Further Exploration of Light Patterns

Goal

We will continue to investigate the properties of LEDs and the incandes­cent lamp by observing and exploring the light spectra emitted by some devices.

Prerequisites

Conservation of Energy

Observation of spectra of some light sources

Introduction

In the previous explorations we saw that incandescent lamps, and LEDs have quite differ­ent properties when the energy supplied to them is varied. Further, LEDs that look alike on the outside can emit different colors of light, even though they are not painted any color. Then when we look at compact fluorescent lamps we see even a different pattern in the spectrum.  The LED, a rather recent invention, acts quite differently from ordinary lamps when we vary the energy applied to them.  teh compact fluorescent, another relatively recent invention, has a spectrum with features that are different from either the LEDs or the incandescent lamp. Our goal for this series of activities is to understand how these devices work in terms of the energies of the atoms in each device. To accom­plish this understanding we need to learn about the emission of light by atoms. Because we cannot see atoms as they emit light, we will need to build a conceptual model of what is happening at the atomic level and use this model to understand the light sources.

The LED is made up of a very small solid consisting of a large number of atoms which are closely packed together and interact with one another in a complex manner. When energy is supplied to the LED, these complex interactions result in the light emitting properties that you have seen. In these solids each atom is very close to its neighbors. Yhe compact fluorescent and incandescent lamps also have solids involved in the emission of light. Just as with closely spaced people the nature of the interactions can be difficult to under­stand at first.Thus, we will begin with atoms that are far away from each other; study how they emit light and then work back to a situation where atoms are close together.

Atoms are relatively far apart in a gas. In fact, one of the defining properties of a gas is that the atoms or molecules have only a few interactions with each other. So, we will supply electrical energy to gases confined in a tube. These gas lamps, which are some­what similar to fluorescent tubes, will emit light. By investigating this light we will be able to build a conceptual model of how gas atoms emit light. We will then extend this model to the closely spaced atoms in a solid and, thus, to other light sources.

In our investigations we will be particularly interested in the energy of the light emitted by the gas.  Two factors — brightness and color — contribute in very differ­ent ways to the energy of a light.  When we think about the definition of energy, the brightness makes sense. A bright light has more energy in it than a dim light. This conclusion matches the observation from the first activity — as we increased the electrical energy supplied to the lamps, they became brighter.

The color connection is not quite so obvious, but we learned about this factor in the Photoelectric Activity.. Atoms emit light in small packets of energy . These packets are called photons.  Each individual photon contains an amount of energy that is related to its frequency.  In turn frequency of visible light is related to its color.  So, if we wish to discuss the energy of one of these photons, we need to know its color. 

For visible light the energy ranges from red at the low energy to violet at the high-energy end. Not visible but still a form of light are infrared photons with an energy lower than red and ultraviolet photons which have energies higher than violet. The order of energies for the various colors of photons is shown below.

Infrared: Low energy photons of light          Red          Orange                  Yellow              Green                     Blue                  Violet    Ultraviolet: High energy photons of light

1  2  3  4  5 6 | Next page