3 ms·

> For perfect reflection, no. That's not true, it wouldn't conserve energy. The kinetic energy of the object that's hit changes, so the light must lose or gain

> For perfect reflection, no. That's not true, it wouldn't conserve energy. The kinetic energy of the object that's hit changes, so the light must lose or gain energy. I think for a reflection this must be because of a red- or blueshift. > What you're referring to is light absorption, which is a different effect that raises the temperature of the target object and red-shifts the incoming photon. Absorption usually means a whole photon is absorbed, not redshifted. A lower energy photon can sometimes be emitted after absorption (however the direction and phase relation with the previous photon are lost)

zepearl 8y ago
> That's not true, it wouldn't conserve energy. The kinetic energy of the object that's hit changes, so the light must lose or gain energy. You mean that as the "path" of the photon is changed by hitting the target object, the vector of the target object must change accordingly? (if yes then how "can" the new vector of the target object computed, if the source object/photon has no mass (I would basically miss a variable for the equation)? IF it does have no mass...)
- typothrowaway 8y ago
  Yes, using its momentum and energy. Here you can read the relations between a photon's momentum and things like wavelength. Without any mass :) https://en.wikipedia.org/wiki/Photon_momentum#Physical_properties https://en.wikipedia.org/wiki/Photon_momentum#Physical_prope... (You need to scroll down a bit for the formulas) These results come from special relativity(and a bit of quantum mechanics), which you really need if things don't have mass.
  - zepearl 8y ago
    Thx a lot - next questions follow... :) (I tried, but the wikipedia chapter is too complicated for me => trying to simplify here) > ...it's momentum and energy. Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right? [puah - for some reason I cannot reply to the my post's - typothrowaway's - reply...]
    - typothrowaway 8y ago
      > (I tried, but the wikipedia chapter is too complicated for me => trying to simplify here) I can imagine. That stuff on the wiki is not even in every physics bachelor. It's definitely not for a general audience, but I thought it might help a little :) > Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right? Yes, or gain it if it slows down the object, if the object is traveling towards the light. Or the internal energy of the object is changed. This happens when a photon is absorbed.

zepearl 8y ago

> That's not true, it wouldn't conserve energy. The kinetic energy of the object that's hit changes, so the light must lose or gain energy. You mean that as the "path" of the photon is changed by hitting the target object, the vector of the target object must change accordingly? (if yes then how "can" the new vector of the target object computed, if the source object/photon has no mass (I would basically miss a variable for the equation)? IF it does have no mass...)

typothrowaway 8y ago
Yes, using its momentum and energy. Here you can read the relations between a photon's momentum and things like wavelength. Without any mass :) https://en.wikipedia.org/wiki/Photon_momentum#Physical_properties https://en.wikipedia.org/wiki/Photon_momentum#Physical_prope... (You need to scroll down a bit for the formulas) These results come from special relativity(and a bit of quantum mechanics), which you really need if things don't have mass.
- zepearl 8y ago
  Thx a lot - next questions follow... :) (I tried, but the wikipedia chapter is too complicated for me => trying to simplify here) > ...it's momentum and energy. Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right? [puah - for some reason I cannot reply to the my post's - typothrowaway's - reply...]
  - typothrowaway 8y ago
    > (I tried, but the wikipedia chapter is too complicated for me => trying to simplify here) I can imagine. That stuff on the wiki is not even in every physics bachelor. It's definitely not for a general audience, but I thought it might help a little :) > Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right? Yes, or gain it if it slows down the object, if the object is traveling towards the light. Or the internal energy of the object is changed. This happens when a photon is absorbed.