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When light is directed towards our eyes we detect the source and locate it. In Huygens' principle it is said that every point on wavefront acts as a source of secondary waves which again travel in all possible directions.

If light is directed not towards our eyes, like path of eyesight being perpendicular to path of light, light from the source of secondary waves can reach our eyes and we can locate that source (that source being in path of light). Then why can't we see the path of light in vacuum?

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The following may be useful to consider.

It is true that Huygen's principle states that every point on a wavefront acts as a source of secondary waves which travel in all possible directions. However, those individual waves are not seen as separate, but only as a superposition.

In other words, say you are looking at a source that is very far away, it will arrive at your eyes essentially as plane waves. If one was to construct Huygen's wavelets at the halfway point between the source and your eyes, those waves would again combine to become plane waves when they reach your eyes. So, if you are looking at the perpendicular direction, the wavefront will not enter your eyes.

However, if you imagine placing a barrier with a small hole half way between the source and your eyes, it might be possible to see the source if you are not looking directly at the it due to diffraction since only part of the plane wavefront is allowed to pass through the hole.

The below figure shows the two cases.

I hope this helps.

enter image description here

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For us to see an object, our eyes' receptors need to absorb photons coming (or scattering off of) from the object.

To see something light must enter the eye and the rods (and cones) must be stimulated sufficiently for the signals to be produced for processing by the brain.

Why can we only "see" reflected light?

In vacuum, there is nothing for the photons to scatter off of, so if the photons are not directed into our eyes, the receptors in our eyes will not be able to absorb them, thus we will not see them.

Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light.

http://farside.ph.utexas.edu/teaching/302l/lectures/node150.html#:~:text=When%20applied%20to%20the%20propagation,all%20of%20these%20secondary%20wavelets.

Maybe you are thinking of this statement about Huygen's principle, but if you read it carefully, you see it states that the secondary wavelets spread out in the forward direction.

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  • $\begingroup$ Arpad, you touched both the photon and the wave model of light. The first with photons does not predict that you will measure light all 180° behind a slit, the last with waves do so (see the sketch in ad2004's answer). I bed that the best sensitiv measurement will not get any light in +/-90° behind a slit like the wave model predict. That let me to a question, I’ll ask. $\endgroup$
    – HolgerFiedler
    Commented Oct 24, 2020 at 6:54
  • $\begingroup$ I placed a question physics.stackexchange.com/questions/589212/… $\endgroup$
    – HolgerFiedler
    Commented Oct 24, 2020 at 7:18

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