Lasers.

Step 1 of 9

One colour

Light is given out and absorbed in packets called photons. Each photon has an energy that depends only on its frequency:

$E = hf = \frac{hc}{\lambda}$$E =$$hf =$$\frac{hc}{\lambda}$⁠

$h = 6.63 \times 10^{-34}$$h =$$6.63 \times 10^{-34}$⁠ J·s is Planck’s constant. The chromium ions in ruby give out light of one wavelength, so all their photons have the same energy. Such energies are handy to measure in electronvolts: 1 eV = 1.602 × 10⁻¹⁹ J.

Step 2 of 9

The threshold

The lamp excites chromium ions, and they fall back to the ground state giving out red light. The Pump power is now fixed at 1.5 kW, below the threshold. Predict what will happen when it is raised.

Step 3 of 9

Copies of a photon

Show the photons shows the ions as points and the light as wave trains. Compare the light glowing in every direction with the light moving along the axis, between the mirrors.

Step 4 of 9

The output mirror

One mirror reflects nearly all the light. The other, the Output mirror, lets a small share of it out at each bounce: that is the beam. This mirror sets how much light stays inside to amplify itself.

Step 5 of 9

A stream of photons

Power is energy each second. Knowing the energy of one photon, you can count how many photons leave each second.

Step 6 of 9

How fast it swings

Light also has a frequency: how many times a second its electric field turns over. It is tied to the wavelength through the speed of light.

Step 7 of 9

Held at threshold

Under Show the photons the excited ions are marked. Whatever more the pump gives the laser, where does it go?

Step 8 of 9

How much gets out

The flash lamp draws kilowatts, and the beam gives a few watts. Most of the energy goes somewhere else.

Step 9 of 9

On the edge

The mirror that lets the beam out is also a leak. The more it lets out, the harder the laser has to be pumped to come on.