Interference

[adrian116]

1. The problem statement, all variables and given/known data
A source S of monochromatic light and a detector D are both located in air a distance h above a horizontal plane sheet of glass, and are separated by a horizontal distance x. Waves reaching D directly from S interfere with waves that reflect off the glass. The distance x is small compared to h so that the reflection is at close to normal incidence.
a). Show that the condition for constructive interference is \sqrt{x^2+4h^2}-x=(m+\frac{1}{2})\lambda , and the condition for destructive interference is \sqrt{x^2+4h^2}-x=m\lambda . (Hint: Take into account the phase change on reflection.)


2. Relevant equations
1. d\sin\theta=m\lambda for constructive interference
2.d\sin\theta=(m+\frac{1}{2})\lambda for destructive interference
3.\phi=\frac{2\pi}{\lambda}(r_2-r_1) phase difference related to path difference


3. The attempt at a solution
I have tried to find d as
d=\sqrt{h^2+(\frac{x}{2})^2}
and the phase difference as
\phi=\frac{2\pi}{\lambda}(\sqrt{h^2+(\frac{x}{2})^ 2}-x)
but i do not know how this related to the equations (b) 1 and (b) 2

[Edgardo]

Hi,

have you made a drawing of the problem? Analyze the geometry.

Hint: What does the condition for constructive interference mean?
What's the general formula for it?

What are equations (b)1 and (b)2?

Note: The formulas that you listed in "Relevant equations" are for interference when light is incident on slits. But your problem is not about slits.

[adrian116]

sorry , not (b)1 and (b)2,
it should be from relevant equations.
As you said, am i necessary to derive other equations
for this problem?
And then following the method as when light is incident on slits
to derive the equations for this problem?

[Edgardo]

The problem asks you to derive the "condition for constructive interference".
How is this condition defined in general? (It has something to do with path difference.)

Then, make a drawing, because then you can calculate the path difference.

You don't need the equations for the slit.

[adrian116]

then , no other equations can be used?
What I need is to derive it myself by that drawing?