Elektroonika Alused
The laser used for the CD player is typically an AlGaAs laser diode with a wavelength in air of 780 nm. (Near
infrared -- your vision cuts out at about 720 nm). The wavelength inside the polycarbonate is a factor of n=1.5
smaller -- or about 500 nm.
The pit/bump is carefully fabricated so that it is a quarter of a wavelength (notice a wavelength INSIDE the
polycarbonate) high. The idea here is that light striking the land travels 1/4 + 1/4 = 1/2 of a wavelength furthe
than light striking the top of the pit. The light reflected from the land is then delayed by 1/2 a wavelength -- an
so is exactly out of phase with the light reflected from the pit. These two waves will interfere destructively -- s
effectively no light has been reflected.
The spacing between pits is equally carefully selected. Recall from basic optics that the image of a beam passi
through a round aperture will form a characteristic pattern called an Airy disk. The FWHM (full-width half-
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