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QUESTION: Does light beams (example: from a bulb in a dark room) hitting a wall split up to form more beams, resulting in the room lightning up? I am generally curious on how light can bright up a whole place since it is beams working in a transverse plane.

EXAMPLE: If you drill a hole in a wall and make light come in, the whole room will not light up, however, if you speak or play music from the outer wall, the sound can be heard no matter where or how you stand in the room due to sound being longitudinal.

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    $\begingroup$ If you reflect upon this question for a while, you'll see the answer. $\endgroup$
    – Carl Witthoft
    Commented Apr 5, 2021 at 18:08
  • $\begingroup$ To your example: if the walls are reflective enough, the light from that little hole will in fact illuminate the whole room. $\endgroup$
    – Carl Witthoft
    Commented Apr 5, 2021 at 18:09

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Remember that light is essentially a wave, like sound (yes yes particle-wave duality, but we're not getting to that today).

Ocean waves diffract through a gap| Credit: Goggle

When waves pass through a gap in an obstacle, it diffracts and spreads out.

Source: wikipedia

The angle at which the first minimum intensity occurs depends on the wavelength. For me singing "ahhhh" at 160Hz, the wavelength is 2.1m, but for visible light, the wavelength is between 380nm and 780nm. For a hole with a diameter of 2cm, visible light can diffract from 7.610^-8 degrees (purple) to 1.5610^-7 degrees (red). On the wall opposite to that hole, this tiny amount of diffraction makes the light show up as a bright 2cm diameter dot. Music, however, has a diffraction angle larger than a full circle (on my calculator it showed up as "out of bounds"), so not just people in the room, even if there's a corpse sealed inside the wall on which the hole was drilled on, it can hear the music too.

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  • $\begingroup$ This is part of the story; diffuse reflectance (of sound or light) at the room walls is the other part. $\endgroup$
    – Carl Witthoft
    Commented Apr 5, 2021 at 18:09
  • $\begingroup$ @Shu Greetings, and thank you so much for your answer. Can I expand your example with the picture? How come, the water waves go straight through the gap and then diffracts? Is this something with gravity to do or how waves are travelling? $\endgroup$
    – Nemanja Vuksanovic
    Commented Apr 6, 2021 at 16:56
  • $\begingroup$ @NemanjaVuksanovic, that is beyond my knowledge, sorry $\endgroup$
    – Shu
    Commented Apr 6, 2021 at 19:34
  • $\begingroup$ @CarlWitthoft, yes, that's precisely true. I was going to mention that, but guess I forgot when I posted the answer $\endgroup$
    – Shu
    Commented Apr 6, 2021 at 19:35
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To the question: No. One light particle (photon) cannot be split up. The room is lightened up completely due to different photons, which come from a source that is always somewhat divergent. This means that the photons come at different angles and are therefor reflected at different angles, lighting up the room.

To the example: The reason for what you described is diffraction, which is basically the spreading of a wave after it goes thrue an opening (which cuts off most of the wave). Diffraction only happens if the Fresnel number $$F=\frac{a^2}{\lambda L}$$ is small enough, which corresponds to the distance between the wall and the opening (e.g. door) $L$ and the wavelength of the wave we are dealing with $\lambda$ being much bigger then the width of the opening (door) $a$. So for visible lightwaves that have very small wavelengths compared to soundwaves, you need a much smaller opening (door) for diffraction to occur. In the case of soundwaves, a normal door is small enough to diffract the wave such that it goes in all directions after passing thrue the opening (door).

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  • $\begingroup$ Hello, and thanks for your answer! I have a curious question regarding your example with the door. If I open my door a bit in a dark room, and the corridor is lit, the light would not cover up the WHOLE room, but only spatial (Maybe some of the floor and wall)? $\endgroup$
    – Nemanja Vuksanovic
    Commented Apr 6, 2021 at 16:52
  • $\begingroup$ @NemanjaVuksanovic Exactly, because the opened door is to big for the light to diffract in all directions. The doorslit you opened would have to be $\rm\mu m$ wide. This would not realistic. If you like my answer, please accept and upvote. $\endgroup$
    – Roger
    Commented Apr 7, 2021 at 9:18

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