When a laser beam is aimed at a wire, a circle of light can be observed on a screen perpendicular to the wire.
Explain this phenomenon and investigate how it depends on the relevant parameters.
pHYSICS
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Reflection
Light transmission
Conditions
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Wire diameter <<< screen distance from incidence
Wire surface is reflective
Wire cross sectional edge is smooth (monotonically and continuously increasing normal angle)
Viewing screen is perpendicular to the wire.
Why Reflection
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1. Circle geometry
2. Wire material
Why Reflection
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Circle geometry
Circle of light always centred on where wire meets the screen.
For beam diameter > wire diameter, point of high intensity (POHI) position remained unchanged with/without wire.
POHI is intersection of straight line parallel and from the light source.
Circle touches POHI
Why Reflection
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Wire Material
Circle only showed up from wires with shiny surfaces
Wire with surface which reflects diffusively creates blurrier circle image.
Wire with surface which reflects speculatively creates sharper image
Imperfect surface (scratches) creates details on circular image.
Rotating the wire rotates details inside the circular image (wire's surface detail causes image details.
How It Works
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Vertical wires will have horizontal normals that are always perpendicular to light ray's vertical component of velocity.
Vertical component of velocities are unchanged by reflection.
Monotonicity and continuous range of the normal angles ensures that the full range of reflected angles (to horizontal) can be achieved without being blocked.
Reflected rays have same speeds and hence they have the same angle to the vertical.
