Waves
and optics
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The model called Wave Optics uses the concept of an electromagnetic (EM) wave to describe the propagation of light.
To simplify the description of many effects, the following notions are often used:
wave front - is the surface to which the wave has reached at a given time
plane wave (an analogous to infinite wide beam of parallel rays in geometric optics)
spherical wave (an analogous to rays diverging from a point).
Obviously, the front of a plane wave is a plane, and the front of a spherical wave is a sphere.
Many unexpected, specific, interesting results are due to the fact that light propagates and interacts with surrounding objects as a wave, and we observe the time-averaged or accumulated energy (intensity) carried by this wave. And here is a big difference with the "usual mechanics". The effect of the fall of two stones is twice that of the fall of one. But with waves it's more complicated.
The simplest wave - of the sin(x) type - has both positive and negative values. When two such waves are added to each other, the result depends on the shift of one wave relative to the other (such a shift is also called the phase difference). If there is no shift, or if it is equal to an integer number of wavelengths (corresponding to a phase difference of 2Οn), then the result is a wave with a double amplitude. And if the shift is half wavelength (the phase difference is Ο) or an odd number of halves, then the waves cancel each other, and as a result we get ... 0. For other shift values, the sum - of course - changes. So, adding of many different waves (with different phases and amplitudes) produces very different results.
Example. Zero result when adding two identical waves with a shift of 0.5 wavelength
The observed value - intensity - is proportional to the square of the amplitude of the sum of all interacted waves at the point of observation.
Example. The square of the sum of two waves for different phase shifts. Maximum amplitude at zero phase shift. Minimum amplitude (0) at phase shift equal to Ο (half wavelength).
Example. The intensity of the sum of two waves depending on the phase shift (from 0 to half a wavelength).
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