Is Huygens's Wave Theory still correct? We have to study on details about Huygens's Wave Theory though we have Electromagnetic theory, quantum theory today. Is it still correct or not?
 A: Huygens's Wave Theory is what you call a first order scalar diffraction theory of light. So what does it describe and what does it fail to describe?


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*First order means that electromagnetic effects like induced currents in surfaces etc. are ignored. These can be described by solving Maxwell's equations for the same system instead of working with the wave picture.

*Solving Maxwell's equations would also remove the second restriction: the scalar nature of the theory means that polarisation effects are ignored. This becomes a problem when working with optical devices that support cross-polarisation scattering.

*Non-linear optical effects can in fact be worked into Huygen's theory to some extent by considering the time dependent form of the wave.

*The most difficult question is to what extent the theory is quantum. The wave picture already describes interference and other quantum effects, e.g. coherence of the wavefield in the spatial and time domain can be included. One runs into trouble when getting into the single photon or pulse regime. That's where a quantum mechanical theory has to be used (or even quantum field theory in some contexts).


Note: this list is supposed to be a summary of the approximations in the theory of Huygens's wavelets and the range of applicability.
A: Huygens worked with scalar, longitudinal waves. This was proved to be incorrect in 1821 by Fresnel, who showed that polarization requires transverse waves.
A (relevant but incomplete) history of light and how well different models explain certain experiments is given in my recent lecture ''Classical models for quantum light''; see
http://arnold-neumaier.at/papers/physpapers.html#CQlightslides
A: Huygen's principle has certain limitations.
However, you cannot say that it is incorrect. Like dalton theory, Thomson's theory, Rutherford's theory, Huygen's theory, they were stepping stones to the study of optics. Even we can consider light as wave, for example - when we light a candle in dark room , it would spread light around all sides thus we can consider light as a wave here. 
