Plane electromagnetic waves of wavelength = 633 nm (red light) from a helium-neo
ID: 1439247 • Letter: P
Question
Plane electromagnetic waves of wavelength = 633 nm (red light) from a helium-neon laser are incident horizontally from the left on an opaque vertical screen having two narrow slit-shaped openings separated by a distance d = 0.104 mm. Light emerging from the two slits forms an interference pattern on a second vertical screen located a distance l = 95.5 cm away. Maximum brightness is observed at the center of the pattern, at a height midway between the two slits. Above that, what is the angle at the first minimum in brightness, i.e. at the center of the first dark fringe?Explanation / Answer
l = 95.5 cm = 0.955 m
d = 0.104 * 10^-3 m
= 633 nm
Condition for destructive interference is given by,
d*sin() = ((m+1/2)*)/l
For 1st minima, m = 0
Substituing values,
0.104 * 10^-3 * sin() = ((0 + 1/2) * 633 * 10^-9)/0.955
sin() = 0.00318
= 0.182o