Showing posts with label Light. Show all posts
Showing posts with label Light. Show all posts

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 100th 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.

  •  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.

  •  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”.


Light is a wave phenomenon (though this statement is not the whole story). Our best evidence for this proposition is interference, as in double slit interference & Refraction is very good evidence, too.

Out of what, is a light wave formed?

The straight answer is from "Electric & Magnetic Fields". A light wave is always formed from both electric & magnetic fields. The fields always point perpendicular to the propagation direction, that is perpendicular to direction in which light is traveling.

When light interacts with electrons, the exchange of energy occurs in a grainy fashion. The amount of energy exchanged is always hv, where 'h' denotes Planck's Constant and 'f' is the frequency of the light wave. The graininess introduces the photon.

In which way should we think of a Photon as a particle? Indeed as a "particle of light"?

a) A photon has energy, and it has momentum. these are nice particle like properties.

b) We cannot ascribe to a Photon, a definite continuous trajectory.

c) It is tempting to think of a light beam as merely a hail of Photons, but it is also incorrect. Don't do it. A wave traveling through space is better picture.

d) The graininess of light manifests itself primarily when light interacts with charged particles. Thus the Photon notion is needed primarily at the start and finish of a Light beam's travel through space.

Indeed, the Title of Einstein's " Photon" paper was this:

"On a heuristic point of view concerning the production and transformation of Light." The word "heuristic" means "stimulating discovery or further investigation" and indicates a tentativeness. More significant for us  is Einstein's specification of "Production & Transformation of Light". We should understand production to mean the emission of light. By "Transformation", Einstein meant the combination of absorption followed by emission.

And in the body of paper, Einstein devoted extensive space to the absorption of light by metals and molecules. Einstein had clearly in mind the interaction of light and electric charges when he introduced his particle theory of Light.

It is better to represent Photon as

as it reminds us the "Particle" aspect of light - The Dot; The wiggly line - "Wave" aspect of the light.

Physics is not able to provide a valid, intuitive picture of Photon, and that is why none appears in this book.

Photons and Waves are inextricably linked.

Major connections are following three

i) Energy of Photon=hv

ii) Momentum of Photon=hv/c

iii) Probability of appearance of Photon ~ square of 'E' of EM wave

Note:- The Photon idea appears on Left and Wave idea on Right.

When to use Wave concept & Photon concept?

use wave picture for describing interference, refraction, reflection and phenomena with Polaroid sheets; use the Photon idea for describing the interaction of light and electric charge, that is, for describing absorption and emission of light.

In a letter dtd. 12 Dec 1951, when Einstein was 72 years old, Einstein wrote these lines to Besso:

"The entire 50 years of deliberate pondering have not brought me closer to an answer to question "what are light quanta(i.e. Photons)? Today, every Tom, Dick and Hary believes that he knows, but he deceives himself."

Does Physicists understand light?

When it comes to equations, Physicists understand light very well indeed. For instance, theoretical calculations in 1950's laid the foundation for the first Lasers(which were constructed in early 1960's). In no way were LASERs accidental discoveries.

Providing pictures to capture the essentials of what the equations have to say is quite a different task. Sometimes a wave picture will do the job; at other times, it will not. And the same is true for Photon description. But where one description fails, the other succeeds thus there in lies the "complimentary" and for us there is a lesson in humility.

Light is a more subtle phenomenon than any single one of our everyday concepts can describe.

Principles of Special Theory of Light


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 same in all unaccelerated reference frames.


The motion of light is not affected by motion of source of light.

How long does it take for moonlight to reach the earth?

Light travels through space at just over 186,000 miles per second. The moon is just under 250,000 miles from Earth, so light from the Moon's surface has to travel more than one second (about 1.3 seconds) to reach us.

The astronauts left reflecting mirrors on the moon. By shining a laser beam on those mirrors from Earth, and measuring the time (less than 3 seconds) it takes to see its reflection, the length of the round trip can be easily calculated to within a few inches!

Speed of Light faster than sound

Sound is generated due to vibrations in atoms/molecules in a medium and thus always require materialistic medium. The physical parameters like Temperature, Pressure etc., effects the sound velocity.

On the other hand Light doesn't require a materialistic medium. It transmits in space through field.

The concept of field is still a mysterious thing to explain meticulously.

Light is transmitted by 'photons' which are treated as zero mass entity. The speed of sound is negligible when compared to speed of light. This is one of the reasons that while thunderbolts, we see flashing light fast and then hear sound. This is a best example.

In fact light falls in different spectrums and light which gets generated at a point travels through the universe changing its presence int he spectrum.