Q.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 explains refraction by assuming the denser medium pulls light corpuscles inward, speeding them up — so it wrongly predicts light should travel faster in water than in vacuum, a prediction experiment later overturned.

Step 1: How the corpuscular theory explains refraction

According to Newton's corpuscular theory, light consists of tiny particles (corpuscles) emitted by a source. When these corpuscles approach the boundary of a denser medium like water, the theory assumes the medium exerts a strong short-range attractive force on them, acting perpendicular to the surface. This force increases only the component of velocity normal to the surface, leaving the component along the surface unchanged. As a result, the corpuscle bends towards the normal on entering the denser medium — reproducing the observed law of refraction — but only if the speed inside the medium is greater than outside, since the normal component of velocity has been increased by the attractive pull.

Step 2: The corpuscular prediction

For this mechanism to correctly bend the corpuscle towards the normal (matching the observed bending of light entering water), the theory requires:

i.e., light should speed up on entering a denser medium such as water.

Step 3: Experimental test

In 1850, Foucault directly measured the speed of light in water and compared it with the speed in air/vacuum. The measurement showed the opposite of the corpuscular prediction:

Light was found to travel slower, not faster, in water — refuting the corpuscular theory's prediction.

Step 4: Which picture is consistent with experiment?

The wave theory of light (Huygens) correctly predicts $v_{medium} = c/\mu$, i.e., light travels slower in a denser medium (higher refractive index), exactly matching Foucault's result. This was one of the decisive pieces of evidence in the 19th century that helped establish the wave theory of light over the corpuscular theory.