# Interference and Diffraction

Waves of all kinds -- light waves, sound waves, water waves, etc. -- exhibit a common set of phenomena due to the manner in which they interact with each other. The general term for these phenomena is interference. Today, you will use two specific types of interference, two-slit interference and diffraction, to determine the physical properties of materials in the lab.

#### Two-slit interference

If light from a coherent source passes through two slits and then falls onto a wall some distance away, it forms a set of light and dark bands. As your lab manual shows, there is a relationship between the spacing between the slits, d, the wavelength of the light, lambda, and the angles theta at which bright bands appear:

```         d * sin(theta)   =  M * lambda
```
where M is an integer: 0, 1, 2, ...

Your job: use this equation to figure out the wavelength of the light emitted by the laser. You will find slides with pairs of narrow slits in your optics kit. I expect that you will

• use at least two different pairs of slits, in order to make two independent calculations
• draw a diagram showing the setup of your experiment
• include an estimate of the uncertainty in each value, and in your final value

#### Diameter of a human hair

Once you have determined the wavelength of your laser's light, you can use it to figure out the diameter of a human hair. If you shine the laser on a distant wall, and place a human hair in front of the beam, you will see a pattern of light and dark spots. Again, there is a relationship between the wavelength lambda, the diameter D of the hair, and the angles at which the dark spots appear:

```         D * sin(theta)   =  N * lambda
```
where N is a positive integer: 1, 2, 3, ...

Once again, I expect each group to

• draw a diagram showing the setup of your experiment
• include an estimate of the uncertainty in their final value for the diameter of the hair

You should hand in all your work before you leave the lab room. In addition to the items specified above, it should include

• Neat tables of all your measurements, including headings and all appropriate units and uncertainties
• All calculations
• A table which lists
• Each type of measurement you make this week
• The tool you used to make it
• The percentage uncertainty in your measurements using this tool

Extra Credit:

Write down your values for the laser light's wavelength and human hair diameter (including uncertainty in both). Go to the RIT Library. Use books (not computers) to answer the following questions:

1. Based on the wavelength of the laser's light, what kind of laser is it? If there are several possibilities, list each one.
2. Is your hair abnormal, or is it within the range of ordinary hair diameters?

You must provide a reference for each answer: book title, author, publisher, page number.