2. BACKGROUND
In 1799, a physicist named Thomas Young performed an experiment
demonstrating that light has wave properties
This contradicted the current view at the time that light ONLY has
particle properties
3. IF LIGHT WAS ONLY A PARTICLE…
If light WERE only particles, light travelling through two narrow slits
would display on a screen as two beams, as the light would be
expected to move in a straight line
This situation would occur:
4. BUT IT’S ALSO A WAVE!
However, since light acts as a wave as well, the contributions from
both slight would constructively AND destructively interfere with
each other
This results in a phenomenon such as this:
Constructive
Interference
Destructive
Interference
5. Here is a slightly more detailed view on how light interferes with itself
through the two slits:
Yellow lines represent constructive interference
Gray lines represent destructive interference
6. BUT DOES THIS ALWAYS WORK?
Does this phenomenon mean that any light that is shined through
two narrow slits can display the demonstrated effect? NO!
The problem is due to “spatial coherence”
Spatial coherence is defined as the ability of light from one point on
a wave front to interfere with another point on a wave front
Lasers do this well, as the light is concentrated to a small area
Flashlights, light bulbs, and most other common light sources are not
as coherent, as their light waves spread out to a larger area
7. THOMAS YOUNG’S EXPLANATION
In a quote from Thomas Young, he explained how spatial
coherence was important:
“In order that the effects of two portions of light may be thus
combined, it is necessary that they be derived from the same origin,
and that they arrive at the same point by different paths, in
directions not much deviating from each other.”
- quote taken from Wikipedia, The Free Encyclopedia
8. PHASE DIFFERENCE
To determine whether a point on the screen would have
constructive or destructive interference, we can determine the
phase difference of the two waves at that point, using this equation:
∆Φ = 2π
∆𝑥
λ
+ Φ0
∆Φ = Phase difference
∆𝑥 = Path length difference
λ = Wavelength
Φ0 = Inherent phase difference (which is usually left out because the
light source is the same)
9. INTERFERENCE
Constructive interference occurs when the phase difference is in
multiples of 2𝜋
∆Φ = m2𝜋, where m is an integer
Destructive interference occurs when the phase difference is half a
wavelength out of phase
∆Φ = (m+
1
2
)2𝜋, where m is an integer