Distinguishing Between Incoherent and Coherent Light

BACKGROUND:

Lasers excite children as well as adults. Lasers are used in the check out stand at the grocery store, in surgery, and in measurement. However, many people do not know what lasers are and how they operate. In this activity, students are introduced to lasers by reviewing light and going over the properties of light that help lasers work. In lab, students will look at lasers more closely.

Lasers are basically excited light waves. Light is a component of the electromagnetic wave spectrum. The name itself stands for Light Amplification by Stimulated Emission of Radiation. Students must first understand two terms: coherent and incoherent light. Light emitted by normal means such as a flashlight or a bulb, is incoherent or the photons of the many wave frequencies of light are oscillating in different directions. It is not a stream of light. Coherent light is a beam of photons (almost like particles of light waves) that have the same frequency and are all at the same frequency. Only a beam of laser light will not spread and diffuse. In lasers, waves are identical and in phase, which produces a beam of coherent light. There are many types of lasers that use gases such as helium, neon, argon, and carbon dioxide. Lasers also use semiconductors (Galiodium and Arsenic), solid-state material (ruby, glass), and even chemicals (hydrofluoric acid) in their operation.

With the cavity of the laser the beam of light is reflected back and forth along the central tube, until the waves of light become coherent.

Movement of light in a laser.

PROCEDURE:

Draw the following diagrams on the board to illustrate coherent and incoherent light waves. Lasers themselves are not a source of energy, they simply convert the energy of a particular source.

Coherent light wave pattern

Incoherent light wave pattern

Searching the web for information on Lasers can be exhausting if you use the "adult" search engines. The subject matter in the adult search engines will give you career and industry information, which is too difficult for young students. We suggest using a children’s search engine. These type of search engines give students sites that may help them understand what lasers are all about. As the students are filling in the worksheet, have them search the web if they wanted to learn more about lasers.


OBJECTIVES: Exploring how lasers work. Distinguishing between incoherent and coherent light. VOCABULARY: coherent incoherent lasers MATERIALS: worksheet Internet	Students plot data on a graph.