Homework Help: Waves travelling through mediums, destructive interference.

1. The problem statement, all variables and given/known data

light travels through air, hits a film (n=1.3) and underneath that film is glass (n=1.5). What thickness of film would result in destructive interference
The lights wavelength is 500nm.

2. Relevant equations

n = c/v

V = F*λ

3. The attempt at a solution

I drew some diagrams, and my attempt is also on the diagram,
the diagram is attached

Also, I was wondering what an increase in n signifies?
is it increase in density?
and also, I believe a wave flips when it reflects off something with higher n? (fixed boundary) and moves as is through something with less n? (free boundary)

could someone confirm this please?

 

hmm.

λ_film = λ_air / n_film = 384.62nm

for minimum thickness, should t = 1/2 λ ?

if so

2t = λ_film
t = 192.3nm =S

 

that would mean T is 384.62nm thick?

why would it not be half of the wavelength?
if you fit in 1/2 a wavelength into the film, and it reflects as an inverted wave, wouldn't it still cancel out?

doesn't 2t = 1/2 λ suggest that an entire wavelength goes in, and then reflects on itself?

 

Ohhh okay.

total phase difference is 2t,
a quarter of the wave gets in and reflects??

T = 96.155nm?

I'm having a bit of trouble visualizing this, I drew another diagram, could you see if it's correct?