Deep Dive: Changing the Speed of Light

Deep Dive: Changing the Speed of Light

Inq's Pyramid Prism

So how can the pyramid prism be both a prism and…not a prism?  It’s because optically, it is a tool called a prism, but geometrically, it’s a pyramid – a distinctly different 3-dimensional shape that is, to be clear, not a prism. 

Geometrically, prisms and pyramids are similar: they are both polyhedrons (3D shapes with multiple sides) whose faces are polygons (2D shapes with multiple sides). Both of them have only straight edges and angles, nothing rounded. And all the sides meet up at the base. The big difference is that prisms have two congruent bases, which are identical and parallel to each other, but pyramids have only one – on the other side, the polygons meet at a vertex, or point.

Inq's Incredible Ruler is a perfect example of a triangular prism vs. this pyramidal shape. 

While the object you have is geometrically a pyramid, optically – or in terms of optical instruments at least - it is a tool called a prism. In this use, a prism is an object of clear material used to redirect or disperse light, and more specifically, one in which light of different frequencies (colors) travels at different speeds, called a “dispersive medium.” So yes, your pyramid prism is exactly that: geometrically a pyramid, optically a prism.

That change in the speed of light has to do with the most famous effect of prisms: they can “split” light into a spectrum, painting a beautiful splash of rainbow colors on your wall. There are two effects at work here. The first is “refraction”: a phenomenon where light changes its angle when it passes from one medium (the air) to another (the glass). One of the interesting properties of waves of light is that different wavelengths have different speeds when traveling through a something, and because of that the angles of refraction are different for each one. This leads to the second effect, “dispersion.” When white light enters the prism, each wavelength (color) gets bent differently: blue light, for example, gets bent more than red light. This causes the colors of the white light to separate out (i.e. get dispersed), as each wavelength is refracted slightly differently: the white light splits into its constituent colors and you see the visible spectrum.

To be clear, the speed of the actual photons in the universe is a constant known as C, about 300,000 kilometers per second, and that doesn’t change. But the overall speed of light when it goes through something does change; in a basic way, when a photon goes through a material, it hits the atoms inside, gets absorbed, and then re-emitted at the same wavelength on the other side. The light keeps traveling through the void between atoms at the normal speed of light (C), but each time it hits an atom and gets absorbed and remitted, the overall speed slightly slows down, and as this happens over and over, it effectively speeds down the light.

How much slower is that light in glass than in air? Actually, quite a bit!  The change in speed is affected by the optical density of a material, and the measurement of how much a substance effects light in this way is known as the refractive index. In a vacuum, with a refractive index of 1.0, light travels at about 300,000 kilometers per second. Air has a refractive index of 1.000293, so light traveling through it is very slightly slower (about 299,912 kilometers/second).  Rainbows are created because dispersion also happens in water where light slows to 225,000 kilometers/second (refractive index of 1.333). And in the glass of your prism, it creeps along at 200,000 kilometers/second (refractive index 1.5). When light passes through a diamond  (refractive index 2.417), things really slow down, to a mere 125,000 kilometers/second – less than half the speed that it moves through air.

By the way, since you now know that each wavelength changes speed slightly differently, you may be wondering what wavelength those speeds are measuring? The standard measurements are calculated for light at a wavelength of 589.3 nanometers, which appears as this bright yellow. 

And of course, your pyramid prism isn’t just a paperweight! You can try the following things (among many, many others):

  • Hold the prism up to your eye and look through it; you’ll get “rainbow vision”! This occurs as the light reflected from objects you’re looking at is diffracted before entering your eye
  • Try different light sources: sunlight, red or black lights, laser pointers. Do any of them also disperse into a spectrum?
  • Study the refractive angles of light through your prism. What’s the best angle for light to come in to create the brightest spectrum on your wall?  How many different ways does light seem to be refracted when it enters and leaves your prism
  • Make a lantern! If you set your prism atop a strong flashlight, the light will disperse around the room.
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