PASCO WA-9899 Ripple Tank System - Manual (Page 19)

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M o d e l N o . W A - 9 8 9 9 E x p e r i m e n t 3 : D i f f r a c t i o n
19
Experiment 3: Diffraction
Purpose
The purpose of this activity is to determine how the diffraction pattern of plane waves
changes as the slit width (gap between barriers) changes and the wavelength changes.
Theory
As a plane wave front passes through a gap or slit in a barrier,
each point on the wave front that moves through the slit gener-
ates a new circular wave front. If the gap or slit is large relative
to the wavelength, the circular wave fronts combine to form a
new plane wave front. If the gap or slit is small relative to the
wavelength, the part of the wave front that moves through the
slit is less like a plane wave and more like a circular wave. The
spreading of a circular wave created as a wave front moves
through a slit as shown in Figure 3.1 is called diffraction.
Setup
1. Mount the light source onto its rod at the back edge of the ripple tank.
2. Pour a small amount of water into the tank and adjust the feet on the legs of the
tank to level the tank.
3. Place the long straight barriers in the middle of the tank and add about 800 mL of
water to the tank (or enough water so that the water level is about halfway up the
long straight barrier.)
4. Use a rod and base to support the ripple generator and position the generator over
the midpoint of one side of the ripple tank. Plug the light source into the ripple
generator and connect the ripple generator to its power adapter.
5. Connect the plane wave dipper to the ripple arms. Adjust the ripple generator
until the bottom of the plane wave dipper is barely in contact with the surface of
the water.
6. Set the light source to ‘STROBE’. Set the ripple generator to 20 Hz and the
amplitude to slightly less than half of maximum.
7. Place a sheet of paper directly under the ripple tank so you will be able to sketch
the images of the waves that are projected onto the sheet by the light source.
Equipment from Ripple Tank System
Ripple Tank Ripple Generator and Light Source
Long Barrier (2) Plane Wave Dipper
Mini Barrier Ruler
Other Equipment and Materials
Large Rod Stand (ME-8735) Paper (about 40 cm by 40 cm)
90-cm Rod (ME-8738)
barrier wave fronts
Figure 3.1: Diffraction
Tip: Make sure that
the plane wave dip-
per is in contact
with the water
evenly over its
length.
Water appears to
curve upward on the
sides of the plane
wave dipper