Quick answer: Physical optics explains light and sound as waves that add together. This AP Physics 2 lab on physical optics gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How does the spacing of double-slit fringes change with the distance to the screen? You will measure the spacing at several distances and use the gradient of a graph to find the wavelength of a laser pointer.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 14
Aim
Measure how the fringe spacing from a double slit changes with the distance to the screen, then use the gradient to find the wavelength of a laser pointer.
Equipment
- red laser pointer (class 2, under 1 mW)
- double-slit slide, slit separation 0.25 mm
- clamp stand and clamp
- white screen or wall
- tape measure and 30 cm ruler
- tape or blu-tack
Safety
Never look into the beam or point it at another person. Block the beam after the screen so it cannot reflect off a shiny surface. Switch the laser off when you are not measuring.
Method
- Clamp the laser and slide so the beam crosses the slits. Tape a sheet of paper to the screen.
- Place the screen 1.00 m from the slide, measured with the tape. Switch on and adjust until a clear pattern appears.
- Mark the centres of 11 bright fringes along the paper (10 gaps). Measure the length of the 10 gaps and divide by 10 to find the fringe spacing Δy.
- Repeat for L = 1.50, 2.00, 2.50 and 3.00 m.
Results
Sample measurements from Bilal in Lahore (slit separation d = 0.25 mm):
| L (m) | Distance across 10 gaps (mm) | Δy (mm) |
|---|---|---|
| 1.00 | 26 | 2.6 |
| 1.50 | 39 | 3.9 |
| 2.00 | 53 | 5.3 |
| 2.50 | 64 | 6.4 |
| 3.00 | 79 | 7.9 |
Analysis questions
Question 1 Calculate the fringe spacings
Show how the distance across 10 gaps gives Δy, and state why measuring 10 gaps is better than 1 gap.Question 2 Plot the graph
Plot Δy (vertical axis, in mm) against L (horizontal axis, in m). Draw a best-fit line and describe its shape.Question 3 Gradient
Calculate the gradient of the best-fit line using the first and last points. Give the units.Question 4 Wavelength
Use the gradient and the slit separation to calculate the wavelength in nanometres.Question 5 Compare with the label
The laser is labelled 650 nm. Calculate the percentage difference between your value and 650 nm.Question 6 Uncertainty
State two sources of uncertainty in measuring Δy and the largest one, then suggest one improvement.Question 7 Predict a green laser
Predict the fringe spacing at L = 2.00 m for a green laser of wavelength 532 nm. Explain your reasoning.
