Explain how Corpuscular theory predicts the speed of light in a medium, say, water, to be greater than the speed of light in vacuum. Is the prediction confirmed by experimental determination of the speed of light in water? If not, which alternative picture of light is consistent with experiment?

Newton’s Corpuscular theory of light theorised the light to consist of small corpuscles and states that, when light corpuscles strike the interface of two media from a rarer (here air) to denser (water) medium, the particles experience forces of attraction normal to the surface. Hence, component of the velocity normal to the point of interaction with the surface increases while the component parallel along the surface remains unchanged. Thus component of velocity of light in one direction will be equal.

Suppose we take

v as the velocity of the light in rarer medium (air)

v’ as velocity in the denser medium (water)

The components parallel to the surfaces can be written as,

v sini and v’sinr


i is the angle of incidence in the rarer medium,

r is the angle of refraction in the denser medium

But, v sin(i) = v’ sin(r)

(where μ is the refractive index). And, we have refractive index ‘μ’ always greater than unity.

Thus, light should travel faster in the denser medium like water than in air.

Note: The experimental results prove that no object can travel faster than the speed of light i.e. c.


No, the predictions of Newton’s Corpuscular theory were not confirmed by any experimental determination of the speed of light in water.

The Huygens wave theory predicts that v’ is greater than v which is consistent with the experimental results.