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Optics (Total internal reflection (Optical fibres (• Used in medical…
Optics
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Double slit interference
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Young's fringes
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• Where bright fringe is formed the light from one slit reinforces the light from the other slit (light is in phase – 2π radians) / λ difference
• Where a dark fringe formed light from one slit out of phase with other slit (cancels out) – phase difference π radians / half a λ difference
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Interference (more)
Coherence
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• Light from two nearby lamps cannot form an interference pattern as two light sources emit waves at random – points of cancellation and reinforcement would change at random
Wavelength and colour
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• Due to red light having longer wavelength than blue light so bigger λ means bigger w when looking at equation
Light sources
• Vapour lamps and discharge tubes produce light with dominant colour eg. sodium vapour lamp emits yellow glow due to λ of 590nm (basically monochromatic light source)
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• As laser beam almost perfectly parallel and monochromatic, convex lens can be used to focus it to a fine point
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White light fringes
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• Outer fringes merge into indistinct background of white light - becomes fainter with increasing distance as where fringes merge colours reinforce so overlap
Diffraction
Observing diffraction
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• Diffraction of light by a single slit can be demonstrated by directing a parallel beam of light at the slit
• Diffracted light pattern forms central maxima (fringe) with subsidiary maxima’s (fringes) which are half the width of the central fringe
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Single slit diffraction
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• W (width of central fringe) = ( λ × 2D) / a where D is the distance between the slit and the screen, and a is the width of the single slit