Physics 20: Intro to Light
Teacher Notes
What is Light?
There is an age-old debate among scientists: “Is light a wave or is it a stream of
particles?”
Isaac Newton believed light consisted of rapidly moving particles. Christian
Huygens demonstrated that light travelled with wave-like properties such as
constructive and destructive interference. Max Plank, Albert Einstein, and others
demonstrated that when light interacts with matter, it behaves like particles or
packages of energy now called photons.
Today, scientists accept the wave-particle duality of light: light travels like a wave
and interacts with matter like a particle!
How do we know light is a wave?
Because it acts like one!
Light reflects in the same manner that any wave would reflect. Light refracts in
the same manner that any wave would refract. Light diffracts in the same manner
that any wave would diffract. Light undergoes interference in the same manner
that any wave would interfere.
a) Reflection of Light Waves
All waves reflect, or bounce off of an obstacle. Light waves
also reflect.
The reflection of light waves off of a mirrored surface
results in the formation of an image. One characteristic of
wave reflection is that the angle at which the wave
approaches a flat reflecting surface is equal to the angle at which the wave leaves
the surface. This characteristic is observed for water waves and sound waves. It is
also observed for light waves.
1
Physics 20: Intro to Light
Teacher Notes
b) Refraction of Light Waves
All waves undergo refraction when they pass from one medium
to another medium. That is, when a wavefront crosses the
boundary between two media, the direction that the wavefront
is moving undergoes a sudden change; the path is "bent."
Light also refracts as it passes from one medium into another
medium. Its refractive behavior follows the same conceptual
and mathematical rules that govern other waves such as water
waves and sound waves.
c) Diffraction of Light Waves
Diffraction involves a change in direction of waves as they pass through an opening
or around an obstacle in their path. Water waves and sound waves have the ability
to travel around corners, around obstacles and through openings.
When light encounters an obstacle in its path, the obstacle blocks the light and
tends to cause the formation of a shadow behind the
obstacle. Light does not seem to bend around the obstacle
and fill in the region behind it with light. But light does
diffract around obstacles.
In fact, if you observe a shadow carefully, you will notice
that its edges are extremely fuzzy. Interference effects
(pattern of alternating rings of light and darkness) occur
due to the diffraction of light around different sides of
the object, causing the shadow of the object to be fuzzy.
2
Physics 20: Intro to Light
Teacher Notes
d) Interference of Light Waves
Wave interference occurs when two waves meet while traveling along the same
medium. Constructive interference occurs when the wave displacement occurs in
the same direction while destructive interference occurs when displacement
occurs in the opposite direction. Both water waves and sound waves show
interference patterns.
In 1801, Thomas Young showed that light also produces an interference pattern.
Young used a single light source and projected the light onto two pinholes. The
light from the source then diffracted through the pinholes and the pattern was
projected onto a screen. Since there is only one source of light, the set of two
waves that come from the pinholes were in phase with each other. The result was a
pattern of alternating bright and dark bands on the screen.
3
Physics 20: Intro to Light
Teacher Notes
The bright spots were constructive interference (a crest meeting a crest or a
trough meeting a trough) producing a “super light wave”. On the other hand, the
dark spots were destructive interference (such as when a crest meets a trough),
canceling out the light waves. Thomas Young's findings provided even more
evidence for scientists that light behaves as a wave.
So what kind of wave is light?
Light is considered an electromagnetic wave.
Electromagnetic waves are produced by a vibrating electric charge, meaning they
consist of both an electric and a magnetic component. The electric and magnetic
fields vibrate at right angles to the direction light is traveling and also at right
angles to each other.
4
Physics 20: Intro to Light
Teacher Notes
Light waves are different from mechanical waves – they do not need a medium to
travel through. They can travel through a vacuum – hence why we can see things in
space!
The Electromagnetic and Visible Spectra
Electromagnetic waves exist with an enormous range of frequencies, known as the
electromagnetic spectrum. The longer wavelength, lower frequency regions are
located on the far left of the spectrum and the shorter wavelength, higher
frequency regions are on the far right.
Electromagnetic Spectrum
Visible Light Spectrum
Though electromagnetic waves exist in a range of wavelengths, our eyes are
sensitive to only a very narrow band, which we call visible light. Visible light
consists of wavelengths that range from approximately 700 nm to approximately
400 nm.
This narrow band of visible light is
affectionately known as ROYGBIV because
each individual wavelength within the
spectrum is representative of a particular
color. That is, when light of that particular
wavelength strikes the
retina of our eye, we
perceive that specific color
sensation.
5
Physics 20: Intro to Light
Teacher Notes
Isaac Newton showed that light shining through a prism will be separated into its
different wavelengths and will thus show the various colors that visible light is
comprised of.
The separation of visible light into its different colors is known as dispersion.
Each color is characteristic of a distinct wavelength; and different wavelengths of
light waves will bend varying amounts upon passage through a prism.
For these reasons, visible light is dispersed upon passage through a prism to
produce the colors: red (R), orange (O), yellow (Y), green (G), blue (B), and violet
(V). Hence ROY G. BIV! (Indigo is not actually observed in the spectrum but is
traditionally added to the list so that there is a vowel in Roy's last name.)
The red wavelengths are the longer wavelengths and the violet wavelengths are the
shorter wavelengths.
When all the wavelengths of the visible light spectrum strike your eye at the same
time, white is perceived. White is not an actual colour - it is the result of a
mixture of two or more colors of light. Thus, visible light - the mix of ROYGBIV is sometimes referred to as white light.
If all the wavelengths of the visible light spectrum give the appearance of white,
then none of the wavelengths would lead to the appearance of black. Once more,
black is not actually a color. Technically speaking, black is merely the absence of
the wavelengths of the visible light spectrum.
6
Physics 20: Intro to Light
Teacher Notes
7