### All GRE Subject Test: Physics Resources

## Example Questions

### Example Question #1 : Interference

A double slit experiment is set up with the following parameters: two slits are a separated by a distance . A beam of light with wavelength shines through the two slits, and is projected onto a screen a distance from the slits. What is the distance on the screen between the central band and the next band on either side? (This distance is marked '' on the figure).

**Possible Answers:**

**Correct answer:**

The condition for constructive interference with double slit diffraction is given by:

Where is 0, 1, 2, ...

Solving for the angle and using the small angle approximation, we get;

The distance in the diagram can be related to the other quantities by simple geometry:

Again, with the help of the small angle approximation. Setting the two thetas equal to each other and solving for , we get:

For the central band, , so the position is also zero. The next band, , yields a distance of:

### Example Question #1 : Interference

Two waves with frequencies: are combined. What is the frequency of the resulting beat?

**Possible Answers:**

**Correct answer:**

The beat from two combined sound waves is: