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. IFLIGHT WAS ONLY A PARTICLE…
▶ Iflight 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’SALSO 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 THISALWAYS WORK?
▶ Does this phenomenon mean that any light that is shined through
two narrow slits can display the demonstrated effect? NO!
▶ The problem isdue 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
+ Φ0
λ
▶ ∆Φ =Phase difference
▶ ∆𝑥 =Path length difference
▶ λ =Wavelength
▶ Φ0 =Inherent phase difference (which is usually left out because the
light source isthe same)
9. INTERFERENCE
▶ Constructive interference occurs when the phase difference is in
multiplesof 2𝜋
▶ ∆Φ =m2𝜋, where m isan integer
▶ Destructive interference occurs when the phase difference is half a
wavelength out of phase
2
▶ ∆Φ =(m+1
)2𝜋, where m isan integer
