Principles of Special Theory of Light

1. Does the speed of light depend on motion of source of light?

No, the motion of light is not affected by motion of source of light.

2. Is photon a particle?

The photon is a particle of light, but it doesn’t possess all essential properties we ascribe to a tiny ball i.e. photon doesn’t behave as a common sense particle but it has got some peculiar properties.

3. When we follow Albert Einstein in developing special theory of relatively, we are developing a theory of space and time.

4.  The principles of special theory of light.

Principle 1:

Colloquial statement: If we are in unaccelerated vehicle, its motion has no effect on the way things happen inside it.

Formal statement: The laws of physics are the same in all unaccelerated reference frames.

Principle 2: The motion of light is not affected by motion of source of light.

5. The special theory of relativity

      

          Special: The word special in name arises because we employ only unaccelerated reference frames, not all reference frames that one can think of. In other words, we special to the way things appear when observed from uniformly moving reference frames.

     

          Relativity:-The word relativity comes from a phrase coined by Henri Poincare, an eminent French physicist and mathematician.

In 1904, Poincare was invited to address the international congress of arts and science, held in st Louis to commemorate the 100^th anniversary of Louisiana Purchase. Poincare spoke of a principle of relativity.

If you are in plane on its way from Chicago to phoenix, another plane making the return flight, over wheat fields of Kansas. A farmer, looking up, notes that you are flying south west at 500 miles/hr relative to his wheat fields.

The pilot of return flight notes that the distance between the two planes is decreasing at about 1000 miles/hr. So far as the pilot is concerned, you are travelling at about 1000 miles/hr relative to his plane.

The essence is this:  statements about uniform motion relative to a specified reference frame wheat fields or another air plane are meaningful.

A quantitative statement about uniform motion without specification of a reference frame is not meaningful. Why? Because our principle 1 says we cannot discern uniform motion without recourse to some reference frame.

Take first the colloquial form of that principle if we are in an unaccelerated vehicle, its motion has no effect on the way things happen inside it. So by just doing experiments inside the vehicles, we have no way to assign a velocity to the vehicle. Only if we look out of window and thereby use wheat fields of Kansas as an outside reference frame. We can decide on velocity (velocity to that outside reference frame).

      

          Theory: It appears because principles 1 & 2 are generations from observation and experiment.

6. THE CONSTANCY OF SPEED OF LIGHT

•  Observes in all un accelerated reference frames measure the same speed for light ( in vacuum) from any given source.

• They all measure 3*10 ⁸m/sec   always for light in vacuum.

• This remarkable property is called “constancy of speed of light”.

Note:-Some factors other than light may be observed differently in unaccelerated frames.

7.  An “event” is anything that happens at some definite locations at some definite time. Proto typical examples are your birth, assassination of Abraham Lincoln etc. In contrast, a forest fire that sweeps across 10000 acres in 5 days does not constitute an “event” because the fire is spread out in space and time.

The adjective “definite” means   “distinct” or  “limited” for any one observing the happening.

8. THE RELATIVITY OF SIMULTANEITY:

•  Spatially separated events that are simultaneous in one frame are, in general, not simultaneous when viewed from other reference frame.

•  Simultaneity is a relative concept, but not an absolute one.

• The concept of simultaneity between two events in different space points has an exact meaning only in relation to a given inertial system i.e.   “Each frame of reference has its own particular time”.

• To measure the length of an object means to locate its end points simultaneously. As simultaneity    depends on frame of reference, the length measurements will also depend on frame of reference.

• Thus, “The length i.e.  Space is a relative concept, not an absolute one”.

• Thus there is no such thing as an absolute, global “now”.

Brewsters Law

When light is transferred from glass surface it may be polarized. It was explained by Brewster. According to Brewster the light reflected from a surface is completely polarized if the reflected beam and the beam refracted into material form a right angle. If the incident beam is polarized in plane of incidence there will be no reflection at all. Only if the incident beam is polarized normal to the plane of incidence it will be reflected.

Reason for above property

In the reflecting material, the light is polarized transversely and we know  that it is motion of charges in material which generates emergent beam which we call as reflected beam.The source of this so called reflected light is not simply that the incident beam is reflected; our deeper understanding of this phenomenon tells us that the incident beam drives an oscillation of charges in the material, which in turn generates reflected beam.

From below fig, it is clear that only oscillations normal to paper can radiate in direction of reflection and consequently reflected beam will be polarized normal to plane of incidence. If the incident beam is polarized in plane of incidence there will be no reflected light.

This phenomenon is readily demonstrated by reflecting a linear polarized beam from a flat piece of glass. If glass is turned to different angles of incidence to polarized beam, sharp attenuation of reflected intensity is observed when angle of incidence passes through Brewsters angle. This attenuation is observed only if plane of polarization lies in plane of incidence. If plane of polarization is normal to plane of incidence, the usual reflected intensity is observed at all angles.