Characteristics of Light
Learning Objectives: After this lesson, you should be able to…
- Define wavelength, frequency, and photon energy, and explore the relationship between wavelength and frequency, between energy and wavelength, and between energy and frequency.
- Recognize the ranges of wavelength of different regions in the electromagnetic spectrum and rank the relative order of wavelength (frequency, photon energy) of different regions in electromagnetic spectrum.
- Rank the relative order of colors and range of wavelength in the visible spectrum.
Light is the final form of energy that we will study. Light behaves like a wave. In fact, light consists of perpendicular electric and magnetic fields that propagate (travel) through space. Since light is a wave, you can think of it as having a wavelength and an amplitude. The wavelength (abbreviated as lower-case Greek letter lambda, λ) of a wave describes the distance between two peaks in the wave. The amplitude of a wave is related to its intensity (the height of a peak). We will work with wavelength throughout the course, but won’t deal with amplitude again.
Waves can also be described in terms of frequency. The frequency of a wave is a measure of the number of peaks that pass by a particular point in space per unit time. The units for measuring frequency are inverse seconds (s), which equals Hertz (Hz). Frequency is abbreviated using the lower-case Greek letter nu (ν).
All light waves move at the same speed in a vacuum (i.e., empty space). This speed is a fundamental constant which is abbreviated with a lower-case c. The speed of light is equal to about 3.0 x 10 m/s. The speed of light is equal to the product of a light wave’s wavelength (λ) in meters and frequency (ν) in s; therefore c=λν (in m/s). An important consequence of this relationship is that the wavelength of a light wave is inversely proportional to its frequency. In other words, as wavelength increases, frequency decreases (and vice versa).
Shown below are the different regions of the electromagnetic spectrum. Where a particular type of light falls on the spectrum depends on its wavelength and frequency. The boundary between each region of the spectrum can be a bit fuzzy, and these regions are characterized mostly by how light within that region interacts with matter (Table 1). The study of light and its interaction with matter is a field known as spectroscopy.
Figure 1: Electromagnetic spectrum
Table 1: How light interacts with matter
| Type of radiation | unit | Interaction with matter |
|---|---|---|
| radio | kilometer (km), meter (m) | Flip nuclear spin |
| microwave | centimeter (cm), millimeter (mm) | Causes molecular rotations |
| infrared (IR) | micrometer (m) | Causes molecular vibrations |
| visible | nanometer (nm) | electronic transitions |
| ultraviolet | nanometer (nm) | electronic transitions, can break certain chemical bonds |
| X-rays | Angstrom (Å) | ionizing radiation (ejection of electrons) |
| gamma rays | Angstrom (Å) | ionizing radiation (ejection of electrons) |
(Note: You don’t need to memorize these details, but you should be familiar with them.)
You’ll need to know the different regions of the spectrum as well as their relative order: radio, microwaves, infrared, visible, ultraviolet, X-ray, and gamma rays (in order of decreasing wavelength and increasing frequency).
You’ll also need to know the relative order of the regions of the visible spectrum. A convenient acronym is ROYGBIV (red, orange, yellow, green, blue, indigo, violet), which lists the regions of the visible spectrum in order of decreasing wavelength and increasing frequency.
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