9) A film with a refractive index of 1.25 strongly reflects light of 550 nm. If the film is on water calculate the minimum thickness.

• Light reflected from thin film undergoes interference. Thus the final intensity might increase or decrease. 

• When interference takes place Constructive interference is when the intensity is increased. Destructive interference takes place when intensity reduces.

• Condition for constructive interference for a film of given refractive index and thickness is given by,

$\boxed{\mathbf{2}\mathbf{n}\mathbf{d}\mathbf{=}\mathbf{(}\mathbf{m}\mathbf{+}\frac{\mathbf{1}}{\mathbf{2}}\mathbf{)}\mathbf{\lambda }}$

Here n is the refractive index of the film, d is the thickness, m is the order, and λ is the wavelength of the source.