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Why do we need lasers? What do we use lasers for? How do you build lasers? Who invented lasers? How big and powerful can lasers be? Why/how do lasers travel over long distances? Are all lasers red? What does the color of the laser mean?Why are lasers dangerous?
Answer 1:

Why do we need lasers? What do we use lasers for?

As you certainly know, a laser is a (usually) very bright and very focused light source. We need and use lasers because there are some tasks which require their brightness and their focused beam. Lasers are brighter and more focused than other light sources. One place where lasers are used is in surgery. The brightness (sometimes called the intensity) of the beam is needed to burn through tissue. The focused beam is also required so that small incisions can be made. Another place where lasers are used are in compact disk (CD) players. The laser is used to carefully read the information on the disk. The tight beam is needed to read the exact spot on the disk that you want to read. Similarly, lasers are used at the supermarket to read prices from bar codes. Lasers are also used in surveying, because they provide straight lines. Lasers are also used in manufacturing for cutting and positioning of equipment.
One thing which I have almost forgotten to mention is that lasers will emit only a single color of light. You may know that white light is made up of light of many different colors: red, orange, yellow, green, blue, violet. A flashlight, for example, gives off many different colors, and your eye sees this as white light. Lasers, as I have mentioned, will only emit light of a single color.
I've simplified things a little bit, and I should point out there are exceptions to some of what I've just said. There are, for example, lasers which do not give off a tightly focused beam.
Who invented lasers?
It turns out that there is something similar to a laser, called a maser that was invented before the laser. A laser provides an intense beam of light, while a maser provides an intense beam of microwaves. When you look at the physics of light and microwaves, it turns out that microwaves are also a form of light. It is just that microwaves are not visible to the human eye.
The maser was invented by Charles Townes and his students at Columbia University in 1954. Since the maser and the laser are basically the same thing, Professor Townes is often thought of as being the "father of the laser."

Townes together with Arthur Schawlow worked out many of the principles needed to make a laser. The first laser was made by a young physicist named Theodore Maiman at Hughes Research Laboratories in Malibu in 1960. Maiman's laser involved the use of a rod of synthetic ruby. The ruby rod was cut so that the laser beam bounced back and forth inside the rod, gaining brightness before emerging from one end.
How do you build lasers? What does the color of the laser mean? Are all lasers red?

There are many ways to make lasers. Some use mixtures of gasses like helium and neon. Others use organic dyes. Still others use semiconductors, somewhat similar to (but different in important ways from) those used in computers.
You may have been told that "laser" is an acronym which stands for Light Amplification by Stimulated Emission of Radiation. The best way for me to explain this is to give an analogy. Think of a two story building. You might be on the ground floor or you might be on the second floor. In all lasers, there is a similar situation where something (you can think of this "something" as being electrons in the laser) has the possibility of being in one of (at least) two positions.
Now, it takes energy for you to climb the stairs to the second floor, so we can say that the second floor is higher in energy than the ground floor. You need some energy to get to the second floor from the ground floor, and this energy comes from the food that you eat. In a laser, we usually put in electricity to move the electrons in the laser to the "second floor." Now, if you run down the stairs from the second to the first floor, you may get warm and start to sweat. The reason for this is that you are using energy to run down the stairs. In a laser, instead of sweating, the electron will release a flash of light as it drops from the second floor to the first.
Now, here's the real trick behind the laser. Imagine that a bunch of your friends were on the second floor with you, and one or two ran down the stairs to the first floor. You would probably follow them. In a laser, a similar thing happens where the flash of light given off by one electron falling down, causes other electrons to fall from the second to the first floor, and in the process they all give off flashes of light. All of these
flashes are pointed in the same direction, and this means that we get a very bright and focused beam. In addition, all of the light flashes are the same color, this is how you get a beam with only one color present.
The color of the laser is directly related to the energy difference between the "first floor" and the "second floor" in the laser. With low energy differences, we get red and infrared (wh

Answer 2:

> Who invented lasers?

Lasers were predated by MASERs (Microwave Amplification by Stimulated
Emission of Radiation), and the first folks to work on lasers used their work. But I'll suppose you can look up who they were in the nearest encyclopedia!

> Why do we need lasers? What do we use lasers for?

Lasers pack a lot of optical power into a very small beam. A laser can be used anytime you want to focus light (or energy) into a small area. You could use this for eye surgery, CD players (the grooves are *really* small), or about 5 other ideas you can come up with. Give it a try.

You can also send messages by turning lasers on and off very quickly (like several billion times per second!)

> How do you build lasers?

A laser needs something to amplify light (like a gas tube, or a ruby crystal) between two mirrors. if the amplifier can make light faster than it can escape (through the mirrors) or be absorbed, you'll have a laser. Crystals are particularly good at amplifying light, and when they break, they can form perfect mirrors at the broken facet. So, the lasers I make are just a (very) fancy crystal that reflects at both sides.

The other thing you need is energy to "pump" the laser. I pump mine with electric current, but light can do it too. Why don't you try to find out how a ruby laser is pumped? (hint: can you get electricity into a ruby? can you get light into a ruby?)

> How big and powerful can lasers be?

Quite big & powerful indeed. By arranging an array of lasers in a grid, I can make it as big and as powerful as I like. But for some purposes(like weapons, or cutting steel, or nuclear fusion), it's more important how intense the beam is (i.e. how much power can you direct at the same spot). To do this, I might put a lens in front of my giant laser grid...as long as I don't melt the lens!

> Why/how do lasers travel over long distances?

Like all light, lasers spread out as they travel. Shine a laser pointer at the far end of the hall and you'll see what I mean. To prevent the light from spreading or being blocked, laser light is usually focused (with a lens) into an optical fiber, and transported within it. What do you suppose fibers are made of?

> Are all lasers red?

I'm making infrared lasers, my friend is making blue lasers, and there's a
green laser in the next room. So I would say no :)

> What does the color of the laser mean?

The color determines the wavelength of the light, with blue having a shorter wavelength than red light. Shorter wavelength light can be focused to a sharper spot, while longer wavelength light is absorbed less by glass optical fibers. Of course, it's also the color our eye sees, and for visual displays (televisions, lasers shows) you want more colors than red!

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