Why if a ray pass though different materials its direction changes?

AI Thread Summary
When light passes through different materials, its direction changes due to the varying speeds of propagation in each medium. The wave front of light is perpendicular to its direction of travel, and when it encounters a new medium, one edge enters first, leading to a change in velocity. This difference in speed causes the wave front to rotate, resulting in a change in the angle of the ray. The phenomenon is explained by Snell's Law, which describes how light refracts at the boundary between two materials. Understanding this principle is essential for explaining optical behaviors in various contexts.
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Hello, why if a ray pass though different materials its direction changes ? couldn't its propagation speed just increase or decrease without having to change the angle ?

Thank you
 
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The way I was taught is that light has a wave front that is perpendicular to the direction of travel. When the wave front hits a different medium, one edge of the wave front enters the medium first and its velocity changes. This causes the wave front to rotate and change the direction of the ray.
 
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