70) A metallic sheet has a large number of parallel slits, 5.0 mm wide and 20 cm apart. This
sheet is used as a diffraction grating for microwaves. A wide parallel beam of microwaves is
incident normally on the sheet. If the intensity maxima occur 2.0° apart in the central region,
what is the wavelength of the microwaves?
A) 5.0 mm
B) 6.0 mm
C) 7.0 mm
D) 8.0 mm
E) 9.0 mm
71) A diffraction grating has 450 lines per millimeter. What is the highest order m that contains
the entire visible spectrum from 400 nm to 700 nm?
A) m = 2
B) m = 3
C) m = 4
D) m = 5
E) m = 6
72) Light is incident perpendicularly from air onto a liquid film that is on a glass plate. The
liquid film is thick, and the liquid has index of refraction 1.60. The glass has index of
refraction 1.50. Calculate the longest visible wavelength (as measured in air) of the light for
which there will be totally destructive interference between the rays reflected from the top and
bottom surfaces of the film. Assume that the visible spectrum lies between 400 nm and 700 nm.
73) A soap bubble, when illuminated at normal incidence with light of 463 nm, appears to be
especially reflective. If the index of refraction of the film is 1.35, what is the minimum thickness
the soap film can be if it is surrounded by air?
74) Light in air of wavelength 500 nm illuminates a soap film that has an index of refraction of
1.3 and air on both sides. What is the minimum thickness of this film that will produce
cancellation in the reflected light when the light is incident normally on the film?
75) A soap bubble with air on both sides has an index of refraction of 1.33. What minimum
thickness of the wall of this bubble will ensure maximum reflectance of normally incident light
having a wavelength of 530 nm?