Why can't infrared rays reach the earth? Why can't they pass through the atmosphere?
[Physics] Why can’t infrared rays go through the atmosphere
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A nephew-friendly, physics-based explanation:
Our brains and nerves work based on electrical impulses, which are little bursts of electrical current. Electricity is what happens when you remove the electrons from one atom or molecule and move them to another one nearby. In some materials, like metals or heavily ionized liquids like blood, it's easy to move electrons around and make electrical current flow. In other materials, like plastic or rubber or bone, it's harder to make the electrons move.
It takes energy to make an electron move away from an atom. In conductors, it takes only a little energy; in insulators, it takes a lot of energy. How much energy it takes to liberate an electron is called the "work function" or the "ionization energy," depending on exactly what you're doing, and is measured in volts. (Well, technically it's electron-volts, but that compound word makes people fall instantly asleep.) If you push the same number of electrons --- the same current --- out of a nine-volt battery, you do about six times the amount of work of a 1.5-volt AA battery.
If you hit an atom with some energy but it's not enough to knock the electron completely free, you can sometimes make the electrons around the atom vibrate. But the atom can't vibrate any old way: only certain frequencies are allowed. If you try to give an atom energy in some amount that's not allowed, the atom's electrons just ignore you. It's kind of like finding a vending machine that says "quarters only": if you have a pocketful of dimes and dollar coins, then too bad for you.
We happen to live in a world where ionization energies for stuff are typically three or five or ten volts, and electronic excitation energies are typically one or two or three volts.
Light is the way that electric charges exchange energy with each other. Light comes in lumps, called "photons," which each carry a certain amount of energy. It turns out that the energy in each lump is directly related to its color: violet light has more energy per lump than blue, blue more than green, green more than yellow, yellow more than red, and red more than infrared. When visible light hits the pigment proteins in the retina, it makes the electrons vibrate; that sets in motion the machinery to send an electrical impulse to your brain. When ultraviolet light hits those pigment molecules it ionizes them, which makes the molecules fall apart and sets in motion a different mechanism ("cleanup on aisle four"). And when infrared light hits those pigment molecules, it doesn't have enough energy to make the electronic vibrations go, so you get zero information about the infrared light: you're at the vending machine, but only with dimes. Visible light photons have energies from about 1.8 volts (red) to about 3 volts (violet).
The whole story is more complicated than this because the different ways a molecule can vibrate depend very sensitively on its shape, but that's the basic idea. This is also why ultraviolet light is more dangerous than visible light: in addition to breaking up pigment molecules, ultraviolet photons have enough energy to break up DNA molecules.
Infrared light can make an entire molecule vibrate, which is what we call heat. (It's easier to make a whole molecule vibrate because molecules are big and floppy, while the electrons are held near their atoms on a short, stiff leash.) The pit snakes have a delicate membrane which seems to detect radiant heat by causing warmed air to flow through a pore; you can see right away that this thermo-mechanical sense is completely different from the electro-optical method that we (and the eyed snakes) use to see visible light.
If you have a metal that is thin enough (i.e. thinner than the infrared skin depth), then you can pass infrared light straight through. Keep in mind this would be metals with thicknesses of the order of tens to hundreds of nanometers depending on the metal. One way this could be easily achieved is careful sputtering of a metal film onto glass. Transparent conducting films somewhat fall under this type of category (though they work quite differently).
Alternatively, you can easily pass infrared light through some doped semiconductors (e.g. Silicon). These aren't as good as usual metals at conducting electricity, but can do the job in some situations.
If the idea is to take any random chunk of metal and try to pass infrared light through it, the best you can do is to drill a hole for a line-of-sight application, or fill that hole with an optical fiber when line-of-sight won't work.
Best Answer
Gases in the atomsphere have natural resonant frequencies at which they vibrate if stimulated to do so. To stimulate the vibration, a frequency at, or near, the resonant frequency must be applied. When you push a child on a swing with the same frequency as the swing's natural frequency, the child/swing will "absorb" that energy and swing higher .
Carbon dioxide molecules, for one, have their resonance frequency in the infrared. So when light from the sun strikes the atmosphere, much of the infrared is absorbed into vibrations of these molecules. It is re-emited again but in random directions instead of the path it would have taken straight to the Earth's surface. Some of re-emision reaches Earth but the majority of the absorbed infrared is re-emited back into space.
Turn this around. Much of the shorter wave radiation that easily passes through the atmosphere strikes the earth and is re-emited as infrared. Instead of escaping into space, much of it is also absorbed by molecules such as CO2. A significant percentage of that randomly directed re-emmision heads back toward Earth to melt glaciers.