Learning Object of 28.3 Double Slit Interference

1. Learning Object: 28.3 Double-Slit Interference
Diagram 1
Diagram 1 shows the set-up for the experiment conducted by Thomas Young in 1803 to show
that light is a wave phenomenon and thus exhibits inference – one of the properties of a wave.
An incident light passes through the double slits and forms certain patterns on the screen.
Describe the effects on the maximum brightness and the separation of the fringes on the
screen when the following changes are made:
1. The distance between the double slits and the screen, D, doubles while keeping the
distance between the double slits, a, and the wavelength of light, λ constant.
The separation of the fringes doubles as well, but the maximum brightness of the
fringes decreases.
2. The wavelength of light, λ is halved while keeping the distance between the double
slits, a, and the distance between the double slits and the screen, D constant.
The separation of the fringes halves as well. However, the number of fringes
increases thus the number of bright fringes increases as well (more constructive
interference). Therefore, the maximum brightness of the fringes on the screen
increases.

2. 3. The distance between the double slits, a, doubles, while the distance between the
double slits and screen, D and the wavelength of light, λ constant.
The separation of the fringes halves while the maximum brightness of the fringes
remains unchanged.
4. The intensity of the incident light on the double slits increases, while keeping the
wavelength of light, λ, distance between the double slits, a, and distance between the
double slits and screen, D constant.
The maximum brightness of the fringes on the screen increases while the separation
of the fringes remains unchanged.
5. What effects are observed on the screen when the distance of each slit becomes
narrower?
The maximum brightness of the fringes on the screen increases, the separation of the
fringes remains unchanged, and the number of fringes that appear on the screen
increases.
6. Based on all these observed effects, deduce an equation that relates all the parameters
on the interference of waves.
λD
𝑎
= Δx
7. A student in PHYS 101 attempts to reproduce Young’s Double Slit experiment. In the
lab, he is provided a double slit, a 632.8nm wavelength laser, and a screen. He wants
to measure the distance between the double slits as it is too narrow to measure
accurately with a ruler. He sets up the experiment as such that the distance between
the screen and the double slits, D, is 1.2m. When the laser is shone through the double
slit, the average separation of fringes that are formed on the screen, Δx is 76mm.
What is the distance between the double slits, a?
λD
𝑎
= Δx
λD
Δx
= a
(6.328e 10−7
)(1.2)
0.076
= 9.991e10-6 m