When a light is passed through water why doesn't it forms ripples?(like a stone does) I am assuming light is particle here. Photon mass ≠ 0 when it's in motion It also have very high momentum

Or is that ripples form are of very small amplitude and can't be seen by naked eye?


3 Answers 3


Absorption of a photon does launch an acoustic wave in water, but only at the point where it is absorbed or scattered. Photons in, e.g., a laser beam passing through water, are absorbed at points more or less randomly distributed along the beam's path. Absorption causes local heating, which causes local expansion, which causes the acoustic wave.

Systems using the photoacoustic effect make use of that acoustic wave. A very brief (nanoseconds) laser pulse can be launched in a thin sheet or line through a liquid solution, and spectral & phase analysis of the resulting acoustic signal provides clues re the dynamics of absorption and relaxation.


When a light is passed through water why doesn't it forms ripples?(like a stone does) I am assuming light is particle here. Photon mass ≠ 0

Photons always have zero mass and move with velocity c

when it's in motion It also have very high momentum

Its momentum depends on the frequency of the light beam to which it belongs. $E=hν$, and since the photon's mass is zero , $E=pc$

Light of frequency $ν$ emerges from the superposition of zillions of photons of energy $E=hν$, they are the building blocks of light, but an individual photon is an elementary particle , not light.

An individual photon passing through water can 1) scatter elastically, 2) scatter inelastically 3) if its frequency coincides with an energy level of the atoms or molecules, it can be absorbed completely. It cannot create waves in the water.

S.McGrew has given an answer for coherent light hitting the water.

Incoherent light just heats up the water due to absorption and inelastic scatters of the individual photons. Transparency in the water means that most of the photons scatter elastically with the medium, keeping their frequency and the images they build up.

  • $\begingroup$ My book states photon rest mass is 0.As i am bombarding photon into water it's mass in motion≠0 $\endgroup$
    – Bhavay
    Nov 17, 2019 at 3:29
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    $\begingroup$ That is not correct. In the standard model of physics the photon cannot acquire a relatiivistic mass because its rest mass is zero. en.wikipedia.org/wiki/Standard_model $\endgroup$
    – anna v
    Nov 17, 2019 at 5:30
  • $\begingroup$ Why is it assumed that a photons momentum depends on its frequency? How about all photons have the same momentum but different energy levels based on their frequency. $\endgroup$ Nov 18, 2019 at 16:50
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    $\begingroup$ @BillAlsept It is called mathematics. the algebra of special relativity as it is applied in modeling data in physics. hyperphysics.phy-astr.gsu.edu/hbase/Relativ/…. the last formula in the first page, put the mass to zero. $\endgroup$
    – anna v
    Nov 18, 2019 at 17:02
  • $\begingroup$ I’m just saying what if the whole idea starts from the wrong Premise. I mean the argument for it involves massless momentum?? Also with c and p the equation involves speed twice. Both seam ad hoc or at least a stretch. On the other hand a particle propagating through space could have two separate forms of energy. A linear momentum and an oscillating energy. That way you would not need to be redundant with the speed. Nor the unexplainable idea of massless momentum. $\endgroup$ Nov 18, 2019 at 18:02

Why do you think photons have very high momentum? Let's do a quick calculation. Let's assume we have a pond, and we want to compare the momentum of a pebble to the momentum of solar insolation on a comparable area (sunlight is pretty bright compared to most of our light sources, so it's a decent upper limit for your intutive experience with light).

Total solar insolation works out to about 1400 W per square metre. Let's imagine a cubic pebble one cubic centimetre in size. We'll compare the pebble's momentum to the momentum of insolation over one square centimetre over a single second (which I hope you agree is pretty generous).

The total energy of incident light would be 0.14 J in that one second. This corresponds to a momentum of 4.67E-10 kg/m/s. Is that a large number? Our pebble would mass about 3 grams, and we could assume it impacts at, say, 10 m/s. Its momentum would be 0.03 kg/m/s. That's eight orders of magnitude more. For most macroscopic earthly purposes, you can completely ignore the momentum of light. It's tiny.

And this is already an extremely generous analysis. Note that:

  • Sunlight actually usually "pushes" on the whole lake equally - so it wouldn't normally create ripples, instead, the lake would be "compressed" under the photons. You would see ripples everytime a cloud passed over the lake, as some of the lake would lose most of the incident pressure, while other parts would still have it. Needless to say, this isn't what we observe :)
  • I took a full second of sunlight. Of course, the pebble's splash against the water is much shorter than that - it will traverse its full "depth" in 0.01s, which is where most of the wave comes from. That's two more orders of magnitude the pebble gains on photons.
  • I assumed the photons momentum would be perfectly transferred to the water, all pointing in the downward direction, as if you got a perfect elastic collision. Needless to say, photons don't interact with water quite in that way. They get scattered, they get transmitted, they get delayed re-emission etc. etc.

And more :)

Can we fix that with taking a very strong light, instead of "mere" sunlight? Yes. There are many, many complications - but the National Ignition Facility is doing essentially that - pushing a huge amount of light into a very tiny space, compressing the fuel pellet into a density of 100 times the normal density of lead. Needless to say, this would produce quite the ripple.

If we want to have the same impact as the pebble, we would need a light pulse of 14 MJ (assuming all of the other conditions in our thought experiment stay the same, i.e. very generous for photons). The NIF laser, which is one of the biggest lasers we have ever built, produces a pulse of about 1.5 MJ. Scaling it up ten times might be a tad difficult, and I don't think you'll find much funding for replicating the effect of a thrown pebble on the surface of a pond with a gigantic death ray :)


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