Materials

HeNe Laser

Small Lab Jack

Green Diode Laser

Optics bench with Multiple Slit Set

Masking tape

Large Paper

Ring Stand with right angle and jaw clamp to hold the diode laser

Safety

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 no shutter then block the laser beam or turn off the laser when not using it.

Introduction

In this activity we will study the some of the phenomena that make up the field of Physical Optics. Previously, we studied the properties of light that resulted from the fact that light travels in straight lines at finite speeds. These properties of light are all that are needed to study reflection and refraction and thus image formation by mirrors and lenses. By contrast, 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

I. Interference from a Double Slit

Use the small lab jack to position the HeNe laser at the end of the optical bench 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.

Q1. In words, describe the pattern of light that is produced.

Q2. 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 bright spots appear to be missing? I.e., is there any point where you expect there to be a bright spot but don't observe one.

D3. Measure the spacing between five successive bright spots on both sides of the central bright spot. Record your values in the space below.

Q4) Based on your measurements describe the spacing of the bright spots in the pattern.

Shutter but do not turn off the HeNe laser. Place the laser pointer in the jaw clamp so that the clamp will keep the laser on. Adjust the height and position of the diode laser so that it replaces the HeNe laser and hits the same pair of double slits as before. If not already there tape the paper back to the wall and trace the pattern for the green laser. Once done, remove the laser from the clamp to turn it off.

D5. Measure the spacing between five successive bright spots on both sides of the central bright spot for the pattern produced by the green laser. Record your values in the space below.

Q6) How is the pattern produced when the green laser shined on the slits similar to the pattern produced by the red laser?

Q7) How are the patterns different?

Q8) Was the spacing between the bright spots larger for the red or the green laser?

Q9) Which has a longer wavelength, the red or the green laser?

Q10) Does there seem to be a direct or inverse relationship between the wavelength and the spacing of the bright spots? Support your answer with evidence from your observations.

Return the red laser so that the light form it hits the slits again. Rotate the slit set so that the laser is normal to the set slits labeled a = .08, d = .25. Record the interference pattern on the paper as before.

Q11. How is the pattern obtained from the second set of slits similar to the pattern from the first set of slits?

Q12) How is the pattern different?

Q13) What type of relationship is exhibited between the slit spacing and spacing of the spots in the pattern?

S14) Write a one paragraph summary of what you have learned about the pattern of light formed when a laser hits two closely spaced slits.