We started class by watching this video by Veritasium on Young’s double slit experiment.

Then we replicated the experiment using a HeNe laser instead of sunlight as our light source.  The students put the slide with the 2 slits in front of the laser and then measured the distance between the slits and the screen where the diffraction pattern is displayed and they measured the distance between the fringes.

The dimensions of the slits is known, so we used this experiment to measure the wavelength of the laser light.  We used a laser pointer for one set up and a nice little HeNe laser that was donated to the class for the other set up.  The lab that we followed can be found here.  Both groups got pretty good values for the wavelength of their laser.

The photo below shows the beautiful interference pattern generated by the double slits.  I printed out this metric scale on cardstock to make the measurements a bit easier.

Once we know the wavelength we can do this same experiment to find the distance between grooves in a CD or the width of a hair!   For the CD experiment we put a hole in a piece of paper so that the laser light could pass through, hit the cd and the interference pattern shows up on the paper.  This pattern looks a bit different than the two slit experiment and its much easier to measure the distance between the maxima.  Both groups found the distance between the grooves to be around 1.5 microns.

Lastly the students took a dog hair and taped it over a hole in an index card and then placed it in the laser beam to generate a diffraction pattern. The photo below shows the laser lighting up the hair.  Just as before the students measured the distance from the screen to the obstacle (hair in this case) and the distance between maxima in the interference pattern.  A bit of math (see lab handout) and they determined the width of the dog hair to be about 50 microns

Another fun thing to do with the CD is demonstrated in this video.  All you have to do is sit with your back to a bright light bulb and hold the CD so that the reflection of the bulb is in the center of the CD (so that you can’t see a reflection) and you’l see a very bright circular rainbow on the CD.