PHYSICS 124 EXPERIMENT NO. 7
INTERFERENCE AND DIFFRACTION
Introduction
In this laboratory you will study the phenomena of diffraction and
interference. In part I, you will observe the diffraction of light by a human hair,
and in Part II, both interference and diffraction of light by various types of slit
arrangements.
Part I -Laser-Light Diffraction
Shine the He-Ne laser beam onto a sing1e human hair. This laser light is diffracted by the
hair giving rise to a let of maxima and minima that can be observe on the wan adjacent to
the laboratory bench. The condition for a diffraction minimum is given be:
where n is an integer, is the wavelength of the laser light, and a is the diameter of the
hair.The angle is shown in the diagram below:
Procedure
CAUTION! Never stare directly into the laser beam or its reflection from a polished
surface! Remove a hair from your scalp and mount the hair on a slide holder. Arrange the
laser light so that it strikes the hair squarely. Observe the diffraction pattern on a nearby
wall. Use a small piece of felt to observe the bright central spot. Use a strip of paper to
record the position of the diffraction minima on each side of the central maximum.
Measure the distance between the hair and the wall.
Questions:
1. Use the equation to determine the diameter of your hair. Be sure to quote error
estimates on your result.
2. Can you observe any ellipticity in your hair? How would you determine this?
3. How precisely do you think you could measure the diameter of your hair with a
conventional micrometer? How does this compare with your diffraction measurement?
Part II ---Light Waves
The glass slide bordered by green tape contains an array of various types of slit
arrangements. Hold the slide with the long demension vertical, and the manufacturer's
trademark (CAL) in the upper right hard corner.
A. On the left hand side of the slide there is a row of single slits of different widths. Look
at the light from a line filament light source ( stand a few meters away ) through each of
the slits. Comment (and sketch) on what you see.
B. The upper right hand corner contains a single slit of width w. Just below it are two
slits, each of width w, separated by a distance also equal to w. Below that the distance
between the slits is increased. Again, look at the line filament light source and comment
(and sketch) on the patterns observed in the each case.
C. The middle row of slits has five different multi-slit arrays. Which of these has the most
slits? How do you know? Notice the way the colors (in the while light) are starting to
separate through this array, which consists of 80 slits.