Introduction:
Aligning a laser beam to reflect and refract through a winding travel path is a skill that the graduate students in Dr. Martin Zanni’s research group have to perfect to be able to do their amazing research. One of the important experiments that involved laser alignment and determining if there was ether in space was the Michelson-Morley Interferometer Experiment.
Purpose:
To gain an appreciation of the skill needed to align a laser beam and to use the understanding of refraction and reflection of light to create a Michelson-Morley Interferometer.
Background:
After the development of Maxwell’s theory of electromagnetism, several experiments were performed to prove the existence of ether and its motion relative to the Earth. The most famous and successful was the one now known as the Michelson-Morley experiment, performed by Albert Michelson (1852-1931) and Edward Morley (1838-1923) in 1887.
Michelson and Morley built a Michelson interferometer, which essentially consists of a light source, a half-silvered glass plate, two mirrors, and a telescope. The mirrors are placed at right angles to each other and at equal distance from the glass plate, which is obliquely oriented at an angle of 45° relative to the two mirrors. In the original device, the mirrors were mounted on a rigid base that rotates freely on a basin filled with liquid mercury in order to reduce friction.
Prevailing theories held that ether formed an absolute reference frame with respect to which the rest of the universe was stationary. It would therefore follow that it should appear to be moving from the perspective of an observer on the sun-orbiting Earth. As a result, light would sometimes travel in the same direction of the ether, and others times in the opposite direction. Thus, the idea was to measure the speed of light in different directions in order to measure speed of the ether relative to Earth, thus establishing its existence.
Michelson and Morley were able to measure the speed of light by looking for interference fringes between the light which had passed through the two perpendicular arms of their apparatus. These would occur since the light would travel faster along an arm if oriented in the “same” direction as the ether was moving, and slower if oriented in the opposite direction. Since the two arms were perpendicular, the only way that light would travel at the same speed in both arms and therefore arrive simultaneous at the telescope would be if the instrument were motionless with respect to the ether. If not, the crests and troughs of the light waves in the two arms would arrive and interfere slightly out of synchronization, producing a diminution of intensity. (Of course, the same effect would be achieved if the arms of the interferometer were not of the same length, but these could be adjusted accurately by looking for the intensity peak as one arm was moved. Changing the orientation of the instrument should then show fringes.)
Although Michelson and Morley were expecting measuring different speeds of light in each direction, they found no discernible fringes indicating a different speed in any orientation or at any position of the Earth in its annual orbit around the Sun. Michelson reluctantly concluded from his experiments that ether must not exist.
Materials:
45-45-90 prism
Helium-Neon Laser
Beam Splitter and vertical mount
2 Adjustable mounted mirror
Wood Blocks or Items to be used as stands
Convex lens and lens mount
Index cards
Procedures:
Task #1
With a laser and a mounted mirror, have the laser beam reflect its beam back onto itself off the mounted laser. Then rotate it to the second mirror and adjust the second mirror to have the beam shine on the designated spot on the wall.
What did you learn about aligning a laser bean from this activity?
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Draw a diagram of your set up.
Task # 2
You are to create a optics set up which a laser beam is to strike a 45-45-90 prism, experience total internal reflection off of the hypotenuse of the prism, strike a mounted mirror, reflect back, and strike the source of the laser beam. Use what you have learned from the Optic unit to create this set up. Have your design checked off by your teacher when you have aligned the laser beam.
Discuss any challenges faced in aligning the laser beam.
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Draw a diagram of your set up.
Task # 3
You are to create a Michelson-Morley Interferometer. Use the diagram on the front page to create this set up. You will need to align the laser beam to strike a beam splitter orientated at 45⁰ with the normal of the laser beam. The laser beam will reflect off and refract through the beam splitter and reflect then reflect back off the two mounted mirrors. The mounted mirrors need to be the same distance from the beam splitter. The beams of light need to be aligned so that they leave and enter the same location on the beam splitter. The two beams need to strike the screen detector at the same location. Have your design checked off by your teacher when you have aligned the laser beams.
What do you see in the combined dot?
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What happens when you place you pushed on the top of the table? Why would this effect the image you see?
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Task # 3 Extra Challenge
If your group has time, place concave lens in front of the laser to defuse the laser beam. This will result in you being able to see the interference pattern better. You will need to do some fine tuning of your apparatus to have the laser beam dots unite on the screen.
How would a convex lens in front of a laser differ from a concave lens?
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What do you see in the combined dot? What happens when you place you pushed on the top of the table?
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Conclusion
Name and describe two things you learned from this Lab activity.
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Next Generation Science Standard:
| HS-PS4-3. | Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other. |