When light travels from a less optically dense medium

[Byrne]

When light travels from a less optically dense medium to a more optically dense medium, the refracted ray bends toward the normal (i.e. air -> water). The reverse is true when traveling from a less optically dense medium to a more optically dense medium.

Now... why is this? Why does a decrease in wave velocity cause the ray to bend toward the normal and not away?

Thanks.

 

[Doc Al]

One way to understand refraction is via Huygens' principle. Here's a useful animation: http://www.walter-fendt.de/ph11e/huygenspr.htm

 

[astrokreng]

I can explain this phenomenon using the principles of optics. When light travels from one medium to another, its speed and direction change due to the change in the medium's optical properties. The speed of light in a medium is determined by its refractive index, which is a measure of how much the light is slowed down in that medium compared to its speed in a vacuum.

In the case of light traveling from a less optically dense medium (lower refractive index) to a more optically dense medium (higher refractive index), the speed of light decreases as it enters the new medium. This decrease in speed causes the light wave to bend towards the normal, which is an imaginary line perpendicular to the surface of the medium. This can be visualized by imagining a car entering a muddy road - its speed decreases and it turns towards the normal direction.

This phenomenon is known as refraction and is governed by Snell's law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the speeds of light in the two media. In simpler terms, as the speed of light decreases, the angle of refraction becomes smaller, causing the light to bend towards the normal.

In contrast, when light travels from a more optically dense medium to a less optically dense medium, the speed of light increases and the angle of refraction becomes larger, causing the light to bend away from the normal. This can be seen when a ray of light passes from water (more optically dense) to air (less optically dense).

In summary, the change in wave velocity due to the difference in refractive index between two media is what causes the bending of light towards or away from the normal. This is a fundamental principle in optics and is essential in understanding the behavior of light in different mediums.