Question

Consider a plane wave impinging on the edge of an opague material(See attached file).Is it possible to determine the diffraction pattern within the framework of Fraunhofer diffraction? Explain your answer.

Answer 1

ya the wave K1 will get break into 2 waves in 2 direction taking some curve will form bigger image then the object the is coxed by diffraction becox the wave is very near to a non transparent object....

Answer 2

TNX for the reply but I need some more explanation.

Answer 3

There are two types of simple diffraction models: Frensel and Fraunhofer.

When you apply the two depends on three variables:
- wavelength, \( \lambda \)
- distance from apeture to screen where you are observing the pattern, \( L \)
- size of aperture, \( a \)

Qualitatively the Fraunhofer diffraction model works best when the screen is far awe from the source of the diffraction relative to the wavelength, and when the aperture itself isn't very large. In this case, it's clear the screen is far away, but the aperture is large, hence Fraunhofer isn't the way to go.

Quantitatively, what we for Fraunhofer diffraction in terms of these three variables is that

\[ \frac{a^2}{\lambda L} << 1 \]
This is clearly not the case.

For Frensel diffraction, the model works when
\[ \frac{a^2}{\lambda L} \geq 1 \]

Hence the appropriate model to use here is Frensel diffraction.

Answer 4

**Correction: Fresnel, not Frensel.

By the way, this quantity
\[ F = \frac{a^2}{\lambda L} \]
is called the Fresnel number.

Answer 5

James again nice job I'm reading it now ;)

Answer 6

good.