Our fingerprints are made up of a series of ridges on the end of the finger. These ridges can be ‘read’ by electronic devices.
The diagram below shows a certain type of fingerprint detector below, which works by scattering light when pressure is applied to a plastic screen. The ridges of the fingerprint press the plastic film against the glass when the finger is pressed to the screen.
When not in use, the plastic screen and the glass have a small air space between them. This is shown in the diagram below which shows two possible paths for the light when the finger is pressed down.
Which line, A to D in the table below, correctly describes the properties of the fingerprint detector that causes the bright and dark patches on the image produced by the camera?
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Bright
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Dark
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A
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identical optical densities of the glass and plastic
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different optical densities between the glass and air
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B
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different optical densities of the glass and plastic
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different optical densities between the glass and air
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C
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the plastic screen has a lower optical density than soft tissue in the finger
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total internal reflection within the glass sends light rays to the camera
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D
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identical optical densities of the plastic screen and soft tissue in the finger
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identical optical densities between the glass and plastic prevent light reaching the camera
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You may assume that plastic and glass have identical refractive indices n = 1.5.