Lesson LVIINature of Light, Huygens’

Principle, Interference: Young’s Double-Slit

Experiment

Nature of Light• Issac Newton – light was particle in nature

(corpuscles)o Lasted until 1830 when scientists found that light traveled in waves,

which explained interference and is not possible with a particle explanation.

o Light has properties of wave-like and particle-like motion.

• Dual Nature of Lighto When we study light we must look at it from a particle or wave like

motion, but never both at the same time (Niels Bohr)

Huygens Principle• Christian Huygens proposed a wave theory of

light that showed the propagation of a light wave could be predicted geometrically by considering each point on a wave front to be a source of a spherical secondary wavelet.

Huygens Principle• Similar to waves in water being created from a

pebble dropped in it, light waves move in concentric spheres from a source.

• All wavelets move from the original wave front at the same speed.

• Huygens’ Principle – Every point on a wave can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets that are tangent to all of them.

Diffraction• Light is bent when it goes through a small

opening

Diffraction• Thomas Young was able to show light diffraction

in an experiment. He was even able to determine the wavelength of visible light from his experiment.

• This showed how there were flaws in the “corpuscular” theory of light.

Interference: Young’s Double Slit Experiment

• In Young’s famous experiment, he used sunlight which fell on a screen containing two closely spaced slits. The slits were spaced a very small distance apart.

Young’s Double Slit Experiment

• If light was a particle, he expected two bright lines to appear. Instead Young observed series of dark and bright lines called fringes.

Young’s Double Slit Experiment

• We imagine light approaching the slits as uniform in wavelength (monochromatic). The waves spread out after passing through the slit.

Young’s Double Slit Experiment

• θ is the angle that a ray is deflected from the middle.

Young’s Double Slit Experiment

• The diffracted waves interact and create areas of constructive and destructive interference.o There is a phase shift in the wave that has to travel a

longer distance.

• One ray travels further than the other, except in the very center of the screen where central maximum occurs.

• Constructive interference occurs when the paths of the two rays differ by whole number multiples of their wavelengths.

• Destructive interference occurs when one ray has to travel an extra distance of ½ a wavelength.

Young’s Double Slit Experiment

• Constructive and Destructive Interference o dsinθ = mλ where m is order of the

interference fringe.• m = 1 is the first fringe on either side of the

central maximum.• Constructive occurs at m=1, 2, 3, 4, etc…• Destructive occurs at m = ½ , 3/2 , 5/2, etc…• The intensity of the light decreases as you move

away from the central maximum.

Young’s Double Slit Experiment

• Through a little geometry, we geto Xm = mλL/d

Example I• Monochromatic light falls on two slits,

0.035mm apart. A fifth-order fringe makes an angle of 9.5° to the central maximum. What is the wavelength of the light?

Answer• 1.6x10-6m

Example II• A screen is placed 1.20m from two

narrow slits spaced 0.100mm apart. Light fron a distant source is incident on the slits. The light has a wavelength of 500nm. A) Find the distance from the central maximum to the first bright fringe. B) Find the distance from the central maximum to the second minimum.

Answer• 6.00mm, 9.00mm