Course:Refraction

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Refraction of a light ray

Refraction means the bending of light inside a transparent/translucent medium, such as a gemstone.
When light travels from air into an optically denser medium (such as a gemstone), the light will slow down inside that gemstone and it will change direction. The amount at which this light slows down and changes direction is dependent on the refractive index of the gemstone.

You will often read about the index of refraction instead of refractive index and think they are the same, but they aren't the same thing.
This has to do with the fact that when we think of light, usually we think of it as the white light coming from the sun (or a lightbulb). As you will learn later in the dispersion chapter, that white light is made up of many colors and all these colors refract differently inside an optically denser medium.
As you can imagine it is not very useful to have to calculate the amount of bending all of these different colored lightrays undergo. Instead we want to measure it for just one color.
When we speak of the index of refraction, we mean the whole range of bending of white light. From red to blue light.
When we use the term refractive index, we refer to the amount of bending for just one color.

Ofcourse we need to pick a color for this refractive index and in gemology we use yellow light with a wavelength of 589.3 nm, which is sodium light. Every reference in gemological textbooks uses this lightsource unless otherwise stated.

In optics, and optics related topics like gemology, the index of refraction is abbreviated with the small letter n and the yellow sodium light is abbreviated with the capital letter D (usually in subscript). So if you read something like nD = 1.714, you know that it is the refractive index measured with sodium light.

Below is a small animation of how light behaves when it travels from air in an optically denser medium and then in air again. Play the video a few times and observe what is happening.
The red arrow represents the light and you will notice that when it reaches the square piece of glass, we draw in the normal (the blue line). Then it travels inside the glass, but it bends and travels slower.
The light will then reach the lower boundary between the glass and air again. You see that it bends again and picks up speed.

The image on the right of the animation shows the path the light takes represented by the black line. It seems that the light before it entered the glass and the light that left the glass are traveling in the same direction, they are parallel. And indeed they are parallel.
In the next image the angles are drawn in and you will see that both these angles are exactly the same. We always measure angles in reference to the normal (the blue perpendicular lines).

click to enlarge
click to enlarge