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Speed of Light – a thought experiment

Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for spaceyou,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours? (Asked Charan)

Deduce an expression for velocity of em waves in vacuum

Please find below a detailed version giving the answer to your question

What are plasmons?

A plasmon is a quantum of plasma oscillation, just like photon is a quantum of light radiation.

Plasmon is a quasiparticle resulting from the quantization of plasma oscillations.

Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric field of the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it. In most metals, the plasma frequency is in the ultraviolet, making them shiny (reflective) in the visible range.

[gview file=”http://www.photonics.ethz.ch/fileadmin/user_upload/optics/Courses/NanoOptics/plasmons.pdf”]

Surface plasmons

Surface plasmons are those plasmons that are confined to surfaces and that interact strongly with light resulting in a polariton

Read more at http://en.wikipedia.org/wiki/Surface_plasmon

[gview file=”http://www.optics.rochester.edu/workgroups/novotny/courses/OPT463/plasmonss.pdf”]

Seeing and photons

If we see an object because of light from the source hitting the surface of the object make its electrons to excite to higher orbits and coming back to same orbit emitting a photon which strikes our eyes, then according to this, at one INSTANT of TIME, an excited electron can emit only one photon and similarly if the object contained (for example) ten electrons on its surface, only ten photons can come form that object at that instant of time and all those photons will have 3 degree of freedom to move in any direction.IF this is the case, we could not see a complete object at a given instant of time as all the photons emitted from that object woudn’t have striken to our eyes and similarly, if 3 people are watching the same object, at a given instant of time, each person would have seen a part of that object as one electron emits only one photon at that instant of time…. but this is not the case in real world….how is it?

SVG of Image:Photoelectric_effect.png


We are not seeing an object not just because of a single of photons or not even just a handful of them.

For example, if we have a 100W bulb emitting light of wavelength 6000 angstrom, the total energy emitted per second is 100 J.

The energy of a single photon is

h\nu =\frac{hc}{\lambda }=\frac{6.6 \times 10^{-34}\times 3\times 10^{8}}{6000\times 10^{-10}}= 3.3 \times 10^{-19} J

Which means that each joule of light energy is caused bu 10^18 photons.

A single photon cannot cause the sensation of vision. Remember, we cannot see in dim light. In order to see there must be a minimum required intensity (the number of photons falling per unit area per second multiplied by the energy of a single photon)

From your question you made a very wild approximation. Please read the following for a farer real life approximation quoted from wikipedia.

It was found that the emission of only 90 photons was required in order to elicit visual experience. However, only 45 of these actually entered the retina, due to absorption by the optic media. Furthermore, 80% of these did not reach the fovea.



Refraction through a prism

For an equilateral prism , find two angle of incidence differing by 20 degrees showing same deviation of 40 degrees?

Asked Atambir Singh


Use the formula \delta =i_{1}+i_{2}-A

Here A = 60 degree (Since equilateral prism)

deviation is 40 degree

This gives A+\delta =i_{1}+i_{2}=100


the values of angles of incidence are 60 degree and 40 degree


Refraction – Disappearing letters

A rectangular block of glass is placed on a printed page lying on a horizontal table. what is the minimum value of u of glass so that the letters on page are not visible from any of the vertical faces of the block.

Asked Kushagr

Where is the light energy going?

where is the sun’s light enery going once the sun has set?


where is the light going when we switch off the torch facing an object. if light gets reflected how long it will do so. and if it gets absorbed, then will it add to the potential energy of the object that is facing the torch?


English: plasma lamp Français : Lampe plasma. ...

Is there any cycle (like hydrocycle)to prove that energy can neither be created nor be destroyed?


Asked Dingu Sagar

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