Refraction

Links and useful resources

Lightning Round Questions

(lang) What is a subordinate clause?
What are zaibatsu? (powerful Japanese family business organizations that controlled whole sections of industry)
What is the difference between an elastic collision and an inelastic collision?
What is a control in an experiment? Why is it important?
What does inheritance mean for object-oriented programming?
If you double an object's momentum by increasing its velocity, what happens to its kinetic energy?

gr7: [lightning:: ]
gr10: [lightning:: ]

Concept summary and connections

Refraction: When light passes through the boundary between different materials, it bends
- Light can also reflect or absorb when it hits a boundary between materials. Often, all three happen.
Bending of light is what allows us to see and make optical instruments.
Snell's law: $n_{1} \sin θ_{1} = n_{2} \sin θ_{2}$
- $n$ is the refractive index of the material, one for each side of the boundary
- $θ$ is the angle that the light "ray" forms with the surface normal of the boundary.
- The normal is a vector that is perpendicular to the surface (just like in 3D modeling)

Lesson content with examples

Wikipedia: Snell's Law
Wikipedia: Refraction
Refraction on youtube

Light bends when it crosses material boundaries, but why?? It happens because different materials change the speed of light that passe through them. Considering light as a wave, if you change its speed in a way that intercepts the wave at an angle to its direction of propagation, it ends up bending the wave. The diagram below was created by Christiaan Huygens in 1678, and it does a great job of illustrating what's happening:

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Lenses work by making this bending happen over a curved surface, so that the wave is bent in a way that sends all of the motion toward (or away from) a specific focal point.

Snell's Law describes how this bending works: Light bends more when its angle is farther away from the normal to the surface. The relationship looks like this:

n_{1} \sin θ_{1} = n_{2} \sin θ_{2}

Where the $n$ terms represent the index of refraction of the material. This number changes from material to material, and represents the amount of bend that the light will be subjected to as it passes through the boundary. But wait, you say, if all of the light bends by the same amount, why do prisms make rainbows?

It turns out that the index of refraction is not the same for all colors of light, so when you get a number like $1.5$ , it represents a sort-of average value. It will be slightly higher for shorter wavelengths, and lower for long wavelengths. That means that colors toward the violet end of the spectrum will be bent more than colors toward the red end, and you end up with the colors of the rainbow (ROYGBIV).

When light enters a material with a lower speed of light (a higher optical density and a greater index of refraction), it bends toward the normal going into the material. When it enters a material with a higher speed of light (lower optical density, lower index of refraction), it bends away from the normal pointing into that material.

One easy way to remember this is to use two rays of light that are close to each other. Pretend they are connected by a string or a ruler, going along parallel to each other. If the light hits the surface at an angle, one of the two will be first to hit. If it slows down, then the light bends toward the new material. If it speeds up, then the light has to bend the other way in order for them to stay connected. REMEMBER THE LAWN MOWER!!!!

Guided practice

What happens when light passes through the boundary between two different materials.
The refractive index of a particular kind of glass is 1.5. If light hits it at an angle of 70 $°$ , what angle will the light be at while inside the glass?
Explain why a fish looks like it's closer to the surface than it really is when you see it from outside of the water.

Homework

#hw (physics) physics classroom refraction Do the beginner level (first one) 2025-03-28