# Homework Help: Wave Optics & Two-Slit Interference Explanation

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1. Dec 4, 2014

### hdp12

I have an online homework question and my classmate told me the answer but I would really like someone to explain to me how that answer was determined. I do not understand.

Q: Consider the electric field observed at a point O that is far from the two slits, say at a distance r from the midpoint of the segment connecting the slits, at an angle θ from the x axis. Here, far means that r≫d, a regime sometimes called Fraunhofer diffraction.

The critical point is that the distances from the slits to point O are not equal; hence the waves will be out of phase due to the longer distance traveled by the wave from one slit relative to the other. Calculate the phase Φlower(O,t) of the wave from the lower slit that arrives at point O.

EQUATIONS:
E(x,t)=Eleftcos[2π/λ(x−ct)].
Φ(x,t)=kx−ωt
ω=2πf
k=2π/λ

SOLUTION:
Φlower(O, t) = 2π/λ (r−ct) + π/λ dsin(θ)

I just really don't understand how that is derived...

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2. Dec 5, 2014

### Simon Bridge

That's the trouble with just having someone tell you the answer, you end up not understanding it.
The reason it was given you as an exercise is that you benefit best from working it out yourself.
Start like this:

What determines the phase of the wave?

3. Dec 9, 2014

### hdp12

I understand that, and I rarely ask others for the solution. However I had to turn in the assignment so I got the answer but am currently going back and redoing it so as to understand it myself. I want to know how to do the problem. I promise I am neither a cheater nor a slacker. I actually enjoy a majority of the problems we are given, this one just tripped me up.

You ask what determines the phase of the wave?
Isn't it derived using the wavenumber or wavelength, distance, angular frequency or frequency, and time? such as in the formula Φ(x,t)=kx−ωt

4. Dec 9, 2014

### Simon Bridge

The phase is a physical characteristic of the shape of the wave.
It is better to think first of the physics or the geometry and then worry about equations.

In terms of the phasor model - the phase is the angle the phasor makes to the real axis.
In terms of the traditional sine wave, points of equal phase have the same instantaneous amplitude and slope.
The equation comes out as $\Phi(\vec r, t) = \vec k\cdot\vec r -\omega t + \delta$.

If you pick the origin for time where $\Phi(0,0)=0$, then, for a spherical wave, what is $\Phi(r,0)$ for some distance $r$ from the origin.

Then keep going - I cannot really be much help if I don't see where you get derailed.
If you are having trouble understahding how the above helps - don't worry: just show me your best attempt.