Mention any one demerit of Huygens' wave theory of light.

Explanation

Huygens' wave theory of light (1678) :
(1) Light emitted by a source propagates in the form of waves. Huygens' original theory assumed them to be longitudinal waves.
In a hormogeneous isotropic medium, light from a point source spreads by spherical waves.
It was thought that a wave motion needed a medium for its propagation. Hence, the theory postulated a medium called luminiferous ether that exists everywhere, in vacuum as well as in transparent bodies. Ether had to be assigned some extraordinary properties,a high modulus of elasticity to account for the high speed of light, zero density so that it offers no resistance to planetary, and perfect transparency.
(4) The different colours of light due to the different wavelengths.
Merits of the theory :
(1) Huygens' wave theory satisfactorily explains reflection and refrection as well as their simultanneity.
(2) In explaining refraction, the theory concludes that the speed of light in a denser medium is less than that in a rarer medium, which agreed with later experimental findings.
(3) The theory was later used by Young (1800-04), Fraunhoffer and Fresnel (1814) to satisfactorily explain interference, diffraction and rectiolinear propagation of light. The phenomenon of polarisation could also be explained considering the light waves to be transverse.
Demerits of the theory :
(1) It was found much later that the hypothetical medium, luminiferous ether, has no experimental basis. In 1905, Einstein discarden the idea of ether completely.
(2) Phenomena like absorption and emission of light by atoms and molecules, photoelectric effect, Raman effect, Compton effect, etc., cannot be explained on the basis of the wave theory.