Unformatted text preview: PHYS 302: Unit 12 Viewing Notes – Athabasca University PHYS 302: Vibrations and Waves – Unit 12 Viewing Notes
Early in the lecture, Dr. Lewin refers to Isaac Newton’s (16431727) espousal of the particle view of light and its
refutation by Thomas Young (17731829) in 1803. While Newton started presenting his results in the 1670s,
Newton and Young’s work was presented together in the important book Opticks only in 1704 (now viewable at
Google books). Christian Huygens (16291695) was roughly the contemporary of Newton. He formulated his wave
theory of light before Newton’s particle view. Newton’s particle view of light explained reflection simply. It also
explained refraction, with many elaborations. Newton’s influence, based on the scope of his experimental work (as
outlined in Opticks), prevailed for nearly 100 years. We now know (as shown in earlier units with discussion of
quantum mechanics) that not only does waveparticle duality apply to light, but also to particles. While a naïve view
of the world is based on particles (allowing for successful analysis at the introductory level), in some sense particles
do not exist, except as an abstraction or limiting case!
A somewhat discouraging discussion about Huygens’ principle (also known as the HuygensFresnel principle in
recognition of Fresnel’s work in building a theoretical framework after Young’s experiment) ends at 5:30. While
Dr. Lewin describes the principle as “giving the right answer for the wrong reason,” in fact, Huygens’principle
relates to the most advanced form of quantum mechanics: quantum electrodynamics (QED). In this approach,
usually associated with Nobel Prize Winner (1965) Richard Feynmann, wavefunctions of an object propagate over
any and all allowed (unobstructed) paths from the source of a target point. The result of interference (addition) of
contributions from all paths defines the amplitude and phase of the wavefunction of the object at the measuring
point, and it defines the probability of finding the object at this point. All particles have wave characteristics—not
only photons, but also electrons, neutrons, protons, atoms, molecules, and in fact, all other objects. So, this “path
integral” approach is, in principle, applicable to all particles. While applying the path integral approach may be
difficult, Huygens’ principle allows simple visualization of a similar behaviour. Thus, it does have a physical basis,
although Huygens could not have imagined this.
Destructive interference, or light+light=darkness, results when a phase difference is ½ λ (6:50). Destructive
interference takes place on hyperboloidal surfaces (as some geometry would show). Constructive interference may
be less surprising than destructive interference, since it results in light+light=more light (however, as we will see
below, more than one might initially expect). Constructive interference takes place when there is no phase
difference. In both destructive and constructive interference, phase differences are cyclical: the angle goes through
the 2π radians that constitute a full circle. The same behaviour results as one adds (through path length changes)
integral multiples of 2π as phase differences (8:30). At a great enough distance, hyperboloidal surfaces appear to be
nearly straight. As such, simplified analysis can proceed on that basis. If one is far away from two slits that are
separated by a distance d and off the axis (perpendicular to the surface having the slits in it) by an angle θ, the
difference in distance from the observation point to the slits is dsinθ (10:30). This is the physical path difference. As
d
a fraction of a wavelength this is sin . Since one wavelength corresponds to a 2π advance in phase, the increase
in phase by this fractional increase in number of wavelengths (in the path) is d 2 sin . (Note that δ is pronounced delta and is the lower case form of Δ.) For constructive interference, where crests of waves add to crests
of other waves to make a higher amplitude wave, one must have δ=n2π, where n=0,±1,±2,… (11:40). Another way
to write this condition is dsinθn =nλ. For destructive interference δ=(2n1)π or d sin n 2 n 1 (13:30). Next,
2
Dr. Lewin demonstrates red laser light (coherent light; see Reading Notes) of wavelength 600 nm going through two
slits separated by 0.25 mm (¼ mm), viewed 5 m away. The constructive interference formula with λ=600 nm,
L=5 m, and d=0.25 mm gives x±1=1.2 cm and x±10=12 cm (19:00). Dr. Lewin draws a sketch with maxima spaced in
units of sinθ by λ/d. Note that for smaller wavelength light, the maxima will be spaced by less than λ/d (23:00). The
sketch shown is for intensity, in units of W/m2. Now the shape of the curve as cos2 can be derived. With a phase
difference of δ, the wave from slit 1 and slit 2 arrive as E1=E0cosωt and E2=E0cos(ωtδ) and sum, using the familiar
“cos half the sum times cos half the difference” identity, to Etot 2 E0 cos t cos (24:30). The
2
2 intensity is the square of the amplitude (the term Poynting vector is not explained in this course), so a cos 2 2 intensity results, with the first term having very rapid variation (of order 1015 Hz), which averages away (25:00). 1 PHYS 302: Unit 12 Viewing Notes – Athabasca University The demonstration is followed by some discussion of the waveparticle and detectability properties of photons
(29:25). A slide shows the effects of wavelength.
Sound interference occurs with all the same equations as for the case of light discussed above, but the wavelength of
sound is much longer than that of light (in this case, 11.3 cm for 3000 Hz, with separation 1.5 m). As such, the
sound in the video lecture demonstration should have 26 surfaces of interference (both minima and maxima) in the
classroom. At a distance of 5 m, the angle of 4.3º from maximum to maximum corresponds to 38 cm. Dr. Lewin
demonstrates sound interference before the break (38:20).
So far, path difference has been obtained through simple geometry. Now we examine a slightly more complex way
of getting path difference, through transmission and reflection in a medium. Consider a thin, horizontal oil film of
thickness d between levels A and B, with index of refraction in the film of 1.5 (that of air is 1.0, above and below)
(41:15). The index of refraction is the inverse ratio of the speed of light in vacuum and in the medium. That is, light
travels 1.5 times slower in oil than in vacuum or in air. The reflection coefficients are the same as those for a paired
rope with different speeds on either side of the joint, and the sign of reflection is negative (42:00). As you view the
remainder of the video lecture, note that you will not be responsible for the mathematical explanations given. You
are expected to know the following about thin films:
1)
2)
3)
4) Path differences can occur through combinations of reflection and phase changes at boundaries.
Path differences give rise to phase differences that can cause destructive or constructive interference.
Phase differences depend on the index of refraction of the medium, its thickness, and its wavelength.
For some wavelengths, constructive interference can give rise to colours in thin films; for other
wavelengths (e.g., long ones), this may not be possible. Antireflective coatings for light or other electromagnetic radiation can be made from appropriate thin films. This
point is not covered in the lecture, yet is of great importance where reflections are not desired (e.g., eyeglasses) or
where they are desired (e.g., heatretentive windows). Also, the ideas presented that focus on optical wavelengths
apply completely to other EM wavelengths. For example, a common cause of poor radio reception is “multipath,” in
which waves that have reflected from various objects (e.g., buildings) recombine (having followed different paths)
and cancel. This effect is not restricted to EM radiation, and is an important part of the acoustic design of buildings. 2 ...
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 Spring '10
 martinconner
 Diffraction, Light, Wavelength

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