The "opening" for light to travel out of a flashlight is circular. Why does the light shine on the objects the flashlight is pointed at over a much larger surface area than the surface area of the aperture? Why doesn't it just illuminate the whole room or wherever the flashlight is?
2 Answers
Light is a wave , an electromagnetic wave classically. When leaving a point source a wave expands isotropically in angle, spherically, and its intensity falls like 1/r^2 where r is the distance from the source
If a point source lamp is set in a room, it will illuminate spherically all of it, as happens with the lamps hanging from the roof.
A flash light has two extra hardware. The light from the point source is focused with a lens that focuses the point source ahead, i.e. does not allow light to disperse spherically . The other is the aperture that also constrains the start of the dispersion of energy to a beam.
Even without the lens the light would start dispersing spherically from the aperture. The lens concentrates the intensity to a beam.
Light does expand, the double slit experiment was incorrect the pattern observed was due to interaction of light with the atoms in the slit. The atoms absorbed and re-emitted the light. This can easily be done with a high lumens LED flashlight so long as it has a condensed beam or an ability to focus it and anything with holes in it. Poke holes in a magazine cover shine the flashlight at the cover directed at the holes and blocking the rest of the light. You will see spheres or shapes, lines, and slits made of multiple spheres narrowing the slits makes a strip of the stacked spheres appear aka interference.
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$\begingroup$ The inverse square law is proof light expands as light gets further from the source its brightness diminishes as it gets more spread out ie expands $\endgroup$ Commented May 18, 2023 at 21:41