[Physics] How does light, which is an electromagnetic wave, carry information

dispersionelectromagnetic-radiationinformationphotonsvisible-light

We see an object when light from a source strikes the object and then reaches our eyes. How does light, which is an electromagnetic wave, gets encoded with the information about the object? Do the individual photons get encoded with this information or is it the wave nature of light that gets modified to carry information about the object?

Also, if light hits an object and then another on the way to our eyes, does it only carry information from the very last interaction it had? How is the information due to all previous interactions erased (if it is indeed)?

Best Answer

You talk about light as if it were a person carrying a clip board writing down things on its way to you. It is a physical phenomenon that gets affected as it propagates.

Depending on the various processes that it goes through before it reaches your eye, its amplitude,polarisation, frequency (or wavelength), pulse time etc. get affected from which we can infer what it must have gone through and get to know of the object it must have reflected off or gone through or originated from.

If the frequency is changed, the photon is said to have a different energy from $E=h\nu$. Since light has both particle and wavelike properties, depending on the situation we are in, we can equally talk about $k=\frac{2 \pi \nu}{c}$

Consider these examples:

  • Say you have a pocket laser source. You shine it on two walls, one at 500 m and one at 1 km. The light travels for more time to get back to you from the second wall. Here, the light is unaffected but only the time is recorded. If you did not know the distance the walls were from you, now you can calculate the distance the walls are at. This is information

  • Leaves are green. This means that they reflect green light and absorb all the other colours that are present in the sunlight. When you go outside and can "see" a leaf, it is information. Now, the frequencies of a light have been partly affected.

  • You see stars at night. Light has travelled for many years and the photons have hit your eye. Now you know how the star looked some few years ago. (the light from the nearest star takes around 4.5 years to come to you). Thus, information on the star's position is being carried, along with it's temperature. The wavelength of the light reaching you is carrying information.

  • Light from objects are also "doppler shifted" : the police use this effect to get the information - the speed of the car that they shine the radar gun on. The frequency is actually changed in this process. This frequency change is carrying information.

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