Download Interference and Diffraction

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INTERFERENCE AND DIFFRACTION
PURPOSE
To investigate the interference and diffraction of light as it goes through one slit and two slits.
THEORY
When light passes through a small slit, different points on the wavefront behave as
sources for secondary waves. These waves will then interfere and produce an interference
pattern called diffraction as shown in Figure 1 below.
If a light wave passes through two slits, it will produce an interference pattern due to the
two waves originating from each slit. Theoretically, each bright fringe should have the same
intensity as shown in Figure 2 below. However, the diffraction effects of each single slit will
affect the intensity as shown in Figure 3 below.
SAFETY
You will be using lasers in the lab. Although these lasers are not particularly dangerous, we
should take a few simple precautions to prevent the unlikely event of eye damage.
1. Never look directly into the laser beam. Laser light has a high intensity and can also be
easily focused. A direct shot of the laser beam on your eye will be focused by your
cornea onto a small spot on your retina and can burn or possibly detach the retina.
2. Never hold a reflecting object by hand in front of the laser beam. This prevents the
possibility of accidentally shining the light into your eyes.
3. Keep your head above the plane of the laser beam.
4. Whenever the light strikes an object, there will be a reflection. At times the reflections
can be almost as strong as the incident beam. Know where the reflections are and
block them if necessary.
5. The laser has a shutter in front of the beam. When not taking data, place the shutter in
front of the laser beam.
SET UP
To begin this lab, set up the optical bench with the laser mounted on one end. Adjust the
optical bench and the laser such that the laser beam points along the axis of the optical bench.
Point the optical bench such that the laser hits the screen. Tape a large piece of paper flat on the
screen centered on the laser beam and mark the point at which the laser hits the paper.
Place the mount holding the single or double slits between the laser and the screen.
Always handle optics only by the edges, as this will prevent them from becoming smudged. This
is particularly important when working with lasers because the smudges can become burned by
the intense laser light that will damage the optics. Carefully position the slits to get a light
pattern similar to those shown in Figures 1 – 3.
PROCEDURE
SINGLE SLIT
1. Mount the single slit in the carrier. What size is the slit aperture (a) (listed on edge of slit
optics)?
2. Turn on the red laser and align the central bright spot on the screen. Observe the diffraction
pattern. Draw a sketch of it below.
3. Are all spots equally bright, or does the brightness vary?
4. Next, choose another single slit but of larger width. What size is the slit aperture (a)?
5. Observe and draw its diffraction pattern.
6. Describe how it is similar to and different from the first pattern.
7. Next, choose another single slit but of smaller width than the previous two. What size is the
slit aperture (a)?
8. Observe and draw its diffraction pattern.
9. Describe how it is similar to and different from the previous two patterns.
DOUBLE SLIT
1. Replace the single slit with a double slit. What sizes are the slit aperture (a) and the slit
spacing (d)?
2. Turn on the laser and align the central bright spot on the screen. Observe the two-slit
interference pattern. Are all spots equally bright, or does the brightness vary?
3. Draw a sketch of what you see. Show the fringes.
4. Choose a double slit with the same aperture (a) but with larger slit spacing (d). What sizes are
the slit aperture (a) and the slit spacing (d)?
5. Observe and draw its two-slit interference pattern.
6. Describe how it is similar to and different from the first pattern.
7. Choose a double slit but with smaller slit spacing than the previous two. What sizes are the slit
aperture (a) and the slit spacing (d)?
8. Observe and draw its two-slit interference pattern.
9. Describe how it is similar to and different from the previous two patterns.
ANOTHER DIFFRACTION PATTERN
1. Choose one other diffraction pattern to view. What is the slit pattern information (size, style)?
2. What pattern results when you use this slit? Draw and describe it.