and diligence to count the number of colours in a rainbow then we’ll be busy counting for a good number of years until the sun explodes. The seven stripes of colours the rainbow is so well-known for is only those which are most apparent and most recognisable but we have to realise that the rainbow is a continuous sweep of colours and not a discrete juxtaposition of seven bands of colours. This continuous arc of colours is called a spectrum of colours and by this we mean that there is an infinite variation of colours in the arc.
Amazing, isn’t it? Suddenly our palette expands from a mere seven to a panoply millions and billions and trillions of colours! Granted, from the three primary colours one can make all other colours in the universe but still to have all those colours at your disposal is pretty amazing. But that’s not the whole story though. Before we get so excited over so many colours of the rainbow let’s just take a step back and understand what this spectrum of colours actually is.
The rainbow is in fact a small portion of an even larger spectrum of colours! Well, I say colours but this is just a play on word. You see, the rainbow, those infinite colours, is just how light manifests itself, putting on his multicoloured garb of dazzling hues and parading before our eyes as a proud peacock with its tail fanned out in full display of its vanity. Light from the sun is a mixture of all known colours and when you do mix all those colours together you get what we call white light. White light when split into its constituents will display the series of colours we see in a rainbow. How can we decompose white light into the myriads of colours? How can we unfurl this peacock tail of sunlight? We can use a glass prism. As the light passes through the prism it bends and because each component, each colour of white light bends by different amounts, we see blue, green, yellow and all the other colours separating from each other and opening up like this colourful fan we call a rainbow. We can use another glass prism to then refocus all the diverging colours back into a single beam of white light. That’s to show, therefore, that not only can white light be decomposed into its constituents but putting back those constituent colours together do give us back the original white light.
Here’s a question for you: if white light is what the sun emits then why is it that we (almost always) paint the sun as yellow? Ask anyone to paint the sun and they will most likely paint it yellow? Why is that? We’ll come to that later. For now let’s carry on with our talk about light and its nature.
What is light? It is a form of energy called electromagnetic energy. We can think of light as being made up of tiny particles called photons. So tiny in fact that their size is insignificant. Not only that but those particles don’t have any mass either! No mass and no size? How can that be? How can they even exist? We are so used to quantities such as mass and length and volume (especially when we talk about objects) that even though a grain of sand or a speck of dust is almost too small to see or too light to weigh in our hand we can still accept these things as stuff, as things made of matter, as material objects. But when it comes to particles of light, with no mass and no size then how can we even call them particles? Well strictly speaking they are not particles in the sense that they are tiny ping-pong balls moving about frantically but that they only display material characteristics in that they have momentum and that when they ‘collide’ with other particles such as electrons they can impart their momentum to these electrons. On such tiny scale, on the quantum scale, what we call particles are not really particle in the everyday sense of reality. They are not solid spheres moving about in distinct paths. However, these tiny constituent of matter and of energy do have certain physical characteristics such as mass, speed, momentum etc. that we associate with more mundane objects.
Light can also display wave characteristics. Think of a tsunami and how this monstrous aquatic beast emerges from the sea waves and rises way high and how it carries such a tremendous amount of energy with it that it can be a destructive force upon land and its inhabitants. This benign sea wave which has turned into a tsunami is what was carrying this initial amount of energy, of kinetic energy. Likewise, a beam of light is like a wave carrying electromagnetic energy. And, to use our marine analogy even further, just as sea waves can be thought of as a ‘disturbance’ in an otherwise calm and flat lagoon, the light waves can be thought of as a ‘disturbance’ in an electromagnetic field. This electromagnetic field we are talking about fills our universe but in some places it’s more ‘concentrated’ than others depending on how many charged particles or magnetic substances are found in a given region of space. A charged particle such as a proton or electron moving in that field will cause a disturbance in that field and this disturbance will spread like a ripple. The ripple or wave in the electromagnetic field can sometimes be perceived as light. I think I’ve pushed this analogy far enough without causing too much distortion to our notion of electromagnetic field. It is a little bit more complex than the sea but this is a relatively good enough approximation to visualising how light waves spread.
So, light is both a wave and a particle. Honestly, we don’t care whether you want to tag it as a particle or as a wave because what matters is what it can do. Don’t judge it by its creed but by its deed. In some instances light can behave as a wave and in others as a particle; it all depends on how we want to see it. Take a cylinder or tube for example and place it on the table with its circular base resting on the table. So it’s standing up not lying down on the table. Now, look directly from above: you see a circle. Next, look at it sideways, it’s rectangular. So which is it? A circle or a rectangle? It’s neither. It’s a cylinder. And depending on how we look, how we choose to observe this cylinder, it can appear like a circle or like a rectangle. Same goes for light. Depending on how we choose to observe this thing called light, sometimes it can look like a wave sometimes like a particle but it’s neither one nor the other. Perhaps we should call it a wavicle but that’s some other debate. What we should understand is that light, the courier of colours, is this: the carrier of electromagnetic energy.
But light is not alone in this business of spreading electromagnetic energy. There is a whole spectrum of other types of radiation or rays other than light rays that carry electromagnetic energy. And within that spectrum of infinite couriers, we’ve identified the DHLs and FedExs and UPSs of the electromagnetic business. Just like in the rainbow example, where we’ve named the seven most recognisable and relevant colours, we’ve classified the whole spectrum of electromagnetic radiation into seven main groups. By no means are these groups completely distinct from each other. They are all of the same nature as light. That is, they all display wavelike and particle-like characteristics, they all carry electromagnetic energy and they all, without exception, move swiftly at the speed of, well, at the speed of light! The speed of light is the universal speed limit. You don’t have to worry about being fined for exceeding this speed limit because there is absolutely no way any body with mass (including you) can travel at that speed. The condition to be met in order to be allowed to travel at the speed of light is this: no mass. If you are privileged enough to satisfy this condition then you may travel at that speed. Photons, the particles of electromagnetic radiation, have no mass and can therefore travel at that speed. We denote this very special speed with the letter c and it has this value: 299 792 458 m/s. So, the seven groups we’ve identified, all have the same characteristics and we cannot draw a definite line which demarcate one from the other. Remember, it’s a continuous sweep of electromagnetic radiation. We know that red light and green light and blue light are but different facets of the same thing: light. Red, green, blue (and all other colours for that matter) have something that differentiates them from each other but their nature is the same. They are part of the big family of electromagnetic spectrum.
The seven members of this electromagnetic spectrum are as follows: Radio, Microwave, Infrared, Light, Ultraviolet, X-Ray and Gamma. Light itself can be split into these sub-members: Red, Orange, Yellow, Green, Blue, Indigo and Violet. This is just a neat way to classify the different members but it is in no way an exhaustive list of available electromagnetic radiation. Even though the spectrum is a continuous array of radiation, what is it that differentiates one member from the other? Well, let’s get back to our picture of light as a wave.
An electromagnetic wave is a vibration spreading outwards from its source. This vibration has three properties: an amplitude, a wavelength and a frequency. The amplitude has to do with how big or small the vibrations are. So if you imagine the vibration as a point going up and down and up and down and up and down, then the maximum upwards displacement of that point from its original position is called the amplitude. The greater the distance, the greater the amplitude. This vibration is not only in the vertical, so to speak, not only up and down, but also horizontal. That point is also moving further and further away from its source as it keeps going up and down. So this vertical motion turns into this wave as it draws out along the horizontal. And now the distance between each up or each down is what we call a wavelength. It’s literally the length of a wave. Both amplitude and wavelength are in the space domain; that is to say, they have to do with displacement and distance. Finally, we have the frequency of the wave and that has to do with the time domain. The time it takes for the point to move from up to down and back up again is called the period of the wave. That is, it’s the time it took to complete one cycle. The longer it takes to complete that cycle, the smaller the frequency. The frequency is exactly the inverse of the period. A wave with a high frequency means that it takes a very short time to complete one cycle. So, we have those three quantities: amplitude, wavelength and frequency. The wavelength or frequency can be used to differentiate between the members of the electromagnetic spectrum.
Radio waves are those which have the largest wavelength, microwaves have smaller wavelength, infrared even smaller and so on and so forth as we move along the list until we reach the gamma rays which have the smallest wavelengths of them all. Again, to emphasise the point, there is no clear-cut distinction of these members but rather a gradual change of wavelengths as we move from one end of the spectrum to the other. So the radio waves gradually morph into microwaves but we cannot say exactly when that change occurs. Similarly, infrared gradually takes the guise of red light but when does it shed its identity to take on another, one cannot tell. It’s like watching an infant growing up and as it moves through the different stages of life, from baby to toddler to child to adolescence and then to adulthood we can recognise when it becomes a child or an adolescent but we cannot pinpoint the exact moment when this change occurs. And the reason for this is because ageing is a gradual change, a gradual and continuous process. The same applies to the electromagnetic spectrum. We recognise that ultraviolet is different from X-rays but where exactly one changes to the other, we cannot say.
Another interesting aspect of the wave is that its wavelength and frequency are related. The larger the wavelength then the smaller the frequency, the smaller the wavelength, the larger the frequency. So radio waves, with their wavelengths of about a kilometre or so have very small frequencies while gamma rays have wavelengths smaller than the size of an atom but their frequencies are extremely high. Not only that, but if you multiply the wavelength by the frequency, you get another interesting aspect of the wave. You get its speed. So multiplying the wavelength of radio waves by their frequency, you get the speed of light. Neat, isn’t it? And regardless of whether the multiplication applies to microwaves or ultraviolet or any other member of the spectrum, the product of these two quantities, wavelength and frequency, always yields the speed of light.
Radio waves are the ones which carry the sounds and images you hear and see on your radio and TV set. The satellite TV signals, however, are from the very same group of waves which heat up your TV dinner in the microwave oven. Microwaves besides being used to transmit TV signals and mobile phone signals have the convenient property that their frequency match the natural frequency of water molecules causing them to vibrate or resonate which increasing agitation. Everything has a natural frequency with which it can be set to vibrate. And if it does so, the vibrations build up into more energetic ones. The same principle applies to water molecules. And as we’ve described in a previous blog, the faster the water molecules move, the hotter the water gets. Most food substances that you heat or cook in the microwave oven relies on the fact that microwaves can shake up those molecules to increase the temperature of the substances.
Next on the list is infrared radiation. To use the example of the TV set, once again, let’s consider the remote control. That little device emits a small burst of infrared radiation whenever you press a button on it. The receiver on your TV set picks up that signal and, depending on the button you pressed, either the volume goes up or the channel switches to one showing a documentary about William Herschel. Of course, you don’t actually see a beam of infrared radiation emerging from the remote control and hitting the TV set. This is because infrared waves are invisible to us. However, we can still detect those waves. And like the remote control, we also emit infrared radiation. In fact, everything emits infrared radiation but of varying intensity and frequency. The hotter the object then the more energetic the infrared waves it emits and the cooler the object, the less energetic the infrared waves. Infrared cameras show this clearly by artificially colouring the hot parts of a body red and cold parts blue. What the camera does is detect those infrared waves then map them to a set of colours. Also, the more intense the infrared radiation is, the brighter it will appear. The intensity of the wave has to do with its amplitude. Large amplitudes imply high intensity.
As a side note to this, let me just mention that heat can be transmitted in three ways: by conduction, convection and radiation. We’ll look at the three in more detail in a separate blog. But what is of interest here is how heat is transferred by radiation. Infrared radiation is exactly how. The sun not only showers us with daylight but also bathes us in its warmth. The heat from the sun reaches us in the form of infrared radiation, crossing the 150 million kilometres of interplanetary space. Even a candle flame emits infrared radiation but the main difference between the infrared waves emitted by the sun and those by the candle is the intensity, not so much the frequency.
Light is the only part of the electromagnetic spectrum which is visible to the human eye. It’s only a fraction of the whole spectrum and just as light carries information, other parts of the spectrum carries other types of information. Bees, for instance, can interpret the information carried by ultraviolet waves which certain flowers reflect. To us, the flowers may look bright yellow but to the bees, they can see other patterns and ‘colours’ invisible to us. Those patterns guide them to where the nectar is stored. Ultraviolet is not all about honey but can also lead to more dangerous consequences. Prolonged exposure to ultraviolet radiation (or UV for short) can be a cause of skin cancer. This brings us back to the sun. On top of infrared and light, it also emits UV. The UV part of the solar radiation is what gives us a nice tan but also, to the extreme, cause skin cancer.
As we move further down the list, we come to X-ray. These have become synonymous with broken bones and sprains and internal injuries. Indeed, every hospital has an X-ray machine that can peer through one’s skin and capture and image of our skeleton. Prolonged exposure to X-ray can be also be lethal. What we should bear in mind here is that, it is not the radiation itself that is dangerous but how often and how long we are exposed to it and how intense it is. Being exposed to intense microwave radiation for a few minutes is more harmful than having a snapshot of our tibia being taken by an X-ray machine that flashes a beam of x-ray radiation on our shin for a fraction of an instant. So even though X-rays have much high frequency than microwaves, it does not necessarily mean that it is more dangerous. Mobile phones also emit microwaves and the more we use them, the more exposed we are to these waves but, again, the intensity of those waves should also be taken into consideration when debating about whether there is any link between cancer and mobile phone usage.
The last member on the list is the gamma radiation. Now, these we have to be very cautious with. Even a small dose can be bad for us. And being exposed to it can apparently turn you into a green monster but I would not explore that possibility if I were you. This is better left to the realm of science fiction and comic book superheroes.
We have glanced over the different types of radiation which constitute the electromagnetic spectrum. But just like the rainbow, even though we group them into seven categories, it doesn’t actually mean that they are discrete bands of radiation. And even though they may have different properties and use, they are all part of the same type of radiation with the same characteristics. I hope that this explanation on light and its companions has been… enlightening!