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We say that to raise an electron to an upper energy level (To excite the electron) you need have a photon with a energy that is equal to the energy gap between the ground state and the next energy level.

But I´m confuse because I cant understand what happens to all the other photons (With different wavelengths) that don´t have the right energy to excite the electron ? Are they reflected ? If so, how is the color of an object defined if a lot of different photons that are within the visible spectrum wavelength are being reflected and reaching our eyes ?

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Photons with "too much" energy are absorbed and can excite electrons as well. The extra energy can be used to create another photon. For crystals, the energy can also be absorbed into the lattice as a mechanical vibration.

Photons with "too little" energy will not be able to excite the electron, and just continue on their merry way. Think about glass (fused silica), which has a band gap of 9eV. It does not absorb visible light because the photons don't have enough energy to be absorbed.

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  • $\begingroup$ Does that mean that the photons will just pass straight trough the objects ? Or will they sort of pinball their way out ? $\endgroup$ – Goncalo Fonseca Apr 2 '18 at 9:19
  • $\begingroup$ The photons will go straight through it they're not absorbed. The light also has wavelike properties where the wavelength will change when it enters a material is a different dielectric constant. This phenomenon is described classically as the Huygens-Fresnel Principle. $\endgroup$ – PaulisDontExcludeMe Apr 2 '18 at 21:27
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When a photon interacts with an atom, 3 things can happen:

  1. elastic scattering, the photon keeps its energy but changes angle.

  2. inelastic scattering, the photon will give some of its energy to the atom, and will change angle.

When the photon changes angle it is 180 degrees in case of a mirror, and same angle in case of a glass

  1. absorption, the photon will give all its energy to the atom

The color you see comes from the reflected and the emitted photons. A body can emit photons even when no light shines on it, like the sun.

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