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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.



QM, Strong Force and Baryons

Could you please explain in a simple way how strong force inside baryons really works? I mean, gluons are messengers of strong force but they are also sensitive to strong force(they are charged) so they are producing other gluons as messengers of their own chrage. What mechanism prevents to grow the number of gluons in lets say proton to go to infinity? They are annihilating with each other so statistically there is still finite number of gluons or is there another mechanism? Thank you!




Asked Peter Stiller


First baryon octet


We are not Psychic – It’s Physics Here!

We receive many questions daily where in questions based on dreams, love, relations etc. are asked.

Psychic CatWe thought initially that those were posted by spammers. But the emails and posts were not from spam-bots but genuine IPs and persons. On analysis, it was found that those posts were made by people who thought that the site name as “AskPsychics” and thus they asked such questions. Some even misspell Psychic as Physic.

When they don’t get their answers, they come back to the site and start looking at the answered questions to find that the site is all about Physics and that there are no Psychichs here!

Thanks to our principle that we do not allow questions to be published directly. All questions posted via the Ask a Question link reach the email inbox of the admin and editors of the site. we scrutinize the question and are posted to the site only if it is related to Physics or education.

Once again I would like to announce –

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Not Psychics

Question Bank in Class XI Physics – Mechanical Properties of Solids

Important Points to remember:

  • Deforming forces are external force applied on a body to change its size or shape.
  • Restoring force is the internal force developed in a body trying to bring it back to the original configuration when it is acted upon by an external force trying to change its size or shape.
  • Elasticity is the property of a body by virtue of which it tries to regain its original configuration when the deforming forces are removed.
  • Perfectly elastic body is a body which regains its original configuration completely on removal of deforming forces.
  • Plasticity is the property of a body by virtue of which it does not regain its original configuration on removal of deforming forces.
  • Perfectly plastic body is a body which does not show any tendency to regain its original configuration on removal of deforming forces
  • Stress: The restoring force developed per unit area of a deformed body is called stress.
  • Tensile stress: If a deforming force is trying to produce an increase in length, then the restoring force developed per unit area of cross section is called tensile stress
  • Compressive Stress: When a deforming force acting normal to a surface tries to produce a change in volume, then the restoring force produced per unit area is called compressive stress.
  • Shearing stress: When a tangential force tries to change the shape of a body whose bottom is fixed, then the restoring force developed per unit area is called Shearing stress.
  • Strain: The ratio of the change in dimension to the original dimension of a deformed body is called strain
  • Linear or longitudinal strain


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


Physics Honours as career


I want to pursue my career in Physics Honours . Can you please suggest me some good institutions ?


Asked Akshat Kanwar






Electrostatics Problem

2 identical metal spheres having charges +15 microC and +25 microC are separated by a distance.If the spheres are 1st brought into contact and then separated to the original distance,what is the ratio of the new force between them to the previous force?kindly explain.

Posted by Ravichandran

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January 2013

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