PHYS 2426 – Engineering Physics II
Physical Optics
Leader: _________________________ Recorder: __________________________
Skeptic: _________________________ Encourager: ________________________
HeNe Laser
Optics bench with the following
Green Laser Diode
Single Slit Set
Multiple Slit Set
Masking tape
Large Paper
Diffraction grating
Laptop
The lasers we are using are just powerful enough that if you took a shot directly in the eye, you could damage your vision. Thus, we should take a few simple precautions to prevent the unlikely event of eye damage.
1. Never look directly into the laser beam. Laser light has a high intensity and can also be easily focused. A direct shot of the laser beam on your eye will be focused by your eye’s lens onto a small spot on your retina and can burn or possibly detach the retina.
2. Never hold an object by hand in front of the laser beam. This prevents the possibility of accidentally shining the light into your eyes.
3. Keep your head above the plane of the laser beam.
4. Whenever the light strikes an object, there will be a reflection. At times the reflections can be almost as strong as the incident beam. Know where the reflections are and block them if necessary.
5. If the laser has a shutter in front of the beam, use it. When not taking data, place the shutter in front of the laser beam. If there is ho shutter then block the laser beam or turn off the laser when not using it.
Introduction
In this lab we will study the phenomena that make up the field of Physical Optics. In geometric optics, we studied the properties of light that resulted from the fact that light travels in straight lines at finite speeds. These two properties of light are all that are needed to study reflection and refraction and thus image formation by mirrors and lenses. In physical optics we study the properties of light that arise because of the wave nature of light. These properties include interference and diffraction.
Procedure and Questions
Position the HeNe laser so that the laser beam is parallel to the tabletop and pointing towards a nearby wall. Place the Multiple slit set on the optics bench and position it in front of the laser so that the beam is normal to the slits labeled a = .08, d = .50. a = .08 mm is the width of a single slit and d = .50 mm is the slit spacing. Tape a piece of paper to the wall perpendicular to the pattern and trace the pattern of light produced on the paper.
1. In words, describe the pattern of light that is produced.
2. You should have observed alternating bright and dark spots. This pattern is called an interference pattern. Each bright spot in the interference pattern is referred to as an order. Do any orders appear to be missing? I.e., is there any point where you expect there to be a bright spot but don't observe one.
We number orders in an interference pattern by integers. The bright spot in the center is 0th order, the one next to it is 1st order and so on. Identify the bright spot in the center.
3. Make a table recording the position of each bright spot from the center and its order. Assign positive orders to spots to the right of the central bright spot and negative orders to spots to the left. Similarly call positions to the right positive and positions to the left negative. If you think you have any missing orders, skip them in numbering your spots.
4. Construct a graph of the location of the bright spots vs. the order. Is the graph a straight line? What does this tell you about the interference pattern from a double slit?
5. Draw a best fit line through the graph, and record the slope for later reference.
6. Rotate the slit set so that the laser is normal to the set slits labeled a = .08, d = .25. Record the interference pattern both on the paper and in a data table as before.
7. Qualitatively, how is the pattern obtained from the second set of slits similar and different from the pattern from the first set of slits?
8. Graph location of the bright spots vs. the order for the second set of slits. Place a best fit line on the graph and record the slope.
9. How did the slope for the second set of slits compare to the slope for the first? Was it larger or smaller?
10. What affect does the slit spacing have on the spacing of the bright spots for an interference pattern in a double slit experiment?
11. What type of relationship is exhibited between the slit spacing and spacing of the spots in the pattern?
The condition that a bright spot occurs in a double slit experiment is give by
ml = d sinq and similarly for dark spots (m + 1/2)l = d sinq where l is the wavelength of the light, d is the slit spacing and q is the angle of the light with the normal to the slits.
If we project the pattern onto a screen a distance L from the slits, then the pattern has spacing between the spots given by ym/L = tan qm. Assuming that the angle is small, we can use that sin q » tan q » q. Combining expressions gives ml = dym/L. Thus by making measurements of screen distance and spot spacing we can either determine slit spacing if we know wavelength or wavelength if we know slit spacing.
1. Set up the HeNe laser in front of the smaller pair of slits so that you project the interference pattern onto the wall.
Measure and record the distance from the slits to the wall. L = _______
Measure and record the distance from the central bright spot to the first bright spot
y1 = _______________
From these measurements and assuming the wavelength of the HeNe laser is 632.8 nm, determine the spacing of the slits, d. Show your work in the space below.
How does this value compare to the value listed on the multiple slit set?
2. Replace the HeNe laser with the green diode laser. Using the result for the slit spacing you have just found, determine a procedure for measuring the wavelength of the diode laser. Describe your procedure in the space below.
3. Carry out your procedure and determine the wavelength of the diode laser.
III. Diffraction
from a Single Slit
1. Aim the HeNe at the wall. Rotate the multiple slit set so that the HeNe laser is aimed at the single slit labeled a = .16. Tape a sheet of white paper on the wall perpendicular to the pattern and trace the pattern onto the paper.
2. Determine the spacing between the central peak and the first minimum and use that to determine the size of the slit. Assume the wavelength of the HeNe laser is 632.8 nm. (Hint: Determine the distance between the first minimum on either side of the central peak and divide by two.) Record your data and show all of your calculations neatly organized in the space below.
3. Rotate the slit to the next smaller size. Describe qualitatively how the pattern changes.
4. Repeat step 3 for the next smaller slit size.
5. Repeat step 3 for the next smaller slit size.
6. In general, how does the diffraction pattern change as you go to smaller slit sizes?
7. Aim the green diode laser at the slit labeled a = .16. Trace the pattern onto a sheet of paper, and determine the spacing between the central peak and the first minimum. Use that spacing and the width of the aperture you’ve just measured to determine the wavelength of the diode laser. Record your data and show your calculations in the space below.
IV. Diffraction
Gratings
1. Observe the fluorescent lights in the hallway outside of the classroom through the diffraction grating.
1) Describe the pattern that you see?
2) Describe how the patterns change as you go further out from the central pattern.
2. Use the HeNe laser to determine the number of lines/cm that the diffraction grating has. Record your data and show your calculations in the space below.
3. Once you have determined the number of lines/cm, use the diffraction grating film to determine the wavelength of the green diode laser. Record your data and show your calculations in the space below.