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So we know the light that's emitted from a torch (flashlight) must be moving in straight lines, so why does it spread out when moving? Why does it cover larger area?

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    $\begingroup$ Why do you think that all light rays from a torch should be parallel? $\endgroup$ – Dvij D.C. Dec 29 '19 at 13:17
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So we know the light that's emitted from a torch (flashlight) must be moving in straight lines, so why does it spread out when moving? Why does it cover larger area?

As the comments says, straight lines are not parallel lines. They can have different directions, and they do:

The flash light, (or any light source from heated filaments) can be considered as an accumulation of point sources.

Point sources of light can be modeled by rays that are opening as $1/r^2$

point source

The light is an overlap of point sources, and depending on the geometry of the flashlight and the filament, it is evident that a multitude of directions will appear.

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So we know the light that's emitted from a torch (flashlight) must be moving in straight lines, so why does it spread out when moving? Why does it cover larger area?

The reason for such spreading out lies in the idea of "Diffraction". You can read more on diffraction at this resource.

The following are some useful points for a quick understanding.

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The roots of why Diffraction happens lies in "Heisenberg's Uncertainty Principle", which you can understand at the following two resources.

  1. Heisenberg's Uncertainty Principle Explained by Veritasium. https://www.youtube.com/watch?v=a8FTr2qMutA

  2. Heisenberg's Uncertainty Principle in action! by Dr. Walter Lewin. https://www.youtube.com/watch?v=0FGo8mi-5w4

Following is a quick summary of Diffraction on the basis of Heisenberg's Uncertainty Principle.

Heisenberg's uncertainty principle tells us that it is impossible to simultaneously measure the position and momentum of a particle with infinite precision. In our everyday lives we virtually never come up against this limit, hence why it seems peculiar. In this experiment a laser is shone through a narrow slit onto a screen. As the slit is made narrower, the spot on the screen also becomes narrower. But at a certain point, the spot starts becoming wider. This is because the photons of light have been so localised at the slit that their horizontal momentum must become less well defined in order to satisfy Heisenberg's uncertainty principle.

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