## Wave Optics

# Diffraction

- Diffraction Confirms wave nature of Light
- The Light waves are diffracted only when the size of the obstacle is comparable to the wave length of Light.
- Condition for observing diffraction.

\tt l\approx\frac{b^{2}}{4\lambda}

l = distance between screen and object

b = size of object - In Fresnels diffraction sources and screen are at finite distance
- No Lenses are required in fresnels diffraction.
- The incident wave fronts in Fresnels diffraction are either spherical (or) cylindrical.
- In Fraunhoffer diffraction source and screen are at infinite distance
- Lenses are required to observe the fraunhoffer diffraction.
- The incident wave fronts in Fraunhoffer diffraction are plane wave fronts.
- Fraunhoffer diffraction is limiting case of fresnels diffraction.
- Condition for minimum intensity in fraunchoffer a sin θ = nλ {n = 1, 2, 3 …….} (a = width of slit)
- Condition for miximum intensity in fraunchoffer \tt a \sin\theta=\left(2n+1\right)\frac{\lambda}{2} {n = 1, 2, 3 …….} (a = width of slit)
- Width of central maxima\tt \beta_{0}=\frac{2D\lambda}{a}=\frac{2f\lambda}{a}

### View the Topic in this video From 00:18 To 8:11

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**Diffraction of a single slit:**1. For minima

*d*sin θ = nλ, where

*n*= 1, 2, 3 .....

2. For maxima

*d*sin θ = (2n + 1)\frac{\lambda}{2}, where

*n*= 1, 2, 3 ....