PASCO WA-9899 Ripple Tank System - Manual (Page 21)

®
M o d e l N o . W A - 9 8 9 9 E x p e r i m e n t 4 : I n t e r f e r e n c e
21
Experiment 4: Interference
Purpose
The purpose of this activity is to determine how the interference pattern formed by
two slits or two point sources changes as the slit width changes and the wavelength
changes.
Theory
When a wave front passes through two slits, the wave front acts like
two point sources. The circular wave patterns that spread from the
slits interfere constructively and destructively. The positions of
maximum intensity (constructive interference) are given by the fol-
lowing formula:
In the formula, ‘d’ is the slit separation,
θ
is the angle between posi-
tions of maximum intensity (the ‘maxima’),
λ
is the wavelength,
and ‘m’ is the order where ‘m’ = 0, 1, 2, etc. See Figure 4.1.
Setup: Straight Barriers
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. 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
Short Barrier Standard Dipper (2)
Mini Barrier
Other Equipment and Materials
Large Rod Stand (ME-8735) Paper (about 40 cm by 40 cm)
90-cm Rod (ME-8738)
barriers
maximum
maximum
θ
d
Figure 4.1: Diffraction
d
θ
sin m
λ
=
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