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Physicists joke:Light is the darkest spot in physics
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James Clerk Maxwell showed that light is a transverse electromagnetic wave with the speed in vacuum c = x10 8 m/s Свет – это поперечная электромагнитная волна The evidence for the wave nature of light was provided by the following phenomena: - Diffraction - Interference - Dispersion - Polarization
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Diffraction – is the spreading of wave at the edges of obstacles (i.e. the wave could spread out from the straight-on direction) Дифракция света – это отклонение света от прямолинейного распространения (огибает препятствия, сравнимые с длиной световой волны)
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In case of diffraction the wave bends around an obstacle or through a gap; it spreads into geometrical shadow For light it could be diffraction by a single slit or a disk – Дифракция может быть от щели или препятствия The resulting pattern of light and dark stripes or rings is called a diffraction pattern This pattern arises because different points along a slit or ring create wavelets that interfere with each other
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The minima of the single-slit diffraction pattern occur when
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A diffraction grating consists of many fine parallel lines or slits (about 1000 or per cm) ruled closely on a piece of glass or plastic The more lines or slits there are, the narrower the peaks
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The maxima of the diffraction pattern are defined by the formula dsinθ = mλ, where m = 0,1,2,3, … – order of diffraction maximum, d = 1/N – slit separation (N – number of lines per unit length of the diffraction grating; it is also equal to the total width of transparent and non transparent parts of the grating), θ – angle the m-th order diffraction makes with the Zeroth order
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Fringe separation, here D – distance from the screen to slits, – slit spacing (distance between the centers of the slits)
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The spread of light waves could be explained by the use of Huygens Principle Huygens principle: Every point on a wave front acts as a point source; the wavefront as it develops is tangent to their envelope Принцип Гюйгенса: каждая точка среды, до которой дошли колебания, сама становится источником вторичных волн
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Huygens principle is consistent with diffraction:
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If light is a wave, interference effects will be seen, where one part of wavefront can interact with another part One way to study this is to do a double-slit experiment:
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If light is a wave, there should be an interference pattern
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We can use geometry to find the conditions for constructive and destructive interference: dsinθ – path difference – разность хода
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Between the maxima and the minima, the interference varies smoothly
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Since the position of the maxima (except the central one) depends on wavelength, the first- and higher-order fringes contain a spectrum of colors
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Visible light – is the light with the wavelengths from 390nm to 760 nm Shorter wavelengths are ultraviolet; longer are infrared
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Prism splits visible light into a rainbow of colors
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Dispersion – is the dependence of the index of refraction of a material on the wavelength (color) of the light Дисперсия – зависимость показателя преломления света от его цвета, т.е. длины световой волны
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Index of refraction n – is a number that describes by how much light slows down when it passes through a certain material The wavelength of the light is also changing when light passes through a material The frequency of the light remains constant, even though the wavelength changes – Частота света не меняется
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Actual rainbows are created by dispersion in tiny drops of water
