TEACHERS GUIDE: DIFFRACTION OF LASER LIGHT

INTRODUCTION

The use of lasers in the high school classroom is rather common place today. Ray optics demonstrations are easily shown using a ray box or the laser. It is very possible that you already use some of the ideas in this packet to teach diffraction. However, some of these lessons should be new and should allow you to deepen the understanding of diffraction among your students. Particle size determination using laser diffraction is a fairly recent development and widely used in industry. The Internet has a wealth of information in this area for the curious mind.

The review section of the lessons is usually covered in the textbook. It is suggested that the teacher cover the equation derivations. They are added here (in the activities) for handy student reference.

The construction, using a compass, of the initial activities should be completed with as much precision as possible. The fact of careful construction and measurement is a daily occurrence in physics research and should be encouraged continuously by the teacher. The initial activities gradually lead the student into thinking particles. The teacher may point out the fact that for a spherical or a nearly spherical particle, Equation 4 rather than Equation 3 would be applicable. Diffraction is a common experience that can be related to the student’s environment; use of the laser to identify small particulate matter is the research we want to introduce to the students.

ACTIVITY ONE

Part I.

In this activity, students will use "given" data to complete calculations on hair width. This lesson could be used immediately after discussion on single slit diffraction or it could be used to introduce single slit diffraction. Another option is to use these activities to teach the unit on diffraction.

It is always interesting to observe students using physics concepts (diffraction) to find something tangible such as the width of a person’s hair. Discussion could always be initiated before this lesson on "How do companies that make hair care products identify types of healthy hair?" or "What conditions are needed for healthy hair?" When physics is related to common occurrences in the student’s life it becomes real to them. The results of this activity are fairly straightforward and parallel many activities and demonstrations used in the current curriculum.

Some students may ask, "How do we know that light is diffracting and not just going straight through the holes?" The answer is that if this were true, the pattern on the screen would get smaller as the edges became closer together. In fact, the reverse is true. As the edges get smaller and smaller, the pattern on the screen becomes farther apart. A piece of silkscreen is included in your packet and can be used to demonstrate this fact. Sometimes thin cotton cloth will also work.

Part II.

Many classroom teachers may think that they do not have equipment that actually allows them to carry out this experiment. We have found that the laser pointers used by professionals during presentations are sufficiently powerful to do this simple diffraction. Again, students should be allowed to construct their own experiment using whatever laboratory reporting techniques that you employ in your classroom. This synthesis and application process is important in developing the skills students need. An argument can also be made that this is how real research is accomplished.

The photographs used in the activity above were taken with a standard 35-mm camera in Dr. Islam’s lab. You may want to try and have students take pictures of what they produce with diffraction around their hair. These techniques often enhance student interest and awareness of physics. Another option would be to allow students to photograph daily occurrences of diffraction around them and then report to the class. The activities are actually boundless with a little thought and imagination.

ACTIVITY TWO

The next logical step in using diffraction is to find the size of small objects or apertures. The diagram that is given was created in Dr Islam’s lab. A small rectangular aperture was used to produce the pattern. The analysis uses equations that may not be familiar to students. If questions arise as to their source (since no derivations are given), students should accept the fact that the equations can be derived with a little effort-using Figure 2.

???????Equation 1

Where:

q = the angle of the dark fringes as measured from the central maxima.

m = the order number.

l = the wavelength of the incident laser light.

a = the width of the obstruction or particle.

Since we are using the first order dark fringes or bands, m = 1.

The scale factor might become a problem. It should be noted that the actual pictures have been reduced in size to fit the page. A sample calculation follows:

2x₁ = 7.5 cm = .075 m \ x₁ = .075m/2 = .0375m and .0375m x 7(scale factor) = .2625m

L = 23’ 6" = 8.08 m

Since sin q = x/L = 0.2625 m/8.08 m = 0.0324

From Equation 1, a = ml / sinq ₁ = 6.33 EE –07 m / 0.0324 = 0.000019484 m = 19.5 m m

Similar calculations can be made for x₂ , x₃ etc. Each time the measured distance must be divided by two to find the value of x since the distance was measured between two minimums on either side of the central bright maxima. If the pattern is uniform, the values of x should increase proportionally and the determined aperture size should remain constant. An average value should be calculated before the students answer the question about the size and shape of the object.

If possible, encourage the students to do additional experimentation of their own on the size of small particles or apertures. It will only improve on their comprehension of how diffraction is used in research.

Questions? Comments??

James Gormley || Dr. Didar Islam