Light of wavelength λ = 450 nm is incident upon two thin slits that are separated by a distance d = 25 μm. The light hits a screen L = 2.5 m from the screen. It is observed that at a point y = 2.8 mm from the central maximum the intensity of the light is I = 55 W/m2.
a) Write an equation for the phase shift between the light from the two slits at the observation point in terms of the given variables.
ANSWER: Φ = 2πdy/(λL)
b) For the given data, what is the phase shift, in radians?
ANSWER: 0.3909 radians
c) What is the intensity of the light at the two slits Io in W/m2?
Small angle approximation: y = mλ/d (for bright fringes)
Young's equation: Φ = kdsinθ
Path Difference Equation: ΔX = λΔΦ/2π
The Attempt at a Solution
I've worked out both of the first two parts, and the answers are correct. I'm not sure how to get the original intensity though. I tried dividing the intensity I at y by the phase shift Φ, which yielded 140.82 W/m2, but for some reason that's wrong.