When I am already getting late for my morning lecture, here at IIT Kanpur where I teach, that is the exact moment my spectacles choose to play hide-and-seek with me. Not in the kitchen nor near the washing machine; neither on the study table nor in the almirah. I struggled through every item on my bed: the winter blanket, the wet towel, and the wordy non-fiction book that helps me fall asleep last night, but to no avail.
As I debated myself if things really could disappear into thin air and, in frustration, placed my hand on my face, I realised the spectacles had been wearing me the whole time.
My students blamed me for the delay in starting the lecture that day, but the real problem lay with the glass – something that’s everywhere but is barely visible anywhere.
Glass, glass everywhere
We are surrounded by glasses. More than half of us wear one at some point in our lives. You are probably reading this on a device whose screen is made of glass. Mirrors, windows, tables, bulbs, and probably the thing you use to drink water – they are all made of glass.
A transparent, shiny object that breaks easily, yet which all of us seem to be obsessed with. There are many mysteries about glass that physicists still struggle to answer. Glass is one of the more magical materials around us. It gives us the illusion of being in an open space even when we are not.
Imagine you are in a car with no glass windows but ones made of a steel frame. Would you still go for that long drive? Most solid things around us are opaque (including ourselves): light cannot pass through them. But glass is different: it feels like a solid and behaves like a solid – but is at the same time transparent to light.
How is it that we can see through glass?
Invisible and visible light
To make sense of glass’s transparency, we need to think about light.
Light is an electromagnetic wave – a wave of electromagnetic force roaming around in space. You may think of waves like in a puddle: when its surface is disturbed, the water moves up and down, and you see a ripple move.
In electromagnetic waves, the electric and magnetic fields are the ‘waters’ that move up and down. The number of times they move up-and-down in a second is called the frequency of the wave. You may not realise it but you know these frequencies quite well. When you tune an FM radio, you’re changing the frequency the radio is ‘hearing’. The waves that reach the FM radio are oscillating around 100 million times every second.
The electrons in the radio’s metal antenna oscillate at that same frequency, transferring your favourite Arijit Singh or Shreya Ghoshal song from the waves to signals that are routed to the speakers. This is the reason the radio on your phone doesn’t work without earphones attached.
Visible light is exactly the same wave but of a different frequency. Like the radio frequency had eight zeroes after 1, visible-light frequency has 14 zeroes after 1 (i.e. 100,000,000,000,000).
Now you see me…
Our eyes can see these high-frequency waves; if they couldn’t, the world would be dark.
If we had eyes that could ‘see’ radio waves instead, this world would look permanently bright. If our eyes could ‘see’ X-rays, we could look inside our own bodies, at our bones, using an “X-ray torch”. We are always surrounded by waves, and just by looking at a tiny fraction of them, we can tell day from night, bright from dark.
If we flip this around, we start to see why glass seems transparent to us: it depends on who (or what) is looking at it. If a material appears transparent to us, it simply lets those frequencies of light with which we ‘see’ pass through itself.
Most metals, including aluminium, which is found in so many household items, are transparent to ultraviolet rays but opaque to visible light, so we can’t see through them with our eyes – but we could have if we’d have eyes that could see in the ultraviolet. Similarly, our muscles are transparent to X-rays but our bones are not. This is why we can use X-rays to look for broken bones.
So the real question we must ask about glass is: why does it block some frequencies of light but let others pass through?
A ‘light’ snack for electrons
Light as we discussed are electromagnetic waves that carry some energy. As it turns out, this energy is carried in small and fixed packets – like the packets in which you get your evening snacks like peanuts or chickpeas. These packets are what the ‘quantum’ in ‘quantum mechanics’ stands for.
And in quantum mechanics, the amount of energy contained in a packet depends on its frequency. The higher the frequency, the more the energy in the packet (like saying bigger the peanut or chickpea). When light falls on any material, the electrons in that material typically absorb that energy. But if for any reason they can’t, those light waves will simply just pass by.
Think of electrons as hungry (not angry) pigeons that can eat food but only of some particular sizes. If you’re throwing nuts at them and a nut is too large, they won’t eat it. They’ll prefer nuts that are just the right size. In the same way, if you’re ‘throwing’ packets of light energy at the electrons, they will absorb packets that are just the right rise. The packets that don’t agree with them will pass unattended.
The electrons in glass for some reason can’t ‘eat’ some frequencies – which happen to be the same frequencies that our eyes can ‘see’. So these light packets pass through the glass and into our eyes. Our brain reads these light signals and finds that the glass is transparent.
Whenever we’re looking for a material that is solid but also transparent, like wind-shields for your car or spectacles for yourself, the first that comes to mind is glass. Common glass also happens to be made of sand, which is abundant and found almost everywhere on the earth. But for being transparent, glass still opens a useful window into our world and the way it works.
If you’re still wondering why light comes in fixed packets and why glass only picks a few packet sizes to absorb, you will need to learn a bit more of quantum mechanics and condensed matter physics, which a course in physics will teach you.
And the next time you lose your spectacles – ask if your vision is clear. If it is, the spectacles are on your face, allowing you to see through a whole universe of electrons playing with light waves.
The author is an assistant professor at IIT Kanpur.
