# Young's Experiment

 Materials: Computer and School Network Time Allotment: 3 Class Days

### Purpose:

The purpose of this lab is to use principles of two-point source interference in order to duplicate the essential method used by Thomas Young to determine the wavelength of monochromatic light using a diffraction pattern.

1. Log on to the student server in the usual manner
2. Open the Multimedia folder; then open the Hypercard application.
3. Once the Hypercard is opened, chose Open... from the File menu.
4. A directory dialogue box should appear. Navigate through the Physics Stacks directory and find the file titled Young's Experiment; open this stack by double-clicking on its icon.
5. Click anywhere on the first card in order to navigate to the simulation card.

The simulation card is divided into two important sections. The top section represents the experimental set-up as it would be seen from above. The set-up consists of a laser beam of unknown wavelength, a card with four sets of double slits, and a screen upon which the interference pattern is projected.

The bottom section of the monitor represents the appearance of the screen as seen from the "front." This view allows you to see the actual interference pattern; the interference pattern consists of a series of bright and dark fringes. These fringes are the result of the interference of the waves which diffract through the two slits.

On-screen buttons can be used to make measurements of the important variables (x and L) which are necessary for determining the wavelength of the laser light. Furthermore, the important variables (d, X, L, and "lambda") can be modified to investigate the effect of a variation of one variable upon another.

### Pre-Lab Analysis:

1. Identify the purpose of Thomas Young's experiment.

2. What measurements did Thomas Young have to make in order to determine the wavelength of light? Explain fully.

3. On the diagram below, indicate the important distances - d, L, and x - which Young measured in order to determine wavelength of light.

4. Explain in detail the best way that Young can determine x for the diagram above.

5. Write the equation which Young used to calculate the wavelength of light.

### Procedure:

Qualitative Investigation:

1. Be sure the lase beam is "turned on" by clicking on the appropriate button at the bottom of the monitor.
2. Investigate the effect of varying "d" upon the interference pattern. Use the buttons at the right of the monitor in order to select different slit separation distances. Describe the effect of varying "d" upon the intereference pattern.

Exactly what effect does doubling d have upon x? _____________ Explain in detail how you determined the answer.

Is the slit separation distance (d) directly or inversely proportional to the amount of separation of bright fringes on the screen? Explain.

3. Investigate the effect of varying "L" upon the interference pattern. Use the "Move Screen" buttons in order to move the screen closer to and farther from the double slit. Describe the effect of varying L upon the intereference pattern.

Is the screen distance (L) directly or inversely proportional to the distance of separation of bright fringes on the screen? Explain.

4. Investigate the effect of varying wavelength upon the interference pattern. Use the wavelength modification buttons in order to increase and decrease the wavelength. Describe the effect of varying wavelength upon the intereference pattern.

Is the wavelength ("lambda") directly or inversely proportional to the distance of separation of bright fringes on the screen? Explain.

Quantitative Investigation:

Now you will investigate the quantitative relationships between the variables and make measurements to determine the wavelength of the laser light.

1. Use the "Modify Wavelength" buttons to increase the wavelength value to its maximum. Click on the up arrow several times until you hear the beep. The beep indicates the wavelength is at its maximum value. For this maximum value, make on-screen measurements of the appropriate values and determine the wavelength of light. Record your data and show your calculations in the space below. Repeat the measurement for various values of length and slit separation distance.

 d (m) L (m) n x (m) Wavelength (nm) 0.00025 ______ ______ ______ ____________ 0.00025 ______ ______ ______ ____________ 0.00030 ______ ______ ______ ____________ 0.00040 ______ ______ ______ ____________ 0.00050 ______ ______ ______ ____________ 0.00050 ______ ______ ______ ____________

2. Repeat the above measurements two more times for randomized values of wavelength. Once you have made the measurements and performed the calculations, check your answer by using the button at the bottom of the monitor. You must receive a print-out stating that you have measured correctly. Use the "Check Answer" button to obtain this printout for two more randomized values of wavelength. Include the printout with the submission of your lab. Record all measurements and show all calculations in the space below.

 d (m) L (m) n x (m) Wavelength (nm) ______ ______ ______ ______ ____________ ______ ______ ______ ______ ____________

### Conclusion:

In a well-written paragraph, summarize the procedure used by Thomas Young in order to measure the wavelength of light. Do a bang-up job!