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**What’s so Special about a Laser?**

A laser is a monochromatic and coherent light source. Monochromatic: One Color… Therefore One Wavelength. Coherent Waves: Same Frequency, similar amplitude and constant phase relationship.

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**How does a laser work http://www.youtube.com/watch?v=y3SBSbsdiYg**

Laser stands for "Light Amplification by Stimulated Emission of Radiation". Stimulated emission, refers to a process where 'electrons that are in an excited state' emit photons and drop to a lower state. This is caused by another photon passing by the excited electron (giving a total of two photons). By nature, both photons are of the same wavelength and are in phase (explanation is not required). Lasers work by pumping lots of energy into the laser such that over 50% of the electrons in a medium are in an excited state. It then has two mirrors, one of which lets 1% of photons through (this is the light we see).

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**Young’s Double Slit Experiment**

If a coherent light source is shone through two slits that are close together, the two slits act like two coherent light sources. Since light is a wave the two sources will interfere. Interference can be constructive or destructive.

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**Waves travel the same distance: still in phase. **

Different distance: may not be in phase. If one wave travels nl further: Constructive interference. If one wave travels (nl+1/2l) further: Destructive interference.

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**Young’s Double Slit Experiment**

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The Geometric Model The distance between the silts d is very small when compared to the distance between the silts and the screen at point P. Therefore we can treat the lines S1P and S2P as parallel. The line S1Z is drawn such that S1P and ZP are identical lengths. This means that the angle S2S1Z is very close to θ.

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**Geometric Model So the path difference of the two sources is S2Z.**

If the path difference =nl : constructive interference/bright fringe

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**x = distance from the center point to the nth bright fringe **

D = distance between the screen and the slits. Because θ is small we can make the approximation: For bright fringes and For dark fringes

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Diffraction Gratings

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**Diffraction and Interference**

Diffraction and interference are similar phenomena. Interference is the effect of superposition of 2 coherent waves. Diffraction is the superposition of many coherent waves.

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**What is a diffraction grating?**

• Consists of a flat barrier which contains many parallel slits separated by a short distance d. • A parallel monochromatic light beam passing through the grating is diffracted by an angle θ similar to two slit interference. However, the intensity of the diffracted light is higher and the peaks are much narrower.

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**The equations are the same for two slit and multiple slit interference.**

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**A diffraction grating can be used as a spectrometer**

Dispersion of light of different wavelengths

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