When light travels from a rarer (less optically dense) medium to a denser medium, such as from air to water or from air to glass, the path difference refers to the difference in the optical path length traveled by light rays entering the medium at different angles. This path difference is due to the change in the refractive index between the two media.
When light enters a denser medium, it undergoes refraction, which means it changes direction and its velocity decreases. The change in velocity is due to the change in the refractive index of the medium, which is a measure of how much the speed of light is reduced in that medium compared to its speed in a vacuum.
The path difference can be calculated using Snell's law, which relates the angles of incidence and refraction to the refractive indices of the two media. Snell's law is given by:
n1 * sin(theta1) = n2 * sin(theta2)
where:
- n1 and n2 are the refractive indices of the initial (rarer) and final (denser) mediums, respectively.
- theta1 and theta2 are the angles of incidence and refraction, respectively.
The path difference is the difference in distance traveled by two light rays that enter the medium at different angles but meet at a common point. This path difference can lead to various optical phenomena, such as refraction, reflection, or interference, depending on the specific circumstances and the angle of incidence.
It's important to note that the path difference refers to the difference in optical path length, not the geometric distance traveled by the light. The optical path length takes into account the refractive index of the medium and is given by the product of the geometric distance and the refractive index.