Why is transparent… transparent?

 Anupam Dahal

Delhi Public School, Dharan

The objects which allow light to pass through them are called transparent objects. We can see through them as though there is no barrier present at all. But how does that work? What makes these objects different from the rest? Why is transparent… transparent?

We know that matter around us is made up of atoms (and molecules) which, indeed, are made up of nucleus (protons and neutrons) which is the center structure, and electrons, which revolve around them. But surprisingly, an atom is mostly empty space, in fact, 99.99% of it. To put it in perspective, if the nucleus were the width of the human hair, an atom would be the size of a kind size bed. This means that there is enough room for light to pass through object unhindered. Now, the real question remains, why all the objects around us aren’t transparent?

Let’s get back to the subatomic level and focus on the different energy levels and orbits of an electron. Each electron has a certain amount of energy associated with it which determines the energy level it can be in. The energy level can be thought as the concentric rings which we see in a typical atomic diagram (Rutherford’s model of an atom). The farther it is from the nucleus, higher its energy level. Now, electrons can jump from one orbital to one of higher energy level if just the right amount of energy is provided to compensate for energy difference. The light we get from the sun or any other artificial source happens to be an excellent agent that can compensate for the difference. So when the light tries to pass through the empty space, it gets observed by the electron. Now, the electrons only observe the light (or the energy) if it can actually ‘jump’ from one orbital to another. If there isn’t enough energy to compensate for the energy difference, it allows the light to pass through it without any hindrance. And it just so happens that the orbitals or the concentric circles of glasses or air are far apart from each other that visible light cannot compensate for the difference. Therefore, they let light to pass through them unhindered or unabsorbed.

This also explains what gives things colors that they have. When the electron makes the jump, they don’t permanently stay there. In fact, after a fraction of a second it reaches to the orbital of higher energy level, it gets back it the orbital it used to be in, called the ground state (for that electron). This also happens with an energy transaction which is in the form of light. It is exactly this light which we preserve as the color of the object. So the color of any object is due to this light which electrons eject when they return back to their ground state.


The Structure of an Atom (Chapter 2), NCERT Chemistry XI Part I

The Structure of an Atom (Chapter 2), Pradeep’s Chemistry XI Part I

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