# PHY 113 Lecture Notes - Lecture 19: Transverse Wave, Standing Wave, Record Producer

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11 Jun 2018
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Nodes and Nodal Lines
A node is a point of minimum amplitude in a standing wave. For example, a guitar string has
two nodes; the points at which the string is anchored to the guitar frame. A guitarist creates
new nodes by pressing down on the string at various points, which changes the frequency, or
pitch, of the sound produced by the string.
A nodal line is a line of minimum amplitude in a moving wave. When two waves of similar
wavelengths meet, and they are not in phase, destructive interference will occur in a line. The
amplitude along these lines is zero.
Anti-nodes and anti-nodal lines exist as well; they are points and lines where amplitude is at a
maximum due to constructive interference.
Interference and Light
In 1801, Thomas Young discovered that when monochromatic light passed through two
narrow slits an interference pattern of bright and dark fringes was produced on the screen.
The bright fringe was the end of the anti-nodal line and the dark fringe was the end of the
nodal line.
This experiment done by Young demonstrated that light diffracts and interferes. As only
waves undergo diffraction and interference, Young’s experiment strongly suggested that light
has a wave nature. Young’s experiment can also be used to measure the wavelength of visible
light.
Doppler Shift
If there is relative motion between the source of a wave and the receiver of the wave, the
frequency received will be different from the frequency of the source. If the source is moving
away from the receiver, the frequency received will be less than the frequency of the source.
Polarization
Polarization is when waves are restricted in their direction of vibration. Only transverse
waves can be polarized. Light can be polarized, which is evidence that light is a transverse
wave.
When light waves vibrate in all modes, it is non-polarized light. When non-polarized light
shines on a polarizing filter, the light that passes through the filter is polarized. The filter has
a polarizing axis called the transmission axis, and will only allow light to pass through that is
vibrating in the same direction as this axis-all other vibrations are absorbed by the filter. If
two such fillers are used, the light passing through can be made dark and bright by rotating
the filters.
Much of the glare reflected from horizontal surfaces is polarized horizontally. This is why
Polaroid sunglasses are used to reduce glare. The polarization axis is vertical greatly reducing
glare. Three-dimensional movies also can use polarization to give the appearance of depth.
Two pictures are taken from slightly different angles. These pictures are projected by using
two projectors. One of these projectors is fitted with a vertical polarizer and the other with a
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