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Whilst observing a lighthouse, I noticed that I could only see the cone of light reflected in the water when the lighthouse light was directly pointed at me. At all other points, I could just about see the cone of light in the air, but no light was visible on the water. Why is this the case?

NB: I would appreciate any answers to include a "laymans" explanation (being more or less a physics layman myself)

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The light that you can see in the air is actually light that has been reflected from particles in the air (dust, water vapour, etc.) towards your eyes. So when the beam is pointing away from you, some of that light will still bounce off those particles and come to you; this is what allows you to see the beam. However, when you look at the water, it acts more like a mirror, i.e. the light is only reflected by the surface. The light's direction of motion will change vertically when it hits the surface (treating it like flat mirror), but the horizontal motion will remain unchanged, so the beam won't be visible from the side, only when you are directly in it.

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  • $\begingroup$ Thanks. However, given that the sea is in motion, wouldn't that mean that the angle of the wave would cause some amount of "scatter", such that (at least in some directions) a small amount of the light would end up reflecting in my direction? $\endgroup$ – Obsidian Phoenix Sep 22 '14 at 8:16
  • $\begingroup$ Yes, and this does happen, this is why you see waves glisten, but this will only be quick flashes, and depending on how bright the beam is your eyes may adjust to the point that you barely notice them. $\endgroup$ – johnpaton Sep 23 '14 at 8:54
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If the beam light or what you say as cone is above the water what you will see is the reflection of light from the lighthouse and not the beam, as you can see it produces a cone shaped light with the help of a parabolic shaped reflector it have or either it uses other type, the point is that the light is focused and under tha law of reflection for specular reflection ( http://en.m.wikipedia.org/wiki/Specular_reflection ) heres the link for you to understand well, the light is reflected on the same angle it has been produced so as the light of the lighthouse is focused the reflections can only be seen if it is directly pointed on you,and on air light is reflected by dust water and any particles in the air so you can se the beam esp. If it is strong.. i hope this helps :)

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