respective slits, this means the waves are ʌ/2 radians (180°) out of phase when they
arrive at the screen. This causes destructive interference, and darkness.
The pattern of bright and dark fringes extends to both the left and the right on the
screen. The light is interfering constructively at the bright fringes, and destructively at
the dark fringes, because of different path lengths to these regions and the resulting
phase differences.
There are a few limitations to showing Young’s apparatus in a compact diagram. First,
the diagram is far from being drawn to scale. The screen should be much farther from
the double-slit barrier than we show here, and the slits should be narrower and closer
together. In actual interference experiments, the interfering rays from the two slits are
practically parallel. Second, we vastly exaggerate the wavelength of the light.
You may have a question about what you would see if you conducted this experiment
yourself. What if, at some instant, two waves meet at the screen and are in phase, but
their electric and magnetic fields both happen to be zero at that point? Would you see
“flickering” as the two reinforcing waves moved from peak to trough and back again?
The answer is no: The frequency of light is so great that you only perceive the average
brightness of a region; the human eye does not perceive changes in intensity due to the
oscillation of a light wave.
You do not even perceive flicker in systems oscillating at far lower frequencies, much
less than the frequency of visible light, which is on the order of 10^14 Hz. For example, a
computer monitor refreshes its display 60 times a second, but you do not ordinarily
perceive any flicker when you look at it.
Pattern of bright and dark
Constructive interference: bright fringes
Pattern of bright and dark
Destructive interference: dark fringes
34.3 - Michelson interferometer
Interferometer: A device
that uses the interference of
two beams of light to make
precise measurements of their
path difference. Historically
used to study the nature of
light.
Albert A. Michelson, an American physicist, is famous
for conducting an experiment that helped Albert Einstein to develop his theory of special
relativity. Michelson’s experiment helped to disprove the existence of the “ether,” an
invisible medium that many scientists believed was required for the transmission of light
waves. In this section, we discuss the design of one piece of equipment he used, now
called the Michelson interferometer, and how it uses the interference patterns of known
wavelengths of light to measure minute differences in the path lengths of two beams of
light.
You see a photograph of an interferometer above, and a conceptual diagram to the
right. We use compass directions like “north” to explain the directions of the various
beams. In the center of the diagram is a beam splitter. It is a plate of glass whose back
is coated with a thin layer of silver. The beam splitter reflects half the light that falls on it,
and lets the rest pass through.
The interferometer works in the following fashion:
- Monochromatic, coherent light strikes the beam splitter from the west side.
- The initial beam splits into two at the beam splitter. The silver coating reflects
beam N to the north, and the splitter refracts beam E, which passes to the east. - Beam N travels to an adjustable mirror, reflects back, and then passes back through the splitter to a viewing telescope.
- Beam E goes through a compensating plate of glass, reflects off a fixed mirror, passes through the compensating plate again and then
reflects directly off the back of the splitter to reach the viewing telescope.
This system causes two mutually coherent beams of light to travel by different paths. An operator can control the length of the north-south path
Michelson interferometer. In this view the telescope is on the left,
the adjustable mirror is on the right, and the fixed mirror is in front.Michelson interferometer
Used for precise length measurements
Interference patterns change with path
difference