Interference and Diffraction

The lasers you will be working with today are very low power.   However, damage to your eye could occur if you look directly into the beam.   If there are two groups at your table, be sure that you have your lasers point away from one another.  The wavelength of the laser beam is 633 nm.

Set-up your laser on a lab jack at the end of the optical bench.   Use the lens holder to hold your multiple slit slide.  Place the slits on the end of the optical bench near the laser.  The exact distance between the slits and the laser doesn't matter.  Place your screen 1 meter from the slits.  Adjust the laser beam so that it passes through the slits by raising or lowering the lab jack.

Aside:  For small angles sin(q) ~ tan(q) ~ q expressed in radians. The approximation is good to 3SF for angles less than 6^o.  You are setting the adjacent side to be 1.00 m, and you will be measuring the opposite side of the right triangle formed by the light going to each bright spot.  For angles here will be too large for the small angle approximation to be valid, you must do more work.  Use your measured values to find the tangent of the angle that the light going to the bright spot makes.  From that you can find the angle and its sine using your calculator.

Part I:  Two slits

• Observe and describe in your notebook the interference pattern of a laser beaming passing through two slits.

• Measure the locations of at least 6 interference minimums.   The easiest way to do this is to hold scrap paper against the screen and make a very narrow mark at the center of the pattern and at the location of each minimum.  Then move the paper to the tabletop and use your ruler to measure the distance of each mark from the center.  (Estimate to the nearest 0.5mm)

• Make a quantitative sketch of intensity.  Use graph paper.   The positions of the minimums should be accurate, but the intensity scale is only relative.

• Calculate the distance between the two slits using all of your locations - i.e. calculate a value of d for each.  The data is more compactly shown in a table with one sample calculation.  Find the most probable slit spacing and the percent error. 

Part II:  Single Slit

• Observe and describe in your notebook the pattern of a laser beam passing through a single slit.

• Measure the locations of at least 6 minimums as you did in Part I. 

• Make a quantitative sketch of intensity. 

• Calculate the width of the slit using each of your locations.  Find the most probable slit width and its percent error.

Part III:  More than 2 slits

• Observe the pattern for two slits again, then observe the patterns for 3, 4, 5, and 6 slits. 

• How are the multi-slit patterns similar to the two-slit pattern?  How are they different?

• Replace the slits with a diffraction grating.  Change the slit to screen distance to 10cm.

• Measure the position of several diffraction maximums relative to the center.

• Calculate the slit spacing using each position.   Find the most probable value and percent error for the slit spacing in your grating.

• By assuming all slits are identically spaced by d, determine the number of slits per centimeter in your grating.  

• Compare your calculation to the number supplied by the manufacturer.