- Coherent. This means the phase difference between the light waves arriving at
any location remains constant over time.
The first condition, monochromatic light, means that all the light passing through the
slits must have the same wavelength. White light, which is a mixture of many
wavelengths (colors), does not produce distinct interference patterns. We often used a
similar condition when analyzing the interference of mechanical waves, restricting our
attention to what occurred when two waves of the same wavelength met.
We also implicitly used the second of the two conditions stated above with waves on a
string. The phase difference between the waves sometimes remained constant
throughout the string, and over long time periods.
For light waves, coherence can be achieved by causing light from a single point source
to pass through two narrow slits, separating an initial beam of light into two beams with
a constant phase difference. You see what happens when coherent light emerges from
two slits and falls on a surface in Concepts 2 and 3.
If the light were not coherent, we would not be able to see interference. If the phase
differences varied over time, then there would be constructive, partial, and destructive
interference occurring at every particular spot at different times, and no overall pattern
of interference could be readily observed. For example, if the coherent light from the
two slits were replaced with incoherent light emanating from two light bulbs, only a
uniform glow of illumination would be visible on the viewing screen.
For complete constructive interference to occur, two light waves have to be in phase,
meaning there is zero phase difference (or else the phase difference is an integer
multiple of 2 ʌ radians, or 360°). At the point shown in Concept 2 the waves have no
phase difference, peak meets peak and trough meets trough, and there is complete
constructive interference. This causes the maxima, or regions of maximum intensity in
the interference pattern.
In Concept 3, you see the result at a different location on the viewing screen where light
waves from the two slits meet out of phase, with a phase difference of ʌ radians, or
180° (the phase difference could also be an odd integer multiple of ʌ radians, such as
3 ʌ radians, í 5 ʌ radians and so on). With this phase difference, the result is complete
destructive interference. Peak meets trough, trough meets peak, and the waves cancel.
Complete destructive interference causes the minima, the stripes of darkness in the
interference pattern.
The regions in between the lightest and darkest points are the result of partial interference. Here, the waves are somewhat out of phase, so
they do not reinforce each other completely, nor do they completely cancel. The visual result is a progression of shades of intensity between
the bright fringes caused by complete constructive interference and the dark fringes caused by complete destructive interference.Constructive interference
Waves in phase
Result: bright region (fringe)Destructive interference
Waves out of phase
Result: dark region (fringe)34.2 - Double-slit experiment: wavelike nature of light
The English scientist Thomas Young explored interference and diffraction in a series of
experiments that demonstrated the wavelike nature of light. Young performed his first,
crucial experiment in 1801. Here, we provide a summary of what he did. His actual
procedure was slightly more complex than the description below due to the pioneering
nature of his equipment.
Young shined light at a pair of barriers containing slits to create the interference pattern
you see on the right. The first barrier had a single slit that acted as a coherent point
source of light. (This single-slit barrier is not shown in the diagrams.) The coherent light
then traveled through two parallel slits in the second barrier, each the same distance
from the single slit, and then reached a viewing screen. In the diagrams you see the
double-slit barrier and the pattern of interference he observed on the screen.
The pattern was one of equally spaced bright and dark fringes. Young knew that water
waves passing through a pair of slits could cause a similar interference pattern. The fact
that both water waves and light produced exactly the same type of pattern supported
his hypothesis that light acted as a wave. Young further reinforced his position when he
demonstrated that he could use interference patterns to calculate the wavelength of the
light that he used in the experiment.
Why do waves create the interference pattern you see here? To answer this question, let’s consider the diagram in Concept 2. Two rays of light
meet at a point that is the same distance from each slit. The drawings of the rays emphasize their wavelike nature.
In this case, the two rays intersect the center of the screen in phase because both travel the same distance to this point. They were in phase
when they passed through the slits, and since they traveled the same distance, they remain in phase at their point of convergence. They
constructively interfere and produce a bright area of illumination. Other bright fringes occur to either side of the central fringe where the path
difference between the waves is not zero, but one full wavelength, two wavelengths, and so on.
Now look at Concept 3. The rays of light reaching the viewing screen intersect at the first dark fringe to the left of center. They travel different
distances to the surface: The ray from S 2 travels a half wavelength farther than the one from S 1. Because they start in phase at theirYoung’s double-slit experiment
Interference pattern appears on screen
Shows wavelike nature of light
Path lengths of rays to screen differ(^628) Copyright 2000-2007 Kinetic Books Co. Chapter 34