PHYS110-How Things Work
Notes on DIffraction of Light
Diffraction occurs for all wave phenomena. In the first part of the lab we saw diffraction
of water waves. Two wave sources sent out circular waves and the two waves interfered
forming what is called a diffraction pattern.
You measured the angles at which you saw constructive interference by sketching the
patterns you saw. Next you calculated the angles by using the equation
θ=λ/D
These two should be close. You should be comparing these numbers. In doing so
you are comparing experimental values (measurements) with predicted values.
In the second part of the lab light was sent through two slits resulting in a diffraction
pattern on the wall. You measured the angle of diffraction by measuring the distance to the
wall, L, and the spacing between the constructive interference fringes, Y. The angle is calculated
from
θ=Y/L
You also calculated the angle from know the wavelength, λ, and spacing of the slits, D.
θ=λ/D
Again, these two should be close. You should be comparing these numbers. In doing so
you are comparing experimental values (measurements) with predicted values.
One of the questions asks if you would observe diffraction from light incident on two slits
which are 5cm apart. From the equation, θ=λ/D, on can see that
light, which has a very small wavelength, (10-10m) will diffract to a very small
angle in this case:
θ=λ/D = 6.3x10-7m/0.05m = 1.2x10-5radians
Converting this to degrees we get:
1.2x10-5radians(180/2π) = 3.4x10-4 degrees = 0.00034 degrees
This is a really small angle, ans thus would not be seen. There would be no noticible diffraction.