Learning Object

The Marching Band Analogy

This cartoon shows you a marching band, walking from an area of solid ground where the musicians can walk fast, into a region of muddy ground where they have to walk more slowly. Each row of musicians carries a long pole. Watch what happens when the first person of a row enters the muddy ground: the direction of the pole changes lightly. As more marchers of the same row come to the muddy ground, the pole rotates further. The only reason for this change in direction is the fact that the people walk at different speeds on the solid and muddy grounds. Interesting, isn't it? You can try this out yourself with some friends by marching in a row, holding a meter stick. This analogy can explain how the kink in the light ray comes about when the light enters a medium of different index of refraction, i.e., a region where its speed of propagation is less. The pole along the row of marchers symbolized the wave front, and the light beam propagates perpendicular to this wave front. As the wave front changes direction because of the different speeds of light in the different media, so does the light beam. And that's how light beams get kinks when you shine then at an angle from one medium into another: simply because light propagates with different speeds in different media. The speed of light in a medium is given by c/n, where n is the index of refraction of the medium. So you can see that the higher the index of refraction of a medium, the slower light moves in that medium.