Lab 8 - Diffraction

# Lab 8 - Diffraction - Physics 8B Lab 8 Diffraction rev 4.0...

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Physics 8B Lab 8 – Diffraction rev 4.0 Lab 8 – Diffraction Warning: You will be using lasers in this lab. Do NOT look directly into the laser beam or at a direct reflection of the beam. Introduction When you have more than one wave (e.g. Electro-Magnetic waves) arriving at a point, you can ask how they interact or interfere with each other. The combined or total E-field is the superposition of the individual E-fields E tot = E 1 + E 2 + E 3 + ! The simplest case is when you have two waves, one of which has traveled farther than the other. In that case, we can see that when the two waves are in phase , they combine constructively , and will give a maximum intensity (i.e. bright spots ). Likewise when the two waves are out of phase , they combine destructively , and will give a minimum intensity (i.e. dark spots ). This can be expressed in terms of the Path Length Difference (PLD) between the two waves and the wavelength. Constructive Interference (Intensity Maxima): PLD = ! , " # ,0, ,2 ,3 , ! Destructive Interference (Intensity Minima): PLD = ! , " 2 , 2 , 3 2 , 5 2 , ! ** Of course, you can have cases in between as well, where the intensity is neither a maximum, nor a minimum. ** This is conceptually the case for situations such as two-slit interference and thin film interference , though for the latter, you must also account for possible phase shifts due to reflection, as well as the wavelength in the medium of the thin film. When there is more than one wave, the principle of superposition still applies, but mathematics/algebra involved can alter the way the equations appear, but fundamentally, we are still talking about whether all the waves cancel giving intensity minima , or combine in some fashion giving intensity maxima . In this lab, you will study Single-Slit Diffraction, Diffraction from Opaque Objects, Circular Diffraction, and Diffraction Gratings . **Each has its own equations, so be careful to keep them straight in your mind.**

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Physics 8B Lab 8 – Diffraction rev 4.0 Part I: Single-Slit Diffraction Here light shines on a single narrow slit of width a. The name may suggest only one wave passes through the slit, but that is misleading. The slit will be treated not as a single point source, but rather as having some finite size. So what you are studying is the interaction between all the waves that pass through this narrow opening. They will result in a diffraction pattern on a distant screen. If you work through the superposition of all the waves passing through the slit for
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## This note was uploaded on 09/28/2009 for the course PHYSICS 8B taught by Professor Shapiro during the Spring '07 term at Berkeley.

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Lab 8 - Diffraction - Physics 8B Lab 8 Diffraction rev 4.0...

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