Making Photons

You may have heard of the photoelectric effect, discovered in the late 1800's and later explained by Einstein (for which he won the Nobel).  To sum it up, the energy of a photon is defined by its wavelength.

The shorter the wavelength, the higher the energy.

So how do you make a photon?  Why, you accelerate a charge!  This acceleration (I have a post on inertia in the making that will help illuminate what it really means to accelerate something) comes at a cost of momentum, and this cost is taken out in the form of a photon.  This means that the energy (wavelength) of a photon is dependent on the strength of the acceleration.  What do I mean?  Well lets take a cue from that image up top and start on radio waves.

Radio waves are the longest wavelength, least energetic photons we can make, ranging from 1mm to 100km (note; this does not mean the photon is that length, merely the wavelength).  As you might expect, the acceleration to create these is relatively gentle.  One way is to generate a current through a wire.  Starting this current accelerates the electrons in the wire, emitting a wave, and then stopping the current accelerates them in the other direction (deceleration is not a thing), emitting another wave.  The energy of the waves are dictated by the strength of the current.

Microwaves!  We know them, love them, and still can't figure out why parts of our burritos are cold (another post, perhaps).  They actually fall under the category of radio waves, so they are made in a similar way.  Your microwave in particular actually uses a magnetron (not a decepticon) to rapidly change the magnetic fields of streams of electrons (another future post, on the relationship between electricity and magnetism).  In fact, it does so at such a rate that the microwaves emitted have a wavelength of 122mm, which makes them perfect for spinning the water molecules in your food.  This spinning bumps them against other molecules, transferring temperature (kinetic energy), and heating your food.

Infrared waves are created a little differently.  More energetic than radio waves at ~1-300um (millionths of a meter), these are created from vibrations of molecules.  For this reason (temperature is the kinetic energy of molecules), it is often considered heat radiation.  It's what you see through infrared cameras, and it is often used in astronomy as the infrared light travels through interstellar dust easier than visible light.

Now onto the tiny band we're all so delightfully familiar with.  Every lovely color you've ever seen fits in a tiny little spectrum between 400-700nm (billionths of a meter), with blue/violet at the lower wavelength and red at the higher(hence 'ultraviolet' and 'infrared').

There are a few ways to make visible light.  One way is similar to infrared light, when something is so very hot, the wavelengths begin to creep in the red visible region (like a stove burner coil).  A much more common way is to excite an atom's electrons, putting them in a higher, more energetic orbit.  When this electron falls back into its ground state, it emits a photon.  Fortunately for us, the energy difference between many excitation states falls generally within the energy of visible light.

You may notice that the energy needed to create these is getting larger.  The 'size' of the vibration or acceleration is getting smaller (first on the order of antennae, then on the order of molecules, then on the order of electron orbits).  You may even notice that this means there's a relationship between the wavelength of the light and the 'wavelength' of the vibration.

Good!  Contemplate that for a while, as we head into the higher wavelengths in tomorrow's post.