## 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.