HELP! PhET simulation: “Wave Interference” (available at http://phet.colorado.ed

Question

HELP!

PhET simulation: “Wave Interference” (available at http://phet.colorado.edu) THE FRINGE OF OPTICS

INITIALSETUP Step 1: When the simulation opens, maximize the window so that it occupies as much screen area as possible. Then click the tab labeled “Light”. Use the button at the bottom of the simulation window to pause the animation. Step 2: In the main (animation) section of the simulation window, click the “Show Screen” button to activate the detector screen. Then click the “Intensity Graph” button to activate the intensity graph. Then maximize the wave animation by clicking the green box with the “+” symbol in the upper right hand corner of the animation box. Step 3: In the control panel section (right hand side) of the simulation window, activate the two-slit barrier. Set the slit width to 525 nm (the second tick mark past zero on the slider). [NOTE: The slider may not settle at the desired tick mark, so just get as close as possible.] Set the barrier distance to 2590 nm (fourth tick— midpoint on the slider). Set the slit separation to 1750 nm (the midpoint on the slider). Step 4: Play the animation (un-pause the simulation). Step 5: In the light source controls (lower left side of the window), change the color to yellow-green (set the selector to the boundary between yellow and green. (SEE Step 6 for using the Measuring Tape. Useful information comes from actually measuring the . Fig. 1 shows where the can be measured.) Watch for several seconds as the light passes through the slits to form a fringe pattern on the screen. Step 6: In Part A, we want to observe changes in the fringe pattern. The pattern may narrow or widen. That is, the peaks (maxima) and valleys (minima) may get closer together or farther apart. As a means for comparison, place a marker on the graph as follows (and as shown in Figure 1): a. In the control panel section, click “Measuring Tape.” Drag the tail end (right handle) up so the tape is vertical. b. Move the tape base so that its red crosshairs mark the minimum immediately below the central maximum on the vertical scale of the intensity graph. (If zoom/magnification is possible, do so now.) c. Move the end of the tape so that it marks the minimum directly above the central maximum. d. The space indicated by the tape measure is actually a minimum-to-minimum distance, but it serves as a good “proxy” for the distance y in Figure 1. A pattern with a larger y would also have a greater minimum- to-minimumdistance

QUESTIONS:

NUMBER 1- PART A: VARYING THE VARIABLES Step 1: Amplitude: A. a. Increase the amplitude to its maximum value using the light source amplitude control (immediately below the Wavelength slider). Allow several seconds for the high-intensity light to travel to the screen. Record your observations. (In other words, what observations seem to be significant and what are the resulting changes in outcomes. Try to be quantitative, if possible. Use the tape measure where possible. Otherwise, use qualitative descriptions.)

NUMBER 2- a. What difference—if any—does increasing the amplitude A have on the fringe pattern spacing y? (Circle your answer.) [REMEMBER: “~” is the symbol for “proportional to.”] (y ~ A) (y ~ 1/A) (y does not depend on A)

NUMBER 3- b. Based on your conclusion, reducing the amplitude will cause the pattern to contract (decrease y). expand (increase y). remain the same

Explanation / Answer

Number 1 : After increasing the amplitude, the amplitudes of the intensity graph on the detector screen increase but the relative position of the minimas relative to each other and the center of the screen does not change

Number 2: Increasing the amplitude has no effect on fringe spacing

Number 3: Decreasing A would not do anything suimiliarly

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