Proving Babinet's Principle w/ Superposition & Diffraction - Help Needed

[KillaMarcilla]

Yo, d00dz

I'm kind of stumped on this problem on my homework: "A monochromatic beam of parallel light is incident on a hole of diameter a >> wavelength. Point P lies in the geometrical shadow region on a distant screen. Two obstacles are placed in turn over the hole. A is an opaque circle with a hole in it and B is the "photograhpic negative" of A (a circle with an opaque hole in it) Using superposition concepts, show that the intensity at P is indentical for each of the two diffracting objects A and B (Babinet's principle)"

I'm just clueless as to where to start on this.. I was about to fire up a point-by-point analysis, but this class doesn't really require knowledge of integration, so I don't think that's the right way to go about finding the answer

Can anyone lend a hand?

I'm going to stay up for a while seeing if I can't help anyone else on their homework, and then I'll get up in the morning early, in case any people in other time zones show up

 

[quantumdude]

I think that page 5 of this document will help you:
http://www.upscale.utoronto.ca/IYearLab/intdif.pdf

 

[M98Ranger]

and help me out

Hey d00dz,

Babinet's Principle states that when two complementary objects are placed in the path of a wave, the diffracted waves from each object will cancel each other out and the resulting intensity at any point will be the same as if neither object were present. This principle can be proven using the concept of superposition and diffraction.

To start, we need to understand what is meant by "complementary objects". In this case, object A is an opaque circle with a hole in it, while object B is a circle with an opaque hole in it. This means that when the light passes through object A, it is blocked by the opaque circle but passes through the hole, while in object B, the light is blocked by the hole but passes through the opaque circle.

Now, let's consider the diffraction pattern created by each object. When a wave passes through a small opening, it creates a diffraction pattern on a screen placed in its path. This pattern consists of a central bright spot surrounded by alternating bright and dark rings. The size and intensity of these rings depend on the size and shape of the opening.

Since both object A and B have the same size and shape of the opening, they will create the same diffraction pattern on the screen. However, in object A, the central bright spot will be blocked by the opaque circle, while in object B, the central bright spot will be created by the hole in the opaque circle. This means that the intensity at the central bright spot will be the same for both objects.

Now, let's consider the rest of the diffraction pattern. In object A, the bright rings surrounding the central spot will be created by the light passing through the hole, while in object B, the bright rings will be created by the light passing through the opaque circle. However, since the size and shape of the opening is the same in both objects, the size and intensity of the bright rings will also be the same. This means that the overall intensity at any point in the diffraction pattern will be the same for both objects.

To summarize, since the diffraction patterns created by object A and B are the same, but with opposite intensities, they will cancel each other out and the resulting intensity at any point on the screen will be the same as if neither object were present. This proves Babinet's Principle using the concept of superposition and diffraction.

I hope this helps and good luck with