Chapter 5. Basic Properties of Light

5.1 Introduction

The goals of this module: After completing this exercise, you should be able to:

1. Describe the different regions of the electromagnetic spectrum (light).
2. Compare the wavelength, frequency, and energy differences among the regions of the electromagnetic spectrum.

In this module you will explore:

1. The different regions of the electromagnetic spectrum.
2. How light is described using wavelength, frequency, and energy.

Why you are doing it: An understanding of the phenomenon that scientists call electromagnetic radiation (light) is crucial in gaining knowledge about our universe. Astronomers use the complete electromagnetic spectrum to investigate the nature of celestial objects.

Two Views of Saturn

5.2 Background

William Herschel studies a spectrum. Copyright SPL/PHOTO RESEARCHERS, INC.

Early astronomers like Galileo, Newton, and Herschel were curious about light. Around 1670, Isaac Newton passed sunlight through a prism and displayed a rainbow of colors. After William Herschel discovered Uranus in 1781, he added to Newton's experiment and investigated additional properties of light. Look carefully at the image to the right that shows Herschel using a prism to create a spectrum. Can you spot the prism in the small window over Herschel's left shoulder? By directing sunlight through the glass prism, Herschel created a rainbow of colors. The order of the colors is always the same.

Summary

Herschel's original prism and thermometers

On the little platform you will notice three thermometers. In 1800, Herschel was able to show that when a thermometer was placed under each color, the temperature was higher than for a thermometer not in the spectrum at all, which serves as a 'control' in this experiment. The placement of the thermometer in the image just beyond the red color also showed a higher temperature than for the control thermometer. Herschel had discovered a new, invisible form of light, infrared radiation!

5.3 Prism

You may have noticed in your home that as sunlight passes through a glass vase, a rainbow is produced on the other side of the room. A beam of sunlight is called "white light" and contains all of the colors of the rainbow. The glass breaks up the sunlight into its component parts and you see the color bands of the rainbow. This rainbow is called the visible spectrum.

A prism produces a spectrum

Look at the glass prism above. A spectrum is produced as the white light shines through the prism. The scale on the right gives the wavelength of the color displayed. The units are in nanometers. One nanometer is equal to a length of 10^-9 meters, one billionth of a meter.

Summary

Light can be described in terms of wavelength, the distance between two successive wave crests. The Greek letter lambda λ , is used to symbolize wavelength.

An electromagnetic wave

5.4 Wavelength and Frequency

Frequency refers to the number of wave crests that pass a point every second and is inversely related to wavelength by the equation, ν = c/λ , where ν = frequency, λ = wavelength, and c = speed of light. The animation below shows two waves of differing wavelengths. There is a "wave crest counter" toward the right side of the animation that will record the number of wavecrests that pass by.

5.5 Wave Energy

The energy associated with light of a specific wavelength is given by the equation, E = hc/λ, where E = energy, h = Planck's constant (6.625 × 10^-34 Joule-seconds), and λ = wavelength.

Investigate the relationship between frequency, wavelength, and energy by sliding the bar over the colors in figure above.

5.6 Electromagnetic Spectrum

Here is the entire electromagnetic spectrum. All of the named regions are forms of electromagnetic radiation and all forms travel at the speed of light through empty space.

5.7 Quick Check Quiz

Indepth Activity: Basic Properties of Light