A. Light of wavelength 640 nm is shone on a double-slit apparatus and the interference pattern is observed on a screen. When one of the slits is covered with a very thin sheet of plastic of index of refraction n = 1.6, the center point on the screen becomes dark. What is the minimum thickness of the plastic in nm?

Enter an integer.

B. A thin film of oil (n = 1.59) is located on a smooth, wet pavement. When viewed perpendicular to the pavement, it appears to be predominantly red (623 nm). What is the minimum thickness of the oil film in nm?

Enter a number with one digit behind the decimal point.

C. Two gratings A and B have slit separations d_Aand d_B, respectively. They are used with the same light source and the same observation screen. When grating A is replaced with grating B, it is observed that the first-order maximum of A is exactly replaced by the second-order maximum of B. Determine the ratio d_B/d_A.

Group of answer choices

A.1/2

B. 2

C. 1

D. 1/4

E. 4