**Young’s Experiment**

Two parallel and very close slits S

**and S**

_{1}**(illuminated by another narrow slit) behave like two coherent sources and produce on a screen a pattern of dark and bright bands – interference fringes.**

_{2}For a point P on the screen, the path difference

Where d is the separation between two slits, D

**is the distance between the slits and the screen and y1 is the distance of the point of P from the central fringe.**

_{1}For constructive interference (bright band), the path difference must be an integer multiple of

*l*, i.e.,

The separation Δy1between adjacent bright (or dark) fringes is,

using which

*l*can be measured.

**Young’s Double Slit Interference Experiment:**

where D is the distance between the slits & the screen d is the distance between the two slits

**Constructive Interference:**

a) Phase difference : D

*f*= 2

*p*

*n*where n is an integer

b) Path difference: D

*X*=

*nl*where n is an integer

**Destructive interference:**

**Diffraction due to Single Slit:**

Where D is the distance of the slit from the screen d is the slit width

**Condition for the Minima on the either side of the Central Maxima:**

**Relation between phase difference & path difference:**

Where ∆

*f*is the phase difference & ∆X is the path difference

**Diffraction:**

a) It refers to light spreading out from narrow holes and slits, and bending around corners and obstacles.

b) The single-slit diffraction pattern shows the central maximum ( at

*q*= 0) , zero

intensity at angular separation

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