Properties of Light
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Printed: December 19, 2014
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C HAPTER
Chapter 1. Properties of Light
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Properties of Light
Lesson Objectives
• Describe the mathematical relationship between the speed, wavelength, and frequency of electromagnetic
radiation.
• Describe the experiments that led to the discovery of the photoelectric effect and how the results were used to
further inform our understanding of electrons and light.
Lesson Vocabulary
• light: A form of energy that behaves both as a particle and a wave.
• frequency: Inversely proportional to wavelength. Photons with high frequency light have more energy than
photons with low frequency light (Zukav 1979).
• wavelength: The distance between two crests of a wave of light. The color of light is related to its wavelength.
This is inversely proportional to frequency.
• photoelectric effect: Occurs when same types of electromagnetic radiation are shined on certain kinds of
matter.
• photon: A description of light as particles.
Check Your Understanding
• What are the general properties of light?
• Are there substances whose color varies with changes in the environment or natural surroundings?
The Nature of Electromagnetic Radiation
Many kinds of waves exist, such as sound waves and water waves. Visible light is also a wave. It is a specific
type of a more general phenomenon called an electromagnetic wave. All waves can be described in terms of the
basic physical properties frequency and wavelength. These two properties are related to the speed of a wave by the
following equation:
speed = λν
where λ is the wavelength (usually expressed in meters) and ν is the frequency (expressed in Hertz, where 1 Hz =
1 s−1 ). All electromagnetic waves travel at a speed of 2.998 × 108 meters/second (about 186,000 miles per hour),
which is known as the speed of light. We commonly abbreviate the speed of light as c when used in equations. In
the case of electromagnetic radiation, this equation becomes the following:
c = λν
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If c is expressed in meters per second, the wavelength must be expressed in meters.
Example 5.1
The brilliant red colors seen in fireworks are due to the emission of light from strontium salts such as Sr(NO3 )2 and
SrCO3 . Calculate the frequency in Hz of red light with a wavelength of 6.50 × 102 nm.
Answer:
λν = c
ν = c/λ = (2.998 × 108 m/sec)/(6.5 × 10−7 m) = 4.61 × 1014 sec−1 = 4.61 × 1014 Hz
Electromagnetism
Much of our understanding of the light and the way it behaves is based on the work of Michael Faraday, James
Maxwell, and Heinrich Hertz. In 1845, the English chemist and physicist Michael Faraday (1791–1867) discovered
that light exhibited magnetic properties. His early experiments measured what happened to light when passed
through magnetic fields. Following Faraday’s work, the Scottish physicist and mathematician James Maxwell
(1831–1879) studied electromagnetic radiation and light. Maxwell calculated the speed of light, which was later
confirmed by other scientists to be the very value Maxwell proposed. From his work, Maxwell inferred that light
was probably a transverse electromagnetic wave ( Figure 1.1). He published this conclusion in 1873.
FIGURE 1.1
The image shows light as a transverse
wave. It consists of oscillating magnetic
and electric fields that are perpendicular
to each other and to the direction in which
the light is traveling.
The Electromagnetic Spectrum
In 1888, shortly after Maxwell published his findings, German physicist Heinrich Hertz (1857–1894) confirmed
Maxwell’s inference, showing that light was indeed an electromagnetic wave. Hertz extended Maxwell’s work and
produced electromagnetic radiation with wavelengths that were not in the visible part of the spectrum. In fact, visible
light makes up only a very small part of the entire electromagnetic spectrum ( Figure 1.2).
Example 5.2
Which type of light has a longer wavelength: red or blue?
Answer:
As is shown in Figure 1.2, red light has longer wavelength than blue light. It is to the left of blue light in the diagram.
Example 5.3
Based on what is displayed in the Figure 1.2, what is the relationship between wavelength and frequency?
Answer:
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Chapter 1. Properties of Light
FIGURE 1.2
In this figure we see the electromagnetic
spectrum. Each form represented on the
spectrum has a unique range of wavelengths and frequencies. For example, all
visible light has wavelengths that range
between ~400 nm to 700 nm.
Wavelength is inversely proportional to frequency. As wavelength increases, frequency decreases. As wavelength
decreases, frequency increases.
Photoelectric Effect
Under the right conditions, light can be used to eject electrons from a solid material. This phenomenon, known as
the photoelectric effect, occurs when some types of electromagnetic radiation are shined on certain kinds of matter.
Figure 1.3 shows light rays of a specific wavelength striking a metal object and causing electrons, or photoelectrons,
to be ejected from the surface.
The photoelectric effect was explored by many scientists in the 1800s. It involves the same fundamental principle by
which modern-day solar cells operate. Whether electrons will be released depends on two factors: the wavelength
of the light source and the material onto which the light is being shined. Here is a video of a simple photoelectric
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FIGURE 1.3
Photoelectric effect.
experiment: http://www.youtube.com/watch?v=WO38qVDGgqw (0:26).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/65299
Light Exhibits Particle Behavior
Further study of the photoelectric effect revealed some perplexing behavior that could not be explained by the
classical view of light as just a wave phenomenon. For example, the intensity of the light did not affect the amount
of energy possessed by the ejected photoelectrons. Photoelectrons emitted with the use of a very bright light had
the same energy as those emitted with the use of a dim light of the same frequency. However, a relationship was
observed between the number of photoelectrons ejected and the intensity of the light source. It was found that the
brighter the light sources, the more photoelectrons were ejected. Another puzzling aspect of the photoelectric effect
was that a minimum frequency of light was required in order to eject any electrons at all, regardless of how intense
the light source was.
Albert Einstein (1879–1955) studied this effect further, and in 1905, he postulated that light can also be thought of
in terms of particles, now called photons. Photons of high frequency light have more energy than the photons of low
frequency light (Zukav 1979), which explained why a minimum frequency was required for electrons to be ejected
by a given light source. Figure 1.4 illustrates this effect.
Materials that eject electrons when illuminated with light, such as potassium, are called photoemissive. Not all
materials are photoemissive, nor are all light sources capable of initiating electron emission from a given substance.
For example, in Figure 1.4, we see that 700 nm light will not initiate electron ejection, while 550 nm light will.
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Chapter 1. Properties of Light
FIGURE 1.4
Wavelength of Light and Photoelectric Effect
Lesson Summary
•
•
•
•
Light behaves both as a particle and as a wave.
Michael Faraday discovered that light exhibited magnetic properties.
James Maxwell demonstrated that the speed of light is constant and that light exists as a transverse wave.
Hertz showed that light was an electromagnetic wave and only one type of electromagnetic radiation in a much
larger electromagnetic spectrum.
• The color of light is related to its wavelength, which is the distance between two crests of a wave of light.
• The photoelectric effect occurs when sufficiently energetic electromagnetic radiation is shined on certain kinds
of matter, causing electrons to be ejected.
• The photoelectric effect provides an example of light acting as a particle instead of a wave.
Lesson Review Questions
FIGURE 1.5
Left: Radio antenna. Right: Long wave
(CB radio) antenna
1. What type of electromagnetic radiation (what wavelength) do you suppose the antenna on each of these
vehicles shown in the Figure 1.5 is designed to receive?
2. Black lights are used for a variety of applications, including sterilization of materials. Why do you suppose
the light is called “black light”? Are there other forms of black light?
3. The laser in an audio compact disc player uses light with a frequency of 3.844 × 1014 Hz. Calculate the
wavelength of this light in nm.
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4. An FM radio station broadcasts at 99.5 MHz. Calculate the wavelength in meters of the corresponding radio
waves.
5. Microwave radiation has a wavelength on the order of 1.0 cm. Calculate the frequency in s−1 of a single
photon of this radiation.
6. As the frequency of electromagnetic radiation doubles, the wavelength ___?
7. As the wavelength of electromagnetic radiation is quadrupled, the frequency ___?
8. The yellow light given off by a sodium vapor lamp has a wavelength of 589.0 nm. What is the frequency of
this radiation in Hz?
Further Reading / Supplemental Links
• Young’s Double Slit Experiment: http://www.studyphysics.ca/newnotes/20/unit04_light/chp1719_light/lesson
58.htm
• National Geographic’s Patterns in Nature: http://photography.nationalgeographic.com/photography/patterns-i
n-nature/
• Following the Path of Discovery: http://www.juliantrubin.com/bigten/hertzexperiment.html
• International Lighting Vocabulary: http://www.cie.co.at/publ/abst/17-4-89.html
References
1. Courtesy of NOAA. http://www.srh.noaa.gov/jetstream/remote/remote_intro.htm . Public Domain
2. Zachary Wilson. CK-12 Foundation . CC BY-NC 3.0
3. User:Afrank99/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Fotoelektrischer_Effekt.svg
. Public Domain
4. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.0
5. Radio antenna: Image copyright Baloncici, 2014; Longwave antenna: User:Junglecat/Wikipedia. Radio an
tenna: http://www.shutterstock.com; Longwave antenna: http://commons.wikimedia.org/wiki/File:CB_ante
nna.jpg . Radio antenna: Used under license from Shutterstock.com; Longwave antenna: Public Domain
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