Diffraction Activity. Laser safety rules

Transcription

1 Diffraction Activity Laser safety rules Note: Failure to follow laser safety guidelines will result in being asked to leave the class for the remainder of the quarter you will fail the course. Lasers are very fragile, always handle with care Do not wear exposed jewelry or other shiny things while working with the laser Block all stray laser light before performing an experiment Between uses, close the laser cover or turn the laser off When you are done with your experiments, turn the laser off and unplug it Never point the laser anywhere other than toward the screen Always place the slit mask close to the laser Experiment 1 1. Write down our equation for the positions of bright fringes from a diffraction grating: 2. Set up the laser and screen across the length of your table. 3. Set up a diffraction grating on the stand, and place it directly in front of the laser. Turn it on and check for stray laser light there will be lots, in all directions. Make sure to block every stray laser beam with books/clothes/etc. 4. Try the different diffraction gratings, and note the different fringe spacing. Which grating gives you the fringes closest together? Does this reflect what we would expect based on our equation?

2 5. Insert the grating with 300 lines/mm. Take whatever measurements you need to take to find the wavelength of the laser. When finished, write your result on the front board. Hint: These calculations have very high precision, so try to keep lots of digits in your calculations.

3 Experiment 2 1. Write down our equation for the positions of the dark fringes, and our equation for the width of the central maximum, for a single-slit diffraction pattern. 2. Set up your laser experiment again, this time with the single slit mask. Again, check for stray laser light and block it. It should not be as bad as the diffraction grating. 3. Try the different slit widths approximate values of the widths are printed on it. What happens to the fringe pattern as you go to larger slit width? Does this match what we would expect from our equations? 4. The written values for slit widths are only approximations you can measure it more precisely. Write down the values you will need to measure to do this.

4 5. Perform the experiment using the ~0.04 mm slit and calculate the true width of the slit. Hint: Minima are much easier to locate than maxima (the center of the maxima is hard to pinpoint). Write your value on the board it should be close to 0.04 mm.

5 Experiment 3 1. Write down our equation for the positions of the bright fringes in a double-slit diffraction pattern. 2. Set up your laser experiment again, this time with the double slit mask. Check for stray laser light and block it. 3. Try the different slit widths and spacings approximate values of the both are printed on it. What changes as you go to larger slit spacing? What about larger slit width? Does this match what we would expect from our equations? 4. This time you will calculate the true value of the slit spacing. Write down the measurements you will need to take.

6 5. Perform the experiment using the ~0.04 mm slit width and ~0.250 mm slit spacing, and calculate the true spacing of the slits. Write your value on the board it should be close to 0.25 mm. Hint: Minima are much easier to locate than maxima, and the distance between minima is the same as the distance between maxima. Hint: Measuring the distance between two minima is tough, measure more is much easier

7 Experiment 4 1. You will now measure the width of a human hair. It turns out that the diffraction pattern for a single object blocking the light (like a hair) is identical to the diffraction pattern for a single slit. The equation is the same too (width of slit = width of hair). 2. Get a hair from someone in your group, make a diffraction pattern, and use it to calculate the width of the hair.