# Cabrillo College

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```							                                       PLC Activity #8
Diffraction
Due: Thursday, April 21 by 4:00 pm

1) Go to Physlet Optics Chapter 38: Diffraction Problem 4 and answer the following:

This animation models light hitting a single slit. You can change the width of the slit and use the
protractor to measure angles (position is given in centimeters and angle is given in degrees).
What is the wavelength of the light?

2) A slit of width a is illuminated with light of wavelength  as in Fig. 36-4 and 36-5 from your
text. However, here we mentally split the slit into three zones of width a/3 as shown in the figure
below. The parallel rays that are shown make an angle  with the horizontal such that the path
length difference between rays r1 and r2 is /2 and that between r2 and r3 is also /2. At that angle
 in the interference pattern, is there a bright fringe, a dark fringe, or something intermediate?

3) Physlet Optics Chapter 38 Problem 5. This animation models a diffraction grating that is a
series of parallel slits in a material. You can change the slit spacing and see the first- and
second-order maxima (position is given in centimeters and angle is given in degrees). What is
the wavelength of the light passing through the diffraction grating?

4) Figure a below shows rays from the top and bottom of a slit in a single-slit diffraction
experiment. The rays, which are at angle  to the horizontal, reach a distant viewing screen at the
first minimum in the diffraction pattern there. (a) What is their path length difference in terms of
the wavelength  of the light? The arrangement in Figure b has the top and bottom rays still at
angle , but now the slit is narrower. (b) Is the path length difference between the top and bottom
rays greater than, less than, or equal to that in Fig. a? (c) What part of the diffraction pattern do
these rays reach on the viewing screen: part of the central maximum, the first minimum, or some

6-1
5) Physlet Optics Chapter 38, Problem 6. This animation models a diffraction grating that is a
series of parallel slits in a material. As you change the wavelength, notice where the bright spots
are. You can move the protractor around to measure angles (position is given in centimeters
and angle is given in degrees). How many slits per millimeter does the diffraction grating
have?

6) For a certain diffraction grating, the ratio /d of wavelength to grating spacing is 1/4.5.
Without the use of a calculator, determine which of the orders beyond the zeroth order appear in

7) The first diffraction minima in a double-slit diffraction pattern happen to coincide with the
fourth interference bright fringes. (a) How many bright fringes are in the central diffraction
envelope? (b) To shift the coincidence to the fifth side bright fringes, should the distance
between the slits be increased or decreased? (c) If that shift is, instead, made by changing the slit
widths, should they be increased or decreased?

6-2

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