White light, with a uniform intensity across the visible wavelength range 430 - 690 nm, is perpendicularly incident on a wate film, of index of refraction `mu = 1.33` and thickness `d = 320`nm, that is suspended in air. At what wavelength `lambda` is the light reflected by the film brightest to an observer?

Explanation

To solve the problem of determining the wavelength at which light reflected by a water film appears brightest, we need to follow these steps:

### Step 1: Understanding the Condition for Maxima
When light reflects off a film, there is a phase change of 90 degrees (or half a wavelength) for the light reflecting off the higher refractive index medium (from air to water). The condition for constructive interference (maxima) in this case is given by:

\[ \Delta x = (2n - 1) \frac{\lambda}{2} \]
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