# Double Slit Interference

(this experiment is to be used with the Waves lab from SPECS)

Around 1801 a gentlemen by the name of Thomas Young demonstrated interference patterns in light waves. Young accomplished this using pin holes rather than slits. In his original experiment he shown a light incident on a screen with a slit (pin hole). The light passing through this opening continued to another screen which had two parallel slits (pinholes). He did this to make certain that the light coming through the two slits would always be in phase having come from the same wave front, the light from the fist slit. The light from the two slits fell onto a screen and produced a visible pattern of light and dark parallel bands called fringes. These light and dark bands are due to constructive and destructive interference of the light coming from each slit.

Purpose

You will be doing something analogous to Young's experiment only using water waves.

Words you should know

amplitude, constructive interference, destructive interference, diffraction, frequency, fundamental frequency, index of refraction, phase angle, and superposition.

glossary of terms

Procedure

Step 1: Create three walls. Make two of the walls the same width 15, height 200. Place one of them so that it is about 3 cm from the left side of the screen and touches the top of the screen.

Step 2: Place the other so that it touches the bottom of the screen directly under the top wall.

Step 3: Place the third wall between the other two so that there is equal space between the top and bottom walls.

Run the program and record your observations. Also sketch the pattern.

Step 4: Next change the size of the space between each of the blocks, making sure that the size remains equal between the top and bottom.

Step 5: Again run the program and record your observations.

Step 6: Continue changing the space and recording your observations.

Does the pattern change? If so, how does it change? What would account for these changes?

What do you think would happen if the spaces between the top and bottom were not equal? Record your prediction.

Step 7: Create it then run it to check your prediction.

Explain any difference between your prediction and the actual run.