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In transverse wave,the particles vibrate perpendicular to the direction of wave propagation.

“light is a transverse wave.”
but light can be propagated without a medium , that is particles.
so, how can light be called as a transverse wave?


Light is an electromagnetic wave which requires no medium for the propagation. In electromagnetic waves including light, electric and magnetic fields vary with time in mutually perpendicular directions and also perpendicular to the direction of propagation.

Propagation of Light, Group velocity and Phase velocity

Prasanna asks:

“how can light travel in the form of waves in space, which has no medium? if light doesn’t travel in the form of wave in the space then in equation. ‘phase velocity=c^2/group velocity ‘ phase velocity becomes zero and also c=0, which indicates velocity of light =0, but light actually travel in space . How can this be possible?”


I feel that there is some sort of misunderstanding in your concept.

Why should phase velocity become zero?

Please remember that light is an electromagnetic wave, where the propagation is due to the mutually reinforcing time varying electric and magnetic field vectors varying in mutually perpendicular directions.

The phase velocity is a the speed at which a point of fixed phase (say the crust of the wave)  propagates, which is not always the speed at which the electromagnetic information travels.

Group velocity is the speed at which electromagnetic information travels.

Hope that the following links will help you sort out the issue.

  1. http://www.microwaves101.com/encyclopedia/phasevelocity.cfm
  2. http://en.wikipedia.org/wiki/Group_velocity
  3. http://en.wikipedia.org/wiki/Phase_velocity
  4. http://en.wikipedia.org/wiki/Speed_of_light
  5. http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SpeedOfLight/FTL.html

Transverse nature of light

Mallika Somnath asks:

it is said that in tranverse waves the particles move perpendicular to the direction of motion of wave.
it is also said that light is a transverse wave.
but how can the particles in light move at all, as light doesn’t require a medium to travel?


Light is an electromagnetic wave, energy propagated as a combination of electric and magnetic fields varying with time in mutually perpendicular directions, also perpendicular to the direction of propagation. This makes light a transverse wave.

What is the principle used in the working of 3D glasses?

Sreeda asked:

What is the principle used in the working of 3D glasses?


3D glasses are actually polaroids. They are not creating the 3D effects. The 3 D movies are filmatographed with two cameras help at two different angles just like we see with TWO eyes.

The two films are projected on the screen with polarized light with their plane of polarization in perpendicular to each other. If we look at the screen without ant 3D glass we will see blurred images only as the two pictures are overlapping each other and both the image are seen together by both the eyes.

3D glasses use Polaroid with the plane of polarization in mutually perpendicular directions. So, when we wear the Polaroid glasses and look at the screen, only one of the image will be seen by one of the eyes and the second eye sees the other image. This creates the 3D sensation, we are seeing the image filmed with the left camera with the left aye and that with the right camera with right eye.

Resolution of Vectors

From: Vakas

Subject: Resolution with geometry
Message Body:
Dear sir,
I m not able to understand that when we resolute mg in a inclined plane that plane having angle theta,how does the vertical component goes through inclined surface & cos comp opp to N.
explain with geometry so that i don’t get confused in future in determining the two comps is right direction.


In a right angled triangle,


When mg is resolved into mutually perpendicular directions, one parallel to plane and the other perpendicular to the plane (opposite to the Normal reaction), the component which is adjacent to θ is the cos component and the one away from θ is the sine component.

Students find difficulty ,mainly in identifying the angle .

See the diagram below and try to analyze


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