Lasers

Lina is playing with her cats using a laser pointer. When she turns it on, a small red dot appears on the wall, and the cats try to catch it. Oh no! It stopped working! Should I change the lightbulb like in a torch?

Actually, a laser doesn’t have a lightbulb and a laser is very different from a torch... When you turn on a torch, the light spreads out in the shape of a cone and creates a big circle on the wall. The further away you get from the torch, the wider and weaker the light becomes. If you place a prism between the torch and the wall, the light will split into different colours, like a rainbow. This is because light is made of electromagnetic waves, which have different wavelengths, and can travel in different directions.

In the light from a torch, these waves are random and mixed. Lasers are completely different! Take Lina's laser pointer, for example. Unlike other sources of light, the light from a laser doesn’t spread or get weaker with distance. Instead, laser light is a tight, narrow beam that can travel over a long distance in a straight line.

The laser rays are parallel to each other — we say that the laser beam is collimated. The light coming out of a laser is one single colour — Lina’s laser beam is red. A prism doesn’t split laser light, because all the light waves in a laser light are of the same wavelength. Laser light is monochromatic. The electromagnetic waves are also perfectly synched!

The peaks of every wave are aligned with the peaks of every other wave. The laser light is coherent. How do lasers produce such light? A laser is usually made up of a tube, filled with a material, such as crystal, liquid, or gas. When you add enough electricity or another form of energy to this material, it affects the atoms the material is made of.

Stimulated by the added energy, electrons inside the atoms switch between their regular lower-energy ground state and a higher-energy excited state. This causes the material to produce light. This light is of one colour, but it’s not quite a laser beam yet. The waves need to be organised first! For this purpose, there are mirrors at the two ends of the tube that the laser is made of.

The mirrors reflect the light, making it bounce back and forth. This helps line up the light waves. Light is a form of energy, so as it travels back and forth through the tube, it stimulates more atoms, so they produce even more light. The light becomes stronger — amplified, and the waves synchronise. One of the mirrors doesn’t reflect all the light but lets some of it through.

The light that comes out through that partly-transparent mirror is: monochromatic — of one single colour, collimated — straight and narrow, and coherent — with perfectly aligned waves. It’s a laser beam! The beam concentrates a lot of energy in a very small point on the surface it falls onto, making lasers very powerful and precise. This makes lasers useful for many things. Lasers are used in medicine: for surgeries, or to seal cuts without stitches.

In manufacturing, lasers make it possible to cut complex shapes out of wood or metal. Thanks to lasers, we can measure distances with great accuracy, even from objects that are far away — like the Moon! Lasers have many applications in everyday life too —they are used to read DVDs in DVD players, or to scan barcodes at the checkout in a supermarket. And they are a great way to entertain Lina’s cats! I guess I will have to get a new one...