The Science Behind a Prism
We’ve seen how raindrops behave like tiny prisms and break sunlight into colors. But did you know that you can do the same thing with just a glass prism?
A triangular glass prism can separate white light into its constituent colors, called a spectrum. Each color is bent or refracted a little differently.
Light Travels at Different Speeds
The speed of light is known as c, and it’s exactly 299,792,458 meters per second. It’s a universal constant that applies to all matter and energy in the Universe.
When light travels through a medium, it slows down as the charged particles in the material interact with it. In the case of a prism, this slowing can cause different colors to separate.
Depending on the medium, light can also slow down by slightly different amounts as it passes through it. When you shine light through something transparent, like water or glass, the more energetic photons will slow down faster than less energetic ones.
This is what creates a rainbow when you shine white light through a prism. As we’ve mentioned, it’s also why a beam of light will bend when it hits glass at an angle.
Light Gets Bent
In the previous Wonder, we learned that when light enters a prism, it bends and separates into its component colors (the rainbow). This optical phenomenon occurs because of something called refractive index.
Every material has a different refractive index. The refractive index of glass, for example, is different from the refractive index of air.
When white light passes through a prism, it bends at different angles because of the difference in refractive indexes. The shorter wavelengths, which make up the different colors of white light (red, orange, yellow, green, blue and violet), get bent more than the longer ones. This causes the color to fan out, making the individual colors distinct from each other.
Light Separates Into Colors
When light enters a prism, it bends and separates into the component colors of red, orange, yellow, green, blue, indigo and violet. The reason for this is due to the differences in refractive index between the air and the glass of the prism.
The refractive index is a measure of the speed that light travels through an object. The higher the refractive index, the faster the light travels through it.
In non-vacuous materials (like air), the longer a wavelength of light is, the lower its refractive index. This means that light with shorter wavelengths, like red, will bend more than light with longer wavelengths, like green.
This is why you see the rainbows we made in our spectoscope experiment. Water droplets in the air cause light to change its angle of refraction and disperse into its component colors.
Light Becomes Focused
When light enters a prism, it is refracted. The amount of refraction depends on the apex angle, the angle of incidence and the refractive index of the material.
When the ray strikes the outside wall of the prism, it is refracted again following Snell's law. This time, the ray is bent away from the normal, because air has a lower refractive index.
The ray then leaves the prism and refracts again, this time in the opposite direction. The refraction causes the red and violet component of the white light to bend a bit more than the other rays.
As a result of this refraction, the seven color components of the light become segregated into their individual wavelengths. These seven colors are the spectral colors of visible light: blue, green, yellow, orange, violet, indigo and red.










