Find the minimum thickness of a film which will strongly reflect the light of wavelength 589 nm. The refractive index of the material of the film is 1.25.

Explanation

To find the minimum thickness of a film that will strongly reflect light of wavelength 589 nm with a refractive index of 1.25, we can follow these steps:

### Step 1: Understand the phase change upon reflection
When light reflects off a boundary from a rarer medium (like air) to a denser medium (like the film), it undergoes a phase change of \( \frac{\lambda}{2} \) (where \( \lambda \) is the wavelength of light).

### Step 2: Determine the path difference
For a thin film of thickness \( t \) and refractive index \( \mu \), the light that travels through the film and reflects off the bottom surface will travel a distance of \( 2\mu t \) (it travels down and back up). Therefore, the total path difference \( \Delta \) between the two reflected rays can be expressed as:
\[
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