Light Waves: Interference, Diffraction, and Polarization Explained
Discover the fundamental features of light waves, including interference, diffraction, and polarization. Explore the concept of waves in daily life and how disturbances in a medium lead to wave propagation. Learn about wave interactions and superposition, as well as the intriguing phenomena of interference based on wave characteristics like wavelength and phase relationships.
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Presentation Transcript
The light is a form of electromagnetic wav e (radiation). But, how do we know? Basic features of the light wave: Interference Diffraction Polarization
There are many examples of waves in da ily life: Water wave, sound wave, human wave in a stadium, , but what is a wave? A wave is a propagating disturbance of s ome equilibrium, quiescent state. A wave needs a medium, like air, water, people in a stadium. The medium consists of individual particles which are normally in a motionless , equilibrium state.
Disturbance? Parts of the medium or some particles in the medi um move away from the equilibrium because of an external force acting on it. In many examples, the motion of the particl es is simple harmonic. Particles in the medium is interactive, when one is off equilibrium, it drags it neighbors into harmonic motion as well (with a little ti me delay:). Then the disturbance of one part icle can propagate in the medium => Wave!
When two waves come together, what hap pens? The displacement (or disturbances) will add to gether (SUPERPOSITION) If at a point in the medium, two waves are pulli ng in the same direction, the displacement will be the sum of the two individual displacements . If two waves are pulling in the opposite directio ns, the resulting displacement is the difference.
When two arbitrary waves are superimposed, th e result is very complicated . If two waves have the same wavelength, t he locations of reinforcement and cancell ation may be fixed in space for a long tim e, making in possible to SEE the superpo sition. We call this phenomenon Interfere nce There are then destructive and constructi ve interferences.
The result of the superposition of two wa ves depends not only on the magnitude of the waves, but also on the phase relatio n. When the waves are in phase, the two crest s coincide, they reinforce each other, the net result is a large net motion. When the two waves are out of phase, th e crest of one wave meets the valley of anoth er, the net result is a cancellation.
When the particles motion is harmonic, the medium can support the simplest w ave: Sine Wave. Frequency ( ) of a Sine Wave = frequency of every particle s oscillation frequency. Wave length ( ) the distance from the nea rest particle which does the same oscillation.
Constructive source at nearly the same pla ce (figure) Destructive source at nearly the same plac e (figure) Small unmounted speakers Two sources separated by half a waveleng th. Same phase. Out of phase.
If the light is a wave, how come that we do n t see much of the interference phenome na? We need two sources of light with FIXED phase relation. If the phase is not fixed and it jumps aro und, the interference gets washed out. Most of t he light sources have very short memory of phas e and are incoherent. If two light sources have a fixed phase relation, we call then Coherent.
When the light is reflected from a thin fil ms, there are two reflections: from the fro nt surface and the back one. The two refl ections have a fixed phase relation and i nterfere coherently. In the first case, the reflection is hard and th ere is a 180 degrees of phase difference. The second reflection is soft, yielding no ph ase difference.
If d is the thickness of the film, 2d is the ex tra distance the second reflection travels. I f 2d is of the wavelength, the two reflecti ons interfere constructively: very little trans mission. If 2d is equal to integer number of wavelen gth, two waves interfere destructively: 100 % transmission.
The thickness of a film may vary, from whi ch we can find interference patterns Applications: Camera lens coated with a thin film to reduce the reflection. Can only do for one wavelength. Question: what do you see when there is a layer of oil on the surface of water?
