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Two-Slit Interference

In the lecture you learned many properties of waves, that they exhibit reflection, refraction, etc. One particular aspect of waves is due to the principle of superposition where the wave functions merely add algebraically. This is unique to waves. In other words, if light (albeit at radio, microwave or ultraviolet wavelengths) exhibits classical interference properties, only a wave model can explain this phenomena. In this lab we allow light to fall upon an opaque screen with two closely spaced, narrow slits. The waves emanating from the two slits then interfere constructively (bright lines) and destructively (dark lines) when the two sources reach the screen. Click here to see what it sort of looks like: And then there was LIGHT!

Apparatus:

Equipment Setup:

Set up the equipment as shown below. The Slit Mask should be centered on the Component Holder. While looking through the Slit Mask, adjust the position of the Diffraction Scale so you can see the filament of the Light Source through the slot in the Diffraction Scale.

Figure 1: Equipment Setup

Attach the Diffraction Plate to the other side of the Component Holder, as shown. Center pattern D, with the slits vertical, in the aperture of the Slit Mask. Look through the slits. By centering your eye so you look through both slits and the window of the Diffraction Plate, you should be able to clearly see both the interference pattern and the illuminated scale on the Diffraction Scale. In some ways this is a bit tricky. You need to focus one eye on the interference patterns seen through the double slit while the other eye on the ruler. With a bit of practice you'll agree this is not so bad.

Figure 2. Geometry of Two-Slit Interference

The geometry is explained in your text. Perform the experiment with the Red, Green and Blue color filters over the Light Source aperture to make measurements for different colors of light. Perform the appropriate calculations:
 

Color m d y L l
Red          
Green          
Blue          

The wavelength is calculated by:
 
 
 l = (d/m) sin (arctan y/L)  

Here the angle l is determined from the inverse tangent and the sin of the angle taken. Slit d spacing is .125 mm. Widths of the slits are 0.04 mm. Your relationship,
 
 
 d sin q = m l  

allows you to determine the wavelength of the three colors of light. Using rough values for these ( Red ~ 600 nm, Green ~ 525 nm and Blue ~ 450 nm) , find rough % error too. These are by no means accurate. The filters are essentially "el cheapos" and do not have any exact values given. They also provide a wide band of light at each color.

Write up the experiment, describing for the reader (mom, dad, spouse, friend, enemy or whoever) including the format explained in class; i.e., Limitations to the Experiment, Data, Data Analysis, Discussion, Summary, Conclusions and any Recommendations. Have fun and learn about the nature of light waves.
 

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