reflects off pits and land. The intensity is measured and interpreted as a series of ones
and zeros (digital information) by photodetectors. This information is then relayed to
other systems that interpret it.
How does interference factor in? The laser beam reflects off of the CD. If all of the
beam hits a land or a pit, then the path length difference back to the photodetector is
essentially the same, and the result is constructive interference: bright light. You see
this case in Concept 1.
On the other hand, when the disc moves and laser light is half on a pit, and half on the
land, the path length difference is significant. The two parts of the laser beam have a
total path length difference of one-half a wavelength, and the result is destructive
interference: darkness.
You see this in Concept 2, where we emphasize “sides” of the same laser beam, and
how one side reflects off of a pit and the other off a land.
DVDs contain more data than CDs and employ a variety of strategies to do so. For
instance, DVD drives use lasers with shorter wavelengths. A shorter wavelength means
smaller pits are possible, and these smaller pits can be placed more closely together,
allowing more data to be stored.
The signal
Interference pattern depends on where
light strikes
Sensor receives brighter or dimmer light34.11 - Gotchas
In a string, complete destructive interference occurs when the peaks of one wave meet the troughs of another wave with the same amplitude.
The same is true with light. This is correct. With a string, the result is zero displacement. With light, the result is darkness.
A double-slit interference pattern is caused by differences in the path lengths traveled by the light emanating from each slit. This is true.
If the path length difference in a double-slit interference pattern equals one wavelength, the result is complete destructive interference. No, the
result is complete constructive interference. When the path difference is one wavelength, the waves are in phase: Peak meets peak, and
trough meets trough. A half wavelength path difference will cause destructive interference.
34.12 - Summary
Light exhibits the properties of both a particle and a wave. The interference patterns created by light can be explained by treating it as a wave.
Interference patterns consist of alternating bright and dark bands called fringes. To create an interference pattern, you need at least two
sources providing light that is both monochromatic (having only one wavelength) and coherent (light from the different sources has a phase
relationship that does not change over time). You also need a screen on which to view the pattern.
The bright fringes in a two-slit interference pattern are the result of completely constructive interference of the light from the two sources, while
the dark fringes are created by completely destructive interference.
Whether a given point on a viewing screen will be a point of constructive, destructive, or intermediate interference depends on the difference in
the path lengths from each of two slits to that point.
An interferometer is an instrument that takes advantage of interference to make precise measurements of length. It relies on the fact that a
specific difference in the path lengths of two monochromatic coherent beams of light causes a specific interference pattern at a point where the
beams meet, and a microscopic change in the path difference causes an easily visible change in the interference pattern.
When a light wave reflects from a material with a higher index of refraction than the one in which it is traveling, it experiences a 180° phase
change. When a light wave reflects from a material with a lower index of refraction, there is no phase change. This affects the nature of thin-
film interference, where waves that reflect from the front surface of a thin film interfere with other waves that refract through the front surface
and then reflect from the back surface.
Diffraction is the expansion or spreading of a wave front as it passes through an opening or past a sharp edge.
Huygens’ principle is a model to explain diffraction. It says that a wave front is made up of a series of spherical wavelets.
Diffraction is most often discussed in terms of light waves. When light passes through a slit in a mask, it diffracts. We can model the light
passing through the slit as coming from a series of point sources which each emit a spherical wave front. The light from each point source
interferes with light from the others. This creates an interference pattern of dark and light fringes.
Diffraction limits the ability of optical systems to distinguish between two objects, to resolve them.