What I love about Physics is how much it permeates everything around us: from the very small to the extremely large; from the trivial to the esoteric; from the birth of the universe right through to its demise and beyond. Physics encapsulates absolutely everything which is fundamental. One trivial thing we can consider is the ubiquitous compact disc. I say ubiquitous for the sole reason that the number of CDs ever sold outnumber the total world population by a factor of 30 to 1! The commercially available CD comes from a long lineage of laser discs. These optical storage devices first appeared on the scene in the late 1950s – yes, they are that old. But CDs have been around for some 30 years or so. The principle on which they are based is simple, really: shine a focused beam of light onto a grooved surface and capture the reflected light from that surface. It turns out that the beam of light is not reflected uniformly from the surface of the CD – unlike the light bouncing off a flat mirror. The light reflected off the CD can tell us about the nature of the surface of the CD; more specifically, it can tell us about the grooves on the surface of the CD.
Think of those curved mirrors typically seen in fun fairs. The distorted images they reflect back at us tell us that their surfaces are not flat. We can even guess whether they are concave or convex mirrors or a funny combination of these curvatures. Likewise, the light reflected off the CD can tell us a whole lot about where the grooves are on the surface. And based on where the grooves are, the light thus detected is converted into an electrical signal. That signal is then interpreted as a series of ‘on’ and ‘off’ pulses. Fed into a computer, these ‘on’ and ‘off’ or 1 and 0 are reshaped into the beautiful melodies embedded in the CD. Were it not for that special type of light reflecting off the CD, the beats and tunes would remain mute. That light is an extremely narrow beam of concentrated radiation. It is called a laser, which is an acronym of Light Amplification by Stimulated Emission of Radiation. Optics, the physics of light and lenses, can unfurl its full plumage in the domain of compact disc players. A lens is required, first of all, to focus the laser onto the surface of the CD; then the physics of light reflecting off a surface comes into play. But also, the type of laser to be used is determined by the types of grooves on the CD. You see, there is another phenomenon of how light interacts with matter that comes into play here. It’s called diffraction. This has to do with how light diverges when it goes through a narrow gap. And if a CD’s surface is a terrain of gaps or grooves then you can assume that diffraction has an important role in this whole setup. It is safe to say that we are all familiar with reflection; the vain ones among us can’t get enough of it whenever they walk past a mirror or any reflective surface for that matter. But what about diffraction? Do we have the foggiest idea of what that is?
Well, diffraction is not confined to light only. In fact any type of wave can undergo diffraction. This comes about as a result of how a wave interferes with itself. I shall go over this in more detail in another post but for now it suffices to say that the amount by which a wave would interfere with itself, and thus diffract, is related to the width of the gap through which it flows. A smaller gap produces a more pronounced diffraction or spreading of the waves as they emerge from that gap. In fact, if the width of the gap is comparable to the length of the wave then the wave will undergo a more significant diffraction than if the width was much larger than the length of the wave. The reason we can hear someone talking from a different room is partly because the sound waves emanated from that room diffracts or spreads out as they pass through the door. The width of the door is of comparable dimensions to the wavelength of the sound wave. However, light coming from that same room will hardly, if at all, diffract as it passes through the door. Here the width of the door is several orders of magnitude larger than the wavelength of the light wave. This is why, for instance, you might hear a vehicle from far away before you can catch a glimpse of the light from the headlights as it approaches from round the corner. There are several other examples of diffractions we encounter everyday without necessarily being aware of it.
If you look at the surface of a CD you’ll notice multiple rainbow patterns. This is yet another example of diffraction in action. What is going on here is the spreading of white light (which is a composite of all colours) into its constituent strands of colours as each strand spreads out or unfurls by a different amount. Again,those grooves on the surface are the cause of this diffraction and thus produces a panoply of rainbow patterns. Red light diffracts more than blue light for a given gap. So on one end of the pattern you see red and on the other is blue and the rest of the colours are distributed in between. Another phenomenon which is also responsible for the production of rainbows is called refraction. But, as mentioned earlier, I will address this in another post.
Now think of those gaps on the surface of a CD. What if we made them smaller? That would allow us to pack more gaps on the same surface area. If a CD can typically carry about 80 minutes worth of music then, perhaps, by compacting those gaps we might be able to fit in 160 minutes or even 320 minutes worth of music. Why not? We can even try cramming in 3 200 minutes, can’t we? Well it’s not that straightforward. A CD is read by a red laser. And red light will diffract by a certain amount given the size of the gap. But smaller gaps cause more spreading. So making those gaps smaller will cause the red laser to diffract by more than the desired amount. If the diffraction is too pronounced then the reflected laser will be too spread out to be able to be detected accurately by the CD player. It has to retain a certain degree of focus following its reflection and slight diffraction. Blue light, on the other hand, has a much shorter wavelength than red light. Which means, they will not diffract as much as red light when they interact with those smaller gaps. Aha! Why not use a blue laser then? A blue laser to read off a compacted compact disc. Genius!
As you’ve probably guessed, such a device already exists and goes by the name of Blu-Ray discs. If CDs have been around for at least thirty years then how come Blu-Ray discs are such late comers among the family of laser discs? It wasn’t because it took a while to figure that blue laser diffracts less than red laser but because of the sheer challenge involved in being able to, first, produce such high-resolution discs and, second, find ingenious ways to encode the data on them. A Blu-ray disc can carry about 4 000 times more data than a typical CD – there is a tremendous amount of data compression going on there. So even if in theory the idea of a Blu-Ray disc sounds simple, the actual engineering of this hi-def disc poses a real challenge. Just one thing, though. The laser used in Blu-Ray players is not actually blue but violet in colour. But Violet-Ray doesn’t sound as cool as Blu-Ray…