Optics: Young’s Double Slit Experiment

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Thomas Young In 1801, Thomas Young conducted an experiment demonstrating the wave nature of light. (Englishman) For 100 years Newton’s

“Corpuscular” theory of light heldsway.

He was 28 years old. Learned to read at 2, knew Latin, Greek,

French and Italian by 13 Attempted to translate the Rosetta Stone

(left the efforts worse off than when he started)

Double Slit Experiment Light from a single source

(Young used the Sun) is shown on two slits spaced closely together.

IF light consists of tiny particles, then you’d expect to see two bright lines placed behind the slits as in (b) of the diagram to the left.

BUT Young observed a series of bright lines as in (c) of the diagram. Young EXPLAINED this result as a wave-interference phenomenon

Double Slit Experiment

Imagine plane waves of light of a single λ, (monochromatic) falling on the two slits as shown here.

Because of diffraction, the waves leaving the 2 small slits spread out [sort of like 2 rocks thrown into a pond]

Double Slit Experiment

How is the interference pattern made on the screen? What is a bright line? What causes the dark lines?

Waves of wavelength λ are shown here entering the slits S1 and S2 which are a distance, d, apart

Double Slit Experiment

Constructive interference occurs whenever the paths of 2 rays differ by any whole number of wavelengths

The waves reaching the center of the screen [θ = 0] have traveled the same distance so are IN PHASE and interfere constructively resulting in a bright area

Double Slit Experiment

The waves spread out in all directions, we are focusing on only three different angles

IF one ray travels at a ½ λ [ or 3/2 λ or 5/2 λ and so on] therefore destructive interference occurs and the screen is dark

Double Slit Experiment To Determine WHERE

the bright lines fall the distance d between

the slits is small compared to L the distanceto the screen

This makes the rays from each slit essentially parallel and θ is the angle they make with the horizontal

From the shaded right triangle in (b), we can see that the extra distance traveled by the lower ray is

d sin θ = mλ, m = 0,1,2...

Double Slit Experiment The value of m is the

order of the interference fringe

Destructive interference occurs when the extra distance d sin θ = (m + ½)λ,

m = 0,1,2... The INTENSITY of the

bright fringes is greatest for the central fringe (m = 0) and decreases for higher orders

Double Slit Experiment We can see that except for

the zeroth-order fringe at the center, the position of the fringes depends on the

Consequently when white light falls on the 2 slits, as Young found in his experiments, the central fringe is white, BUT the first-and higher-fringes contain a spectrum of colors like the rainbow.

Young could then determine the wavelengths of visible light.