Light - Other Applications of Light

Laser light is described as being monochromatic and coherent which means that the crests and the troughs of the individual rays of light are perfectly aligned and so the light beam doesn't spread out much and is very intense. Scientists working for the NASA space agency have been able to measure the distance of the Moon extremely accurately by reflecting laser light off special reflectors left behind after the Apollo Moon landings.

You can probably think of several uses of lasers - laser pointers, light shows and even cat toys; astronomers fire lasers into the sky to create an artificial star that they can use to improve the quality of the images they record. The uses that you are supposed to know about in the senior high school are cutting, cauterizing and burning. Industrial lasers are extremely powerful, so powerful in fact that they can be used to cut through many different materials, including metals. The advantages of laser cutting are that the material is easier to hold in place as there is no cutting edge that is biting into it; the laser beam doesn't wear out; the cut is very smooth; very complex shapes can be cut as the laser is computer controlled and one or two others. The main disadvantage is that they don't have the sort of power to be able to cut through very thick materials like steel plate.

Cauterizing is a medical term that refers to the use of heat to either remove part of the body or to seal part of it in order to reduce bleeding and to avoid further damage. Doctors and surgeons are beginning to use lasers to remove growths known as skin tags in place of the conventional method involving a heated wire. The heat from the laser beam cuts through the skin tag and seals the blood vessels, reducing the bleeding from this minor operation. You are probably more familiar with laser eye surgery which is used to correct a number of different problems with the eye by re-shaping the cornea. In most people, laser eye surgery means that they no longer have to wear contact lenses or glasses although for a few people, it creates more problems that it solves. Shining any laser, however low powered, has dangers, even in the hands of doctors carrying out eye surgery with them.

Visible light can be sent along glass fibers. This is because of total internal reflection (TIR). TIR occurs when light inside an object hits the surface at an angle that is greater than the critical angle. At angles less than this, light only partially reflects and the rest refracts at the surface, leaving the medium. Glass fibers are long and thin so light rays already inside the fiber will always hit the outside surfaces at angles greater than the critical angle and will remain inside the fiber. It is only at the end of the fibers is it possible for light to leave.

The critical angle depends on the refractive index of the transparent material so it will be different for glass and perspex. The refractive index is equal to the reciprocal of the sine of the critical angle.

As the name suggests, total internal reflection means that all the light is reflected inside the glass fiber, so it is a very efficient method of moving light from one place to another. It is used to transfer digital signals in high speed broadband and in endoscopes (devices used to look inside of body cavities).