SAT Subject Physics Practice Question 202: Answer and Explanation
48. Which of the following changes to a double-slit interference experiment with light would increase the widths of the fringes in the diffraction pattern that appears on the screen?
A. Use light of a shorter wavelength
B. Move the screen closer to the slits
C. Move the slits closer together
D. Use light with a lower wave speed
E. Increase the intensity of the light
Correct Answer: C
Relative to the central maximum, the locations of the bright fringes on the screen are given by the expression mL(λ/d), where λ is the wavelength of the light used, L is the distance to the screen, d is the separation of the slits, and m is an integer. The width of a fringe is, therefore (m + 1)L(λ/d) – mL(λ/d) = λL/d. One way to increase λL/d is to decrease d.