## 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?

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

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

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)

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?

Use the formula $\delta&space;=i_{1}+i_{2}-A$

Here A = 60 degree (Since equilateral prism)

deviation is 40 degree

This gives $A+\delta&space;=i_{1}+i_{2}=100$

Therefore,

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.

## Where is the light energy going?

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

or

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?

or

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