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If photons are massless, how do they get reflected, blocked when something comes in its way? Shouldn't the particle just pass right through any object?

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  • $\begingroup$ E=mc^2 and m act as gravytational mass $\endgroup$
    – trula
    Aug 13, 2023 at 12:50
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    $\begingroup$ They're massless, but they still have momentum and energy. There is an energy transfer (to the object's electrons), and then the energy gets re-radiated back as photons. The 'reflection' here is actually scattering. $\endgroup$ Aug 13, 2023 at 13:00
  • $\begingroup$ @PrathamHullamballi. Why does the object re-radiate it, and that to so quickly and in such a specific direction/angle? If a photon hits an object, why doesn't it get get re-radiated in an angle unequal to the angle of incidence? $\endgroup$ Aug 13, 2023 at 13:09
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    $\begingroup$ Regarding why electrons re-radiate photons, the answer is very non-trivial. There are many good answers on StackExchange, so look them up. Why is that the photon gets re-radiated back at an angle equal to the angle of incidence is due to the conservation of energy and momentum. Answers to this can range from Fermat's Principle to properties of electromagnetic waves that one can use to study reflection. $\endgroup$ Aug 13, 2023 at 13:43

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Special relativity says that an object that moves at the speed of light - like photons - cannot be slowed down or stopped. However, there is nothing that forbids a photon from being reflected (moving at the different direction at the speed of light) or absorbed (completely disappearing).

Photons are the gauge boson associated with electric charge, and as such interact with any electrically-charged particles. In particular they interact with electrons, which are plentiful in dense materials. A photon hitting a material is usually absorbed by one of its many electrons (raising its energy level), and then either quickly thrown back out (reflection) or turns into movement of the atoms and heat (black body absorption). In certain materials, the energy levels allowed (by quantum mechanics) to electrons do not allow absorbing photons of particular energy (i.e., wavelength), so this material is transparent to these wavelengths.

It is not true that photons are absorbed by anything that has mass. A neutron, for example, has mass, but does not have a charge and as such cannot absorb or reflect or otherwise interact with photons (I'm limiting the discussion to low-energy photons to avoid more elaborate second-order effects). In theory a big ball of neutrons will be transparent. In practice, neutron stars are not 100% neutrons and not transparent (see Are Neutron stars transparent?).

Something unrelated that mass can do is do deflect or attract photons - i.e., gravity also curves light rays. This is general relativity. But it doesn't "reflect" or "block" photons - just slightly modifies their trajectory.

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    $\begingroup$ The neutron might be neutral but it has a nonzero magnetic moment so it can interact with photons. It's just that it's much weaker than those of particles like protons and electrons. No need for high energy photons here either. $\endgroup$
    – Triatticus
    Aug 13, 2023 at 16:26
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It's all due to something that we can never observe directly ... it is the EM field (Electro Magnetic field). We know it is present because magnets and electric charges attract/repel and electric currents make magnetic fields as well. The term photon refers to the real energy that can pass thru a vacuum like space .... we see it as visible light. (There are also something called virtual photons which we use to describe EM forces).

All atoms/molecules are surrounded my moving electrons and they are always working with the EM field .... atoms/molecules creat many EM energy transfers called photons .... and they also absorb photons.

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