The Nature of Laser Light: Comparisons with a Regular Household Light Bulb

So you’ve heard about the infra-red pump diode and the crystals in a laser but that still didn’t really tell you much about what laser light actually is? In this article we hope to help you out. Without going into how a laser is constructed, we’ll try and explain how it works!

In a laser a source of energy is used to cause a material to produce light. Just like matter is made up of atoms, light is made up of photons. So, in other words, the source of energy is causing the material to produce photons. How does this happen? In very simple terms, the atoms in the material get “excited” by the energy into a higher atomic state. Think of yourself carrying a box up the stairs. Using your energy, you have carried the box against gravity up the stairs. Without energy the atoms cannot remain in the higher atomic state and so they “drop-down” to their former lower atomic state (called a ground state). Similarly if you hold the box over the staircase and let go, without the energy to keep the box up there, it will drop back down toward the ground. Now, the box dropping down releases the energy into mechanical energy ie. it impacts the floor with energy! When atoms drop down to lower atomic states they release the energy by the production of photons!

Picture: photons being produced from the lasing material

The special thing about a laser is the use of mirrors (which come in different sizes and types) which direct the photons that are produced right back into the material, causing even more photons to be produced! Instead of the energy causing the material to produce photons, it’s the photons themselves that cause the material to produce more photons. This is called stimulated emission.

Picture: mirrors (optical resonator) directing photons back into the lasing material

In a regular household light bulb electricity is the energy that passes through the material (metal filament) of the light bulb causing the atoms in the material to jump to higher atomic states and to release photons (light) upon dropping down.. There are no mirrors in a regular light bulb however to direct the photons back into the material to make the material produce even more photons.

Picture: regular household light bulb

Another major difference between laser light and light from a regular household light bulb is that a light bulb produces light consisting of many different kinds of wavelengths. Wavelength corresponds to the color of the light. A light bulb produces what appears to the human eye as white light, which in actuality is many different colors of light joined together. This can be seen by shining white light through a prism such as the image below:

Picture: white light through a prism

Laser light is different than the white light produced by a regular household light bulb in that the laser light is not composed of many different wavelengths. Laser light has a much smaller range of wavelengths and appears to the human eye as monochromatic (one color). Why is this so? The answer lies in the nature of the material that is producing the photons of light. The material used to produce laser light emits photons in a much smaller range of wavelengths than does the material used in a regular household light bulb.

And now for the big question . . . so, why can’t you use your regular household light bulb to do all the cool things that you can do with your laser? The answer lies in the source of the photons and the ability of those photons to be collimated (focused) into a smaller area. In a regular household light bulb the photons are emitted over a relatively large surface area and there is nothing in their path to stop them from going every which way. This can be observed by the general white glow coming off of a light bulb. This is quite useful when trying to illuminate a room but won’t get you very far when trying to do the experiments and tasks required of a laser. In a laser the photons are emitted over a much smaller surface area, called a point source. In addition, optics “capture” the photons, causing the photon beam to be collimated. In other words, the optics direct the photons into a column, otherwise known as a beam. This concentrates the light in a smaller area and is the key to the power of a laser. Think of a mischievous child using sunlight and a magnifying glass to incinerate ants. This is the same principle. A magnifying glass “captures” sunlight shining over the entire circular area of the glass and focuses it down into a smaller area, concentrating all of the power of the sunlight into a beam coming out the other side of the magnifying glass. In a laser the same thing is happening. Laser light is emitted over a small area to begin with and then focused into a column using optics.

Another key to the power of a laser as compared to a regular household light bulb is that laser light is more coherent than light from a regular household light bulb. Photons travel in waves. You can visualize light (photon) waves as waves in the ocean. Ocean waves have peaks and valleys. Light waves also have peaks and valleys. When a peak and a valley from a wave interact with one another in a point in space they cancel each other out! But if a peak and a peak interact the peaks add to one another. Light from a light bulb is traveling every which way, the peaks of the waves sometimes canceling with valleys and sometimes adding to other peaks. Laser light, on the other hand, travels in the same direction, the peaks of the waves adding together. Laser light is not completely coherent, but it is much more coherent than a regular household light bulb. What makes laser light coherent, you ask? Well, that has to do with the photons being re-directed by the mirrors back into the material. When the photons get redirected back in, they cause more photons to be emitted from the material. The photons that get redirected back into the material have a certain wavelength and direction. When these photons cause other photons to be emitted those photons have matching wavelength and direction (are coherent).    

The following is a picture summary of the concepts discussed in this article:

In a laser a source of energy causes a material to emit photons. Those photons are then redirected back into the material causing the material to emit even more photons of the same wavelength and direction (coherent) as the incoming photons. Those photons are then collimated (focused) by optics into a beam. This is why a laser is so much more fun than a regular household light bulb!fdfdsafdsafdsafds

Click here for our more detailed explanation of how a laser is constructed, including what components go into making a laser and how they work.

NOVAlasers wishes you fun exploring!

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