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# P5 Revision

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• 1. Written by MJ BradleyP5 SPACE FOR REFLECTION incident ray i i-angle of incidence air Refraction The critical angle 1 glass refracted ray When light passes 2 Light refracts (bends) when it enters a substance of differ- form a more dense ent density because it changes speed. Refractive index is substance to a less glass a measure of how much a substance bends light r r-angle of refraction dense substance there is a critical 2 Calculating refractive index (n):n = light speed in vacuum n = sin i angle (c) where the 1 c Dispersion of emerging ray light speed in substance sin r Light In a grazes the bound- At angles of incidence prism light is ary surface greater than the critical Question : What is the refractive index of glass for light at an incident angle dispersed into angle TIR occurs of 30o which refracts with an angle of refraction equal to 20o? different col- ours because The Endoscope Answer: n = sin i/sinr = sin30/sin20 = 0.5/0.34 = 1.47 each colour (or frequency) is In an endoscope some opti- refracted differ- cal fibres totally internally How Optical Fibre works entlyTotal Internal Reflection reflect light down towards less dense layer the stomach (in this exam- cats-eye. Light is ple) and other optical fibres i internally reflected inside dense the glass prism i take the reflected light from Violet is the stomach back to be TIR occurs when the refracted the incident ray is at an angle, i, most and red viewed by a camera and greater than the critical The light totally internally reflects at the the least screen angle inside a dense boundary of the two layers. i must be material greater than the critical angle Convex lens: these refract light rays inwards Optical fibre and Communications Digital signals (on and off) can be transmitted using Fibre optic cable optical fibre. Infra-red pulses in digital code are totally internally reflected along the fibre principal axis C f Light is internally reflected down the cable. Endoscopes Diffraction: This is the spread- f use fibre optic cable The fatter the ing out of waves when they pass lens the shorter through a gap or pass an object. the focal lengthWave revision: The effect is greatest when the The greater wavelength equal the gap width the ampli- Principal focus: rays parallel to the principal wavelength amplitude tude the brighter axis are refracted towards the principal focus (f). the light The distance cf is the lens FOCAL LENGTH or the louder the Images from a convex lens sound Rule1: A ray parallel to the principal axis is refracted through the principal focus FInterference: Waves can interfere with each other constructively Rule 2: A ray through the optical centre does not change direction.(add) or destructively (cancel each other out) peaks and The peaks of tree lens troughs one wave add F F match to the troughs of the other 2F image of tree the image is upside down and smaller,It is a REAL image - it can be destructive interference projected onto a screen constructive interference tree An Interference Experiment F F The sound from B quiet 2F the image isWaves can interfere with each other has travelled 1/2 a u p s i d eto produce patterns as shown exam- wavelength further loud down and isples are light, sound and microwaves than from A. Peaks magnified match with troughs quiet The image is now and destructive inter- the correct way up It is a real ference occurs loud and magnified. The image A eye traces back quiet The sound from A where the rays loudspeaker and B has travelled seem to have come A magnifying glass! loud the same distance. from. It is not a real loudspeaker Peaks match with image we call it a quiet peaks and con- virtual image Other loud areas are heard structive interfer- 2F F F because the path differ- loud ence occurs B ence between the two Magnification waves is a whole number quiet of wavelengths. Peaks magnification = height of image match with peaks and con- structive interference oc- loud height of object curs. Why are other quiet A lens Camera quiet eye lens Projector film - not digital! convex lens bulbtree The lens can move forwards or backwards The lens is moved in screen to make a sharp focus. The closer the ob- or out to focus the ject the further the lens must be pulled image on the screen outwards slide