Some things about Light . . .

Light is one of the most familiar things we encounter in our lives , yet one of the most mysterious things around us.Β  What do we know about light? Well, white light is composed of seven primary colors, VIBGYOR. But what are colours? They represent various wavelengths of light. Different wavelengths also means different frequencies! Note that here, we are attributing wave parameters to light. So light is a wave. But what is fluctuating in this wave ? The answer is electric and magnetic fields. They oscillate in two perpendicular planes which are in turn perpendicular to the direction of propagation( see figure 1).

There is also one peculiar thing about the speed of propagation of light. The speed of light in vacuum , a well -known number (3 x 108Β Β  m/s)Β  is the fastest possible speed attainable. Not only that, imagine you are in a car moving at 30 kmph along the east . To you, a bike moving at 60kmph along the east,with respect to an observer at rest outside the car, would appear to move at 60-30 = 30 kmph. However, this is not true for light. The speed of light of light would be the same for all observers. This fact was the founding principle for Einstein’s theory of relativity!

Also, we all have heard of Snell’s law or have atleast seen it in action. Light bends when it enters a different medium(figure 2). The equation describes this is not just applicable to light, but to many other situations!Imagine you are a life guard on a beach and there’s a person drowning at some distance in the ocean. The speeds with which you swim in the water and the walk on the land are known .What will the path will you take so that you can reach the drowning person in the shortest time possible?The answer is simply given by snell’s law(I’ll leave it to the reader to work it out! )

We all have also seen some videos/lectures on whether light is a wave or particle. There are experiments like the photoelectric effect which indicate that a beam of light is composed of particle and not waves! The particle picture of light, at a very basic level , describesΒ  a light beam as being composed of little packets of energy which are called photons. Photons have zero mass (as opposed to electrons and protons which have very tiny yet non-zero masses). Another interestinf thing about photons is that there can be infinitely many of them in a single point in space. This is not true for electrons, protons etc (there can be only one electron/proton of a kind at a point in space). However, photons still carry momentum. How can a massless thing have momentum? If a massless truck hits you, will you feel any shock? Light definitely does carry momentum as is evident from the pressure of light on a surface.Β  Light from the sun exerts about 10 micro newtons per square meter of area of the earth ( which is a very tiny amount!).

All in all, we can say that light is definitely very interesting, and it takes a long time and some really unintuitive arguments to correctly describe it!

By Kushan

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