The method and the results are on the physical optics lab. Do the analysis yourself first, then check the answers below.
Quick answer: Physical optics explains light and sound as waves that add together. These answers go with the AP Physics 2 lab on physical optics: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Also, model answers use the sample table above. Also, your own values will differ a little, and the key skill is to show the gradient method, the unit conversions and an honest comment on uncertainty.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 14
Δy = (distance across 10 gaps) / 10, so 26 mm gives 2.6 mm and so on, as in the table. Measuring 10 gaps spreads the reading error of about ±1 mm over 10 fringes, so the error in Δy falls to about ±0.1 mm.
Question 2 Plot the graph
Then, the points lie close to a straight line through the origin, which shows that Δy is directly proportional to L, as Δy = (λ/d)L predicts.
Question 3 Gradient
Gradient = (7.9 − 2.6) mm / (3.00 − 1.00) m = 5.3 / 2.00 = 2.65 mm/m = 2.65 × 10⁻³ (no unit, mm per m).
Question 4 Wavelength
λ = gradient × d = 2.65 × 10⁻³ × 2.5 × 10⁻⁴ m = 6.6 × 10⁻⁷ m = 660 nm.
Question 5 Compare with the label
difference = (660 − 650) / 650 × 100 = 1.5 %, within the experimental uncertainty.
Question 6 Uncertainty
In addition, sources: (1) judging the fringe centres on the paper (largest, ±1 mm over blurred edges); (2) measuring L from the slide to the screen (±5 mm). Improvement: use a longer L and mark more fringes, or photograph the pattern against a ruler.
Question 7 Predict a green laser
Δy = λL / d = 532 × 10⁻⁹ × 2.00 / 2.5 × 10⁻⁴ = 4.26 mm, smaller than the red value of 5.3 mm, because Δy is proportional to wavelength and green light has a shorter wavelength.
