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Lasers

by Ron Kurtus (revised 15 February 2016)

A laser is a light source that creates coherent electromagnetic radiation.

One of the most interesting and clever optical inventions is the laser. Its use has opened the doors to such devices as your CD player and automatic checkout scanners in stores.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



How does a laser work?

The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. That expression means that the light is formed by stimulating a material's electrons to give out the laser light or radiation.

The way a laser creates its radiation is by using a rod of made of a certain material such as ruby that has its surfaces partially mirrored. The rod is bombarded with light or sometimes electrons that cause atoms in the rod to emit light of a specific color. The mirrors cause the light to be reflected back and forth. Each time the light passes through the material, it stimulates the atoms' electrons to give off more light. Once the amplitude is great enough, an intense beam of light energy is emitted through one of the partial mirrors.

The most common laser color seen is red, which comes from ruby lasers and some diode lasers. There are some lasers that emit in infrared. Getting a laser to emit blue light has been very difficult to do.

What is special about laser light?

If you have ever shined a flashlight at night, you can see that its beam spreads out, thus limiting its effectiveness. Although the reflector around the light bulb sends the light in a parallel beam, the wave nature of light causes it to spread out.

Laser has coherent light

In a light bulb filament, light is sent from its various parts in short bursts of energy. These packets of waves randomly come off the filament, such that the light beam is an incoherent mixture of all these bursts of energy.

On the other hand, every time a wave of light bounces back and forth inside the laser, it stimulates electrons rotating around the atoms to give off photons or light waves all at the same time. Thus, instead of being a random collection of bursts of light energy, laser light consists of waves all being emitted and amplified at the same time. This is called coherent light.

Beam does not spread

One amazing characteristic of laser light is that it does not spread out. This is because it is coherent. While a powerful flashlight or a spotlight may shine effectively several hundred yards or meters, laser beams have been bounced off the surface of the Moon and detected back on Earth.

Beam can be focused

Another result of being coherent and of traveling in a narrow parallel beam is that laser light can be focused to a very small point. If you have ever used a magnifying glass to focus sunlight to a point, you can see that you are only able to get a point of light that is 1/8 inch or 0.5 cm in size. A laser beam can be focused to 0.01 cm or smaller.

Where are lasers used?

There are many interesting uses for lasers, depending on the special characteristic being applied.

Focusing to a point

Since laser light can be focused to a very small point of light, lasers are used in CD players, in surgery and in welding.

An audio CD or a CD-ROM has information coded in tiny pits on its surface. A laser light is focused to get information on the sequence of those pits. Since regular light cannot be focused to a small enough point, it is not effective for reading CD information, while laser light is.

When light is focused, the point of focus can become hot enough to burn the skin or human tissue, if the intensity of the light is great enough. This characteristic can be used to perform surgery that results in less damage than it a knife or scalpel was used.

Focusing a very high-energy laser beam on a piece of metal can actually melt the metal. Equipment employing high-energy lasers is used in industry to accurately cut and weld sheets of metal.

Going in a parallel line

Everyone has seen laser pointers that will shine a narrow beam across the room. Other devices that use this characteristic are laser range-finders, price scanners, surveying equipment and laser weapons.

A straight beam from a laser can be bounced off items as far as the Moon to measure their distance. They are also used in surveying equipment to establish a straight line.

A parallel beam of red laser light is used in the scanners seen in most stores. The beam is scanned across a bar code and reflected to a detector.

Very high-energy beams of laser light have been used in space to try to destroy enemy missiles. The fact that the beam does not spread and lose its energy is important in being able to damage a target.

Coherence

The fact that laser light is so coherent allows holograms to be made and viewed. We have all seen the artificial holograms that show somewhat 3-dimensional figures from normal light reflected off a shiny, etched surface. A true hologram is a film material that is created using laser light. It usually also requires laser light to create a realistic 3-dimensional figure in space.

Warning of danger

You should be careful when dealing with lasers, because they can injure your eyes. Since a laser beam focuses to a point that can get very hot, a beam shined in your eyes may damage your retina.

Even though many lasers are supposed to be safe, do not let someone aim a laser pointer at your eyes. There have been cases of people becoming partially blind from laser pointers.

I even avoid staring at the light from the scanners used in stores, just to be on the safe side.

Summary

A laser outputs light that is coherent and in a highly parallel beam. Laser light can travel great distances without the beam spreading and can be focused to a very small point. This type of light has many modern applications. Also, you should use caution when looking at laser beams.


Be focused


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