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My understanding is: when light hits an object, it can do one of three things: transmit through the object, be absorbed into the object, or reflect from the object.

However, I have also learned that when light hits atoms, it can excite electrons, leading to another emission of light as the electrons go back down to their ground states.

The first paragraph, while intuitive, does not seem to be logically explained by the second. Can someone please help me understand the first in terms of the concepts of the second?

Specifically, is absorption of light exciting an electron with more energy than necessary, and some energy remains in the atom, or is the entire energy of the photon absorbed?

Is reflection just hitting the atom with a photon that is just the right amount of energy to get it excited, so no net energy is gained?

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  • $\begingroup$ Not quite right. Light can be transmitted, absorbed or scattered. Reflection is a particular form of scattering. en.wikipedia.org/wiki/Scattering $\endgroup$ – MaxW Apr 16 '16 at 3:29
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While I can't explain the phenomenon of light transmission, I can say a few things. The energy levels correlating to physical states and the gaps between the states are quantized. This an all or nothing reaction. The frequency of light must exactly match the energy (E=hv) needed for the excitation event (electron is pushed into a higher orbital, a vibrational mode to expressed, etc) for absorption to occur. When the light frequency is mismatched and does not absorb, it will reflect. The frequency that induces excitation is theoretically the frequency of emission, but phenomena such as intersystem crossing may create multiple paths by which the excitation energy will disperse. Hope this helped.

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If light is neither absorbed nor scattered, it is transmitted through the substance. This occurs when the energy from the light is transferred into the substance, and the vibrations of the electrons are passed on through the substance and re-emitted on the other side. Different arrangements of atoms in substances can cause these vibrations to propagate differently through different substances. This is the cause of Snell's law.

Different substances absorb specific wavelengths of light. This is determined by the differences in energies of the different atomic or molecular orbitals. The energy of the photons promote electrons from the ground state to an excited state, and is then dissipated as vibrational energy as the electron returns to the ground state. All other light that does not match these energy differences are cause electrons to vibrate on a much smaller scale for a short amount of time before re-emitting the light, thereby scattering it.

White light consists of all the wavelengths of the visible spectrum. When all wavelengths are scattered by a substance, it appears white. When all visible light is absorbed, it appears black. The dissipation of all this energy absorbed by black substances explains why black cars get hotter in the sun than white cars.

absorbed vs observed

When only some wavelengths of light are absorbed, the resulting scattered wavelengths constitute what our eyes perceive as color.

It's also important to note that visible light isn't the only part of the electromagnetic spectrum that can interact with matter. The spectrum ranges from high frequency gamma rays and x-rays to low frequency microwaves and radio waves. If you were wondering if there was anything particularly special about visible light, check out this physics post!

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  • $\begingroup$ Hmm, the way I learned it was that light is emitted when an electron falls from an excited state to the ground state. However, you are saying that if a photon exactly matches the energy difference necessary to excite an electron, the light will instead be absorbed. Is this correct? Also, What exactly happens in the process of scattering? Does scattering mean that the original photon hitting the object never excites an electron in the first place? $\endgroup$ – extremeaxe5 Apr 21 '16 at 2:17

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