during reflection of light energy transfer is not 100%. when light from a rarer medium like air strikes a denser medium like glass slab, some part of light is reflected back and some of it is refracted.
But during total internal reflection (TIR) when a beam of light from a denser medium like water strikes a rarer medium like air, it is 100% reflected and there is no loss of energy. why?
what is the reason that no energy is lost in TIR but lost in glass slab?
Firstly, your point about total-internal-reflection (look into the tag wiki for further info about the phenomenon) resulting in 100% transmission of light energy and hence 100% intensity of incident light being preserved during reflection is conventionally considered true, but is not entirely correct.
Let me explain:
Reflection of any type, even if it is total internal reflection, cannot reflect 100% of the incident light energy. Total Internal Reflection nearly 100% (99.9999% maybe, but not 100%) efficiency compared to conventional reflection from a surface.
An interface between 2 optical media has the property of a "critical angle", the angle of incidence beyond which if light is incident on the interface from the optically denser medium into the optically rarer medium, it will be reflected back entirely into the denser medium, with no refraction into the rarer medium. Normally, at an interface between 2 transparent media of different optical densities, the light wave will be partially reflected into the medium from which it was incident and partially refracted into the medium into which it was incident, until the angle of incidence goes beyond this critical angle. This phenomenon of total internal reflection is usually observed for light, but is also applicable for other types of waves, like sound and waves on a string. This phenomenon is explained by Huygen's Wave Theory of Light in a classical sense. It occurs at the interface between 2 media of different densities in which the wave travels.
Hence, the intensity of incident light is preserved nearly entirely in total internal reflection, except for that minor fraction which may be absorbed by the denser medium itself, which is low due to the medium being transparent. There is also some energy loss due to photon tunneling across the interface, but again, it is not conventionally significant (total about $10^{-3} \%$). There are also evanescent-waves across the interface, but they do not result in net energy transfer across the interface. Refer Wikipedia here.
In conventional reflection, 2 surfaces are involved:
1) A transparent unsilvered surface
2) An opaque silvered surface
For light transmission through the unsilvered surface, similar energy losses are applicable as in case of total internal reflection.
However, for light reflection at the silvered surface, the surface being opaque, will absorb a significant portion (say about 1%-2%) of the incident light energy, while it will reflect most of the light energy due to it being silvered and reflective.
This 2nd significant energy loss of incident light at the silvered surface is not applicable for total internal reflection, hence we commonly say that total internal reflection forms images at 100% the brightness (intensity) of the incident light.
I got my answer.
When light enters in glass from air, it likes to reflect back by wave front theory (although 1-2%). In case of from denser to rarer medium first below critical angle some part of it is reflected back and some is refracted, but above critical angle some is reflected and other part (which was earlier refracted) is even reflected back causing 100% reflection back.
