White light is incident normally on a thin glass plate of refractive index 1.5. Calculate the minimum thickness of the film for which the wavelength `4200 Å` is absent for the reflected light.

Explanation

To solve the problem, we need to find the minimum thickness of a thin glass plate (film) for which the wavelength of 4200 Å is absent in the reflected light. Here is a step-by-step solution:

### Step 1: Understand the condition for absence of wavelength in reflected light
For a thin film, the condition for destructive interference (which leads to the absence of a particular wavelength in the reflected light) is given by the formula:
\[ 2 \mu t = (m + \frac{1}{2}) \lambda \]
where:
- \( \mu \) is the refractive index of the film,
- \( t \) is the thickness of the film,
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