If I hold up a (round) flash light very close (2-3 cm) to a surface (a wall or a floor), I can see a "circle" of light on the surface with a diameter just a bit larger than that of the flashlight, If I take the flashlight further away from the surface the diameter of the circle on the surface becomes larger. This means there is a "cone" between the flashlight and the surface. What characteristics of light causes this cone? If I aim the flashlight to a reflector, and the reflector is aimed at a surface, when the flashlight is close, the surface will have a lighted circle; when I take back the flashlight, the shape on the surface becomes more like that of the reflector...
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$\begingroup$ In short, that is because light usually propagate in rectilinear paths in the air. $\endgroup$– SpirineCommented Oct 24, 2016 at 10:29
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Your flashlight has a round lens in front of the light source and usually a spherical reflector behind the light. If there was no lens the light would propagate away in all direction like a candle or a burning match. The lens and reflector collect this scattering light and focus or expands it in one general direction. Try this. Picture a candle burning. The light is leaving the flame like an expanding sphere. Now picture just 1/2 of the sphere leaving the flame. Let the 1/2 sphere get about a foot from the flame. Now draw lines from the 1/2 sphere's perimeter back to the flame. It's a cone.
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$\begingroup$ Thanks! Is the "spherical expansion" a general characteristic of light, regardless of the light source? Or is it also related e.g. to the shape of the flame or something other? Is this a gif a "good" illustration of generally how light flows/expands? upload.wikimedia.org/wikipedia/commons/7/7e/Spherical_Wave.gif (Taken from Wave_equation#Spherical_waves on Wikipedia). $\endgroup$– Yuval A.Commented Oct 25, 2016 at 13:19
