How can lasers cook something?

posted: 04/11/12

You're probably familiar with the lasers used in scanners at checkout counters and pointers in classrooms, but how is it that Kari, Grant and Tory were able to actually pop a popcorn kernel with a laser?

When you see a laser, you can immediately tell that it isn't like "normal" light -- the kind that comes from the sun or an incandescent light bulb. Lasers yield long, narrow beams of light that don't diffuse like light. A laser beam can be relatively weak and harmless, like in a laser pointer, or it can focus enough heat to cut through steel. It can also cook food.

But before we get hungry, let's start with the simple science behind laser cooking, starting with the tiniest piece of any element that maintains the chemical properties of that element: the atom.

The Excitement is Building

Atoms, tiny physical particles of all elements, can gain and release energy, which can bump their electrons into different levels or states of excitation. An electron in an excited state (with lots of energy) that falls down to the ground state (the lowest amount of energy) releases that energy in the form of a photon: a particle of light. The process of electrons releasing photons can be disordered and spontaneous, as in a light bulb, or it can be controlled and stimulated, as in a laser.

Let There Be Light

The word "laser" stands for "light amplification by stimulated emission of radiation." In stimulated emission, when a photon encounters an atom in an excited state, sympathetic vibration causes the atom to fall down to the ground state. When this happens, the atom releases a second photon of the same frequency and enters the same electromagnetic wave as the first, making coherent light. By contrast, normal light is incoherent, consisting of photons in multiple wavelengths that follow different paths.

Seeing Double

Mirrors positioned on either side of a laser toss the photons back and forth, encouraging this replication process to repeat over and over again. As each photon joins the wave, it amplifies and reinforces the others. Because one of the laser's mirrors is partially transparent, it allows a fraction of the light hitting it to escape.

What escapes? A concentrated laser beam.

Population Control

In order for this process to work, however, the laser must meet a certain condition -- there must be more atoms in the excited state than in the ground state -- a condition known as population inversion. To attain population inversion, the laser must pump the atoms with energy, through such means as an electric current or an external, intense light source.

Now We're Cooking

Through its ability to focus energy sharply into a coherent beam of light, a laser can come in handy. The laser medium, or the kinds of atoms and molecules inside the laser, can be solid, gas or liquid. The nature of the laser (how much heat or energy it transmits) depends on the laser medium. Helium-neon lasers used to be common in pointers and scanners, but now they're being replaced by semiconductor laser diodes. Other kinds of lasers use such materials as argon, carbon dioxide and liquid with organic dye. Lasers that use a medium like carbon dioxide transmit microwaves and infrared waves and easily become hot enough to cook food.

So... To Laser Cook or Not to Laser Cook?

Although Kari, Grant and Tory were able to successfully pop a popcorn kernel with a laser, that was because they had a very powerful laser and because one kernel is pretty small -- about the size of the narrow laser beam. Because such lasers can be dangerous, and cooking anything bigger than a popcorn kernel with a laser is pretty impractical anyway, it's probably wise not to heed the famous warning: Don't try this at home.

More on