# Light (Constructive/Destructive Interference)

## Homework Statement

This is not a homework question but I would like if someone could explain to me when to consider what equation constructive/destructive? I learned these two equations very vaguely in class:
dsinθ = mλ or d(ym)/L=mλ
dsinθ = (m+1/2)λ or d(ym)/L = (m+1/2)λ

The power point provided for us in class says that dsinθ=mλ is constructive and the other eqn is destructive. However when we started discussing thin film interference, suddenly the equations turned to
2t = mλ
2t= (m+1/2λ)

and in some situations 2t = mλ an equation for destructive interference and vice versa. When do I also know which equation to use for specific situations?

## Homework Equations

dsinθ = mλ or d(ym)/L=mλ
dsinθ = (m+1/2)λ or d(ym)/L = (m+1/2)λ
2t = mλ
2t= (m+1/2λ)

## The Attempt at a Solution

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tech99
Gold Member
For the thin film case, the light passes twice through the film, first as the incident ray and then as the reflected ray, so the time delay is doubled.

• Vanessa Avila
ehild
Homework Helper

## Homework Statement

This is not a homework question but I would like if someone could explain to me when to consider what equation constructive/destructive? I learned these two equations very vaguely in class:
dsinθ = mλ or d(ym)/L=mλ
dsinθ = (m+1/2)λ or d(ym)/L = (m+1/2)λ

The power point provided for us in class says that dsinθ=mλ is constructive and the other eqn is destructive.
The equations have no meaning without description of the situation, and definition of the symbols. So, what was that representation about, and what are d, m, λ, and θ?

However when we started discussing thin film interference, suddenly the equations turned to
2t = mλ
2t= (m+1/2λ)

and in some situations 2t = mλ an equation for destructive interference and vice versa. When do I also know which equation to use for specific situations?
Again, what is the situation and what are d, m, λ?

The equations have no meaning without description of the situation, and definition of the symbols. So, what was that representation about, and what are d, m, λ, and θ?

Again, what is the situation and what are d, m, λ?
d is the distance, m is the number that represents dark spots/bright spots(?), and λ is wavelength.

For the thin film case, the light passes twice through the film, first as the incident ray and then as the reflected ray, so the time delay is doubled.
So this phase shift changes which equation is constructive and which is destructive?

ehild
Homework Helper
d is the distance, m is the number that represents dark spots/bright spots(?), and λ is wavelength.
distance of what from what? And what is θ?