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Is it necessary that for an E/M wave of given frequency which can pass through a medium of given refractive index, it should lose some of its intensity. Practically, this must be necessary because of imperfections in the crystal or other defects.

Also I think that if the photons collide with the nucleus, there has to be a loss in intensity, but I intend to ignore the effect of the direct collision with the nucleus which would be very small as is expected. So, are there any other reasons for intensity loss? Assume that we are talking about glass and red (monochromatic) light.

Edit: I read that glass is transparent in the visible range of light because in the visible range, photons cannot supply exactly the energy to the electrons for the electrons to absorb it and jump to a excited state. If we assume this, then a monochromatic light will either get reflected or refracted. How can both occur in the same situation?

Hence, can we choose a monochromatic light of such energy that it is less than the energy required to excite the electrons present in the material, and hence the material is unable to absorb the photons at all? (assuming complete uniformity in the crystal design of the material throughout). Hence there will be no intensity loss, theoretically.

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When light hits a barrier, even transparent ones, some light is reflected and some is refracted. This is often described by the transmission coefficient for that material, and at that wavelength. This can happen at the macroscopic barriers and at the smaller barriers between crystals or grains within a material. It is a simple property of waves which does apply with light waves.

You can think of this as light being absorbed and re-emitted by a material in random directions, but with a preference for the direction of the incoming light. Due to the fact that some photons will go through and others will be emitted in other directions, the intensity will decrease. Additionally, some of those photons which are absorbed do not actually get re-emitted and converted to heat.

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  • $\begingroup$ Can we choose a monochromatic light of such energy that it is less than the energy required to excite the electrons present in the material, and hence the material is unable to absorb the photons at all? (assuming complete uniformity in the crystal design of the material throughout). Hence there will be no intensity loss, theoretically. $\endgroup$
    – Prem
    Apr 13, 2015 at 17:20
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    $\begingroup$ @Raja In theory, yes. In practice, this rarely happens. We can get pretty close, though. $\endgroup$
    – PipperChip
    Apr 13, 2015 at 17:35
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Since your title starts with "Theoretically", I will give the theoretical answer: no.

Intensity does not need to decrease. If you send polarized light into a slab of transparent material at the Brewster angle, then there will be NO reflection, and ALL the light will be transmitted. Theoretically, this means that you might get 100% transmission. In practice, materials will have defects that may result in not all the light being transmitted, but there is no fundamental reason why this should always be the case.

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