# What will happen if light is longitudinal wave?

Topic edited by mfb (original was "What will happen if light is transversal wave? ")

Will people see the world in a completely different view?

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davidbenari

Orodruin
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Light is a transverse wave...

Light is a transverse wave...
Well. I mean longitudnal ... Sorry

Orodruin
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Then the question is akin to asking how a black pot would look if it was red. There simply is no longitudinal wave solution to Maxwell's equations and so such a solution is not allowed by the physics that we use to describe the electromagnetic field.

PFuser1232, davenn and mfb
Khashishi
Of course, inside matter and in waveguides, light can have some longitudinal components.

A Gaussian beam (which is in free space) has a significant longitudinal component of the electric field at the center of the beam.

In an imaginary world, if the equivalent of light was a longitudinal wave, probably we would see sounds rather than lights.
This is already possible in biomedics with ultrasounds to see through the tissues by echoes; moreover raising up with frequency, about some Th, the wavelength of ultrasound becomes comparable to the light one.
But a description of how we would see that world, seems to my mind really too demanding...

Drakkith
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In an imaginary world, if the equivalent of light was a longitudinal wave, probably we would see sounds rather than lights.
I doubt that. The frequency of audible sound is much too low to use 'optically'. You can't get a good image with something the size of your eyeball since the diameter of the pupil is so much smaller than the wavelength of audible sound. For example, the wavelength of a sound wave of 10 KHz is 1.35 inches. In the daytime your pupil is typically only 3-5 mm in diameter at best, which is 0.12 to 0.2 inches. For a good image your optical system needs an aperture much larger than the wavelength.

You can't simply move the frequency up either, as the air cannot support a wave with a frequency anywhere close to the frequency of visible light. You just can't get the air molecules to accelerate back and forth that quickly (not to mention the source producing the sound waves).

The frequency of audible sound is much too low to use 'optically'. .
Here I was talking about ultrasound and their actual and future use in biomedical equipment.
The idea of "to see the sound" simply liked to me.
All the rest belongs to an imaginary world

Drakkith
Staff Emeritus
Here I was talking about ultrasound and their actual and future use in biomedical equipment.
Uh, that's not what it looks like from your earlier post, but okay.

Pierce610
Actually it looks just like that, Pierce610 began the post with "In an imaginary world"...

Pierce610
Drakkith
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Actually it looks just like that
That's certainly not what I'm seeing, but what people say and what people mean to say are sometimes not the same.

In any case, I'm sure there would be at least some slight changes to the behavior of light if it were a longitudinal wave, but I don't what they would be. Interestingly there is an article about creating longitudinally polarized EM waves that are able to propagate several wavelengths through free space: http://www.nature.com/nphoton/journal/v2/n8/full/nphoton.2008.127.html

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so if light was longitudinal. we would be looking at the rearafication and compression of the E.M field. how about looking at it in reference to a completly different question. how would sound behave if it was transverse? Or how about comparing the behaviour of P waves and S waves through the earth? same medium different type of wave. one transverse the other longitudinal. It won't answer the question complettly but might give you some incite.

Drakkith
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so if light was longitudinal. we would be looking at the rearafication and compression of the E.M field
Well, light is a transverse wave in the sense that the electric and magnetic field vectors point perpendicularly to the direction of travel and oscillate in their respective axis. In other words, if we graph the EM wave and set the X-axis as the direction of propagation, the Y-axis as the magnetic field vector, and the Z-axis as the electric field vector, the electric field vector oscillates back and forth along the Z-axis while the magnetic field vector oscillates back and forth along the Y-axis. These vectors represent the force exerted on a charged particle, so the oscillation means that the strength and direction of the force is changing over time.

I honestly don't know what the field vectors are doing in a longitudinal mode, but I don't think it can be described as rarefaction and compression since there's nothing being compacted or rarefied.

how about looking at it in reference to a completly different question. how would sound behave if it was transverse?
No need to guess. Sound can be transmitted through solids as transverse waves: http://en.wikipedia.org/wiki/Sound#Longitudinal_and_transverse_waves

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. It requires a medium to propagate. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation.

okey. so therefore we can look at just sound waves. and the question would be: How does the behaviour of a transverse sound wave differ from a longitudinal one, in the same or similar medium?

Drakkith
Staff Emeritus
okey. so therefore we can look at just sound waves. and the question would be: How does the behaviour of a transverse sound wave differ from a longitudinal one, in the same or similar medium?
Exactly like the quoted paragraph from wiki says. I honestly don't see the point in comparing sound waves to light waves. Aside from obeying the wave equation, they have very little in common.

year. but I was thinking in terms of if you were a scifi writer and you are writing a story about inter-dimensional communication or something and you invented a way of making light waves propagate longitudinally for what ever reason. maybe refer to sound waves as a rough analogy. at least something to base some believable assumptions on.

What will happen if light is longitudinal wave?

Nothing, the world will be the same, as always.

Drakkith
Staff Emeritus