How did Thomas Young and Augustin-Jean Fresnel explain double refraction?

Thomas Young and Augustin-Jean Fresnel explained double refraction by proposing that light waves are transverse waves, meaning their oscillations occur perpendicular to the direction of propagation. This was a significant departure from the earlier, incorrect assumption that light waves were longitudinal, like sound waves.123

By identifying light as transverse, Young and Fresnel provided a framework to understand polarization effects. In this model, the polarization of an incoming light wave can be described as a combination of two perpendicular polarization components. When this light enters a birefringent material like calcite, these two components travel at different speeds. Because they experience different indices of refraction, they refract differently through the crystal, resulting in the two offset images observed in double refraction.1

Fresnel subsequently developed a comprehensive mathematical model of these transverse waves, which successfully accounted for double refraction and various other optical phenomena, ultimately leading to the universal acceptance of the wave theory of light.31