Activity 3

Light and waves

Goal

We will look at a property of light and learn how scientists conclude that light behaves as a wave.

Prerequisites

A route to help us understand why each element emits a certain energy of light.

Introduction

The light from atoms indicates that only certain energy transitions occur in each atom. We were able to explain these results using energy diagrams. However, we have not yet explained why only certain energies occur in each atom or why these energies are different for different elements. To get to the reasons will take us a little time. We begin with a short diversion about the nature of light.

Interference of Light: Young’s Experiment

Approximately 200 years ago, scientists (then called “natural philosophers”) argued about the fundamental nature of light. In his book Optiks, Isaac Newton assumed that light consisted of a collection of minute particles. With this model, he explained most of the known optical experiments. Others thought that light must take the form of a wave. In 1801, Thomas Young completed an experiment that seemed to end this controversy. In this activity, you will begin with an experiment very similar to the one done by Young. Place a laser about two (2) meters from a sheet of white paper taped to a wall. With the room as dark as possible, aim the laser so that its beam is perpendicular to the paper. Remember: Never look directly into the beam coming from the laser. Then place a slide with two very small, closely spaced slits in the path of the laser beam, also perpendicular to the beam as shown in Figure 3-1.
  
 
 Figure 3-1: Diagram for Young’s double-slit experiment. 

You may have to adjust the position and orientation of the slits so that the laser light passes through both slits and forms a clear pattern on the wall.
The double slit can be difficult to construct at home.  The video below shows how to make one with a wire and pieces of electrical tape.



When you are satisfied with the pattern take a photo and save the results for future references.  We call such a pattern an interference pattern.

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