physicsa.com
 class code: SPS22 teacher: Mr. Elert classroom: A314 office: A214 test day: Friday phone: (718) 724–8500 ext. 2141 email: elert@midwoodscience.org

## Physics A: Problem Set 16: Diffraction and Interference

 Barron's Let's Review: 12.7 Diffraction and Interference of Light physics.info: Diffraction & Interference (Light) Wikipedia: Diffraction, Interference (wave propagation), Double-slit experiment, Discrete spectrum (physics), Continuous spectrum HyperPhysics: Diffraction of sound Khan Academy: Interference of light waves YouTube: The Brightest Part of a Shadow is in the Middle

### in class

1. Suppose you were the owner of a radio station and that you wanted to extend the range of your broadcast. Would it be wise to erect a second transmitting antenna to broadcast simultaneously with the first? How will a second transmitter affect reception? Explain your reasoning.
The signals from two simultaneously transmitting antennas are going to interfere with one another. The interference pattern they produce will have alternating bands of constructive and destructive interference. In the constructive regions, the signal will be strong. In the destructive regions, the signal will be weak. This is unlikely to be an advantage — unless the constructive regions happened to fall on populated areas and the destructive regions happened to fall on deserted regions.

### at home

1. For what portion of the electromagnetic spectrum could a picket fence be used as a diffraction grating? Explain your reasoning.

The ridges in a typical diffraction grating that students might use are spaced about 10−6 m apart. This is on the same order as the wavelengths of visible light. The pickets in a picket fence are about 10 cm apart (or 10−2 m, if you prefer). According to one trusted reference, this is solidly in the microwave portion of the electromagnetic spectrum.

2. For each of the following examples, state whether the source of light produces a spectrum that is mostly continuous or mostly discrete. Compile your results in a table like the one below.
Identify the spectral type with a check mark (√)
source continuous discrete
blow torch
candle flame
cathode ray tube
emission nebula
firefly
fluorescent tube
glow stick
incandescent bulb
laser
LED
neon sign
plasma display
the sun
red hot metal
Identify the spectral type with a check mark (√)
source continuous discrete
blow torch
candle flame
cathode ray tube
emission nebula
firefly
fluorescent tube
glow stick
incandescent bulb
laser
LED
neon sign
plasma display
the sun
red hot metal