Simple Nature - Light and Matter

object that diffracts it, so the triangle is long and skinny. Most realworldexamples with diffraction of light, in fact, would have triangleswith even skinner proportions than this one. The two long sides aretherefore very nearly parallel, and we are justified in drawing theright triangle shown in figure o, labeling one leg of the right triangleas the difference in path length , L−L ′ , and labeling the acute angleas θ. (In reality this angle is a tiny bit greater than the one labeledθ in figure n.)The difference in path length is related to d and θ by the equationL − L ′d= sin θ .Constructive interference will result in a maximum at angles forwhich L − L ′ is an integer number of wavelengths,L − L ′ = mλ . [condition for a maximum;m is an integer]Here m equals 0 for the central maximum, −1 for the first maximumto its left, +2 for the second maximum on the right, etc. Puttingall the ingredients together, we find mλ/d = sin θ, orλd = sin θm. [condition for a maximum;m is an integer]Similarly, the condition for a minimum isp / Cutting d in half doublesthe angles of the diffractionfringes.q / Double-slit diffraction patternsof long-wavelength red light(top) and short-wavelength bluelight (bottom).λd = sin θm. [condition for a minimum;m is an integer plus 1/2]That is, the minima are about halfway between the maxima.As expected based on scaling, this equation relates angles to theunitless ratio λ/d. Alternatively, we could say that we have proventhe scaling property in the special case of double-slit diffraction. Itwas inevitable that the result would have these scaling properties,since the whole proof was geometric, and would have been equallyvalid when enlarged or reduced on a photocopying machine!Counterintuitively, this means that a diffracting object withsmaller dimensions produces a bigger diffraction pattern, p.788 Chapter 12 Optics