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Light, Time and Vacuum

Why does the light needs time to travel through vacuum?Is there something makes it needs time? How did we prove that  there is nothing in vacuum?


When there is no medium, there is nothing to oppose the propagation of light and therefore it moves with a constant velocity. All electromagnetic waves including light travel in vacuum with the same velocity (3 x 10^8 m/s). Since the energy is to travel from one point to another, a finite time is also required.

English: Electromagnetic waves can be imagined...

The velocity of light is experimentally determined by many different methods. (Romer’s methos, Fizau’s method, Foucault’s method, Michelson’s method etc.)

When light travels in a medium, whenever light interacts with the matter, the atoms absorb light and are re – emitted and the average speed in a medium will be less than in vacuum. In vacuum, there is no matter t interact and that’s why there light travels with maximum velocity.

(If you remember the structure of atom, most of the space inside an atom is vacuum. The speed of light decreases only when it encounters matter. So, when we consider a particular medium, the speed of light is a constant in it)

Problem from Ray Optics

A diverging beam of light from a point source S having divergence angle alpha falls symetricaally on a glass slab as shown in fig.The angles of incidence of the two extreme rays are equal.If the thickness of the glass slab is t and the refractive index is n,then the divergence angle of the emergent beam is,
c)sin inverse 1/n
d)2 sin inverse 1/n


Light Amplification

If you passed directional light from a device such as a laser through the back of a 2 way mirror at a 45 degree angle and then directed that beam to 3 highly reflective mirrors at 45 degree angles in series so that the beam was directed back to the exact point it emerges from the two way mirror and creates an infinate loop what would happen?  Would the beam change color since the particles would be out of sync because of the time delay?  Would the light and heat of the beam amplify itself if the reflective surfaces were reflecting more than 50% of all the light emitted and more than 50% of the original sources light were transferred through the 2 way mirror?  I included a drawing to simpilify the questions.


Asked Jeff

Why light is not accelerated in vacuum?

If vacuum has no mass …. then why isn’t a light beam accelerated in space ?? – Asks Akil Raj


The speed of light in a medium or in vacuum is constant. It is neither accelerated or decelerated while propagating. But, the speed of light in a (optically) denser medium is less than the speed of light in an optically rarer medium.

The speed of light is maximum in vacuum and it is constant and is equal to 299792458 m/s as theoretically established by electromagnetic wave theory too.

9If you look at it this way; “In vacuum there is no particle to oppose the propagation of light, therefore it should travel with a constant velocity, because to increase or decrease the velocity some external influence is essential)

Huygens Wave Theory and rectilinear propagation of light

As per 12th Physics textbook, it has been mentioned that according to Huygens wave theory of light, light does not travel in straight line.

But I have heard that it is wrong and rectilinear propagation of light is one of the merits of Huygens theory rather than demerit please clear my doubt

Asked by Mohammed Faiz Ansari


The wave nature of light becomes prominent only when the size of the aperture of the obstacle is comparable to the wavelength.

The light of wavelength λ coming through an aperture of width a can be considered to be in rectilinear motion up to a distance of


where ZF is called the Fresnel distance.


Thus, wavelength λ decreases, the limit for rectilinear propagation increases.This is the reason why we say, ray optics is valid when the limiting case wavelength λ tending to zero is applied.

Time Travel Again !

Davide Writes: (The email is posted as such. SO, sorry for the telegraphic language and poor spelling and grammar)

“Hello there i have one more question,a crazy one:) according to Einstein we need a enormous amount of energy to push matter at a speed of light,now for me the real problem there  is that an atoms at a speed of light will break a part right?so even if we will find a way to push it,we will still failed.I was wander are we looking to the right way?is it photon, so the world of small with no mass only work? now here my crazy question,lets speculate a bit,we have two red dwarf pretty close to earth Barnard’s Star” only  “5.9 ly and Wolf 359 7.7ly a baby step compere the universe,if we can send a same kind of mission the we have done with Stardust Nasa’s comet sample and collect a piece the size of a soccer ball back to earth,we could work on it outside earth atmosphere in the space base.Since we can’t go in the first place at a speed of light let’s says that the all deal will last 150 year to go and back,(assuming that we use a new energy to go bit faster)in the mean time the space base will be pretty developed and maybe we even found a new power to push to the speed of light(i remind you that we are speculate)we could put our hands on the sample using electromagnetism so that we avoid any risks and then send it at a seed of light.What i try to say is that a single atom is weak but billions of it together is a different story,here an example, lets imagine to make a spherical shape out of Lego(the toy) and send it in to space at a great speed,it will break pretty soon,now we make another one but we glue the piece together,it will last bit longer,again another with glue and a put net around it will last even longer so instead of a Lego we use our sample,the glue in the inside is the immense mass that will have (piece of red dwarf) net outside the  immense gravitational pull ,now do you think that it will be enough to keep it together at a speed of light?regardless the energy that we need to push it?  after all even in the past we believed that going faster than a  speed of sound a plane will brake a part or something bad could happen,instead only a big BUM wen we reach it and nothing more,so maybe if we can go faster than light nothing will happen,or maybe we find a new way to travel,we can even give a name for fun “a speed of matter” what do you think about it?i know it sound like sci fi but hey physics it self is blowing mind science right?   Thank you kindly for your time:)”

I don’t like to respond now.

Responses from visitors invited

black hole

Subinjith asked

“if there is an black hole then how does it so much of gravitational pull to never even let go of the light tht passes by  its surface…????”

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