3.2 The Electromagnetic Spectrum

Learning Objectives

  • Describe the categories of electromagnetic energy, and be able to place these categories relative to each other in wavelength and energy.
  • Describe the wavelengths of visible light and name a color for a given wavelength range.
  • Convert wavelengths of electromagnetic energy between commonly used units (meters, micrometers, nanometers, Angstroms).

Prior Knowledge and Skills

  • None

Key Terms

  • Wavelength
  • Frequency
  • Visible, Infrared, Ultraviolet light

Guided Inquiry

Electromagnetic Spectrum

Watch:

A tour of the Electromagnetic spectrum:

And a crash course on light (relevant portion is 0-4 minutes):

After reviewing the videos, look closely at the following diagrams of the electromagnetic spectrum:

Figure 3.2.1 The Electromagnetic Spectrum. National Aeronautics and Space Administration, Science Mission Directorate. (2010). Introduction to the Electromagnetic Spectrum. Retrieved March 9, 2019, from NASA Science website: http://science.nasa.gov/ems/01_intro and https://smd-prod.s3.amazonaws.com/science-pink/s3fs-public/thumbnails/image/EMS-Introduction_0.jpeg

Figure 3.2.2. The Electromagnetic Spectrum. OpenStax, College Physics. OpenStax CNX. Mar 4, 2019 http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@14.48  Figure 24.9.  Also at: https://courses.lumenlearning.com/physics/chapter/24-3-the-electromagnetic-spectrum/

 

3.2.1. Does ultraviolet light have longer or shorter wavelengths than infrared light?

3.2.2. Do X-rays have larger or smaller energy than visible light?

3.3.3. A Raman spectrum includes a peak at 10 micrometers wavelength.  In which region is this peak located?

 

Figure 3.2.3. The visible light spectrum, showing wavelength ranges for colors in nanometers.  From: OpenStax, College Physics. OpenStax CNX. Mar 4, 2019 http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@14.48, Figure 24.16.

Rendered Spectrum.png
By SpiggetOwn work, CC BY-SA 3.0, Link Figure 3.2.4 The visible light spectrum.

3.2.4. What is the range of wavelengths (in nanometers) for the color green, according to Figure 3.2.3?

3.2.5. What color is a typical laser pointer with a wavelength of 650 nm? Hint: Figure 3.2.4 might be easier to use for this question.

Frequency and Wavelength

The number of crests that pass a given point within one second is described as the frequency of the wave. One wave—or cycle—per second is called a Hertz (Hz), after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second (as shown for the top wave in Figure 3.2.5) has a frequency of 2 Hz.

Electromagnetic waves have crests and troughs similar to (but not identical in behavior to) those of ocean waves. The distance between crests is the wavelength.

Figure 3.2.5. Frequency and wavelength of a wave. From National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved March 9, 2019, from NASA Science website: http://science.nasa.gov/ems/02_anatomy

Energy

An electromagnetic wave can also be described in terms of its energy—in units of measure called electron volts (eV). An electron volt is the amount of kinetic energy needed to move an electron through one volt potential. Moving along the spectrum from long to short wavelengths, energy increases as the wavelength shortens. Consider a jump rope with its ends being pulled up and down. More energy is needed to make the rope have more waves.

Figure 3.2.6. Higher and lower energy waves. From National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved March 9, 2019, from NASA Science website: http://science.nasa.gov/ems/02_anatomy

3.2.6. Does a wave with a high frequency have a high energy?  Explain your answer.

Following from the answer above, we can write an equation to describe the relationship between wavelength and frequency as:

Equation 3.2.1              c = νλ

The speed of light (electromagnetic energy) is a constant c = 3.00 × 108 m/s.  Wavelength is represented by lambda (λ), and frequency is represented by the Greek letter “nu” or ν.  Sometimes “f” is used to replace nu.

3.2.7. Calculate the wavelength of a 1530-kHz AM radio signal.

Solution
Rearranging gives:


For the ν = 1530 kHz AM radio signal:
λ=3.00×108 m/s / 1530×103 cycles/s =196 m

3.2.8. Does your answer match the range of AM radio wavelengths?

Summary

The electromagnetic spectrum is divided into different categories of light energy: gamma rays, X-rays, ultraviolet, visible, and infrared light, microwaves, and radio waves.  The energy and wavelength of electromagnetic energy changes across the spectrum, with gamma rays having short wavelength and highest energies, and radio waves having longest wavelengths and lowest energies.  Humans can detect only a narrow range of electromagnetic energy with our eyes, called visible light.  Energy and frequency of light are positively correlated with each other, but are negatively correlated with wavelength.

References

Lumen Learning Physics Textbook: https://courses.lumenlearning.com/physics/

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Introduction to the Electromagnetic Spectrum. Retrieved March 9, 2019, from NASA Science website: http://science.nasa.gov/ems/01_intro

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved March 9, 2019, from NASA Science website: http://science.nasa.gov/ems/02_anatomy

OpenStax, College Physics. OpenStax CNX. Mar 4, 2019 http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@14.48