Date created: December 30, 1997
Last modified: December 30, 1997
Maintained by: Peter Watson
watson@physics.carleton.ca

Derivation of formula for Youngs slits:

D₁² = D² + (y-d/2)² = D² + y²-yd + d²/4

and obviously

D₂² = D² + (y+d/2)² = D² + y²+yd + d²/4

subtracting..

D₁² - D₂² = yd

so

(D₁- D₂)(D₁+ D₂) = yd

but

D₁+ D₂ ~ 2D

and for constructive interference

D₁- D₂ = nλ

giving

y = nλ D       
     d 

d sin(θ) = nλ or

θ = nλ
    d