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Updated September 2026, 5 days ago

College Level Quantum Mechanics (Zero Prerequisites)

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$Does light actually slow down in water? Supported by Screen Australia and Youtube through the Skip Ahead initiative. 3Blue1Browns video: https://www.youtube.com/watch?v=KTzGBJPuJwM Part 2: https://youtu.be/uo3ds0FVpXs?si=zW8nyuZpn8NshfqC Experiment: If you’d like to try this experiment at home then you’ll need a phone with Lidar or you can get a laser meter quite cheaply. The app I found most reliable for the measurement was this one: https://apps.apple.com/us/app/lidar-measuring/id1535032210 Code: The 3D simulation is here: https://github.com/mithuna-y/speed_of_light_in_a_medium/tree/main/multiple_layers References: The Feynman lectures- “Ch 31: On the origin of refractive index” and “Ch 48: Beats” Matter and Interactions 3rd Edition - The quote is from section 24-4 on page 1001. It’s a long passage but I tried to paraphrase it accurately. Here’s the full quote: How might we measure the speed of propagation of an electromagnetic wave? One can think of two different approaches: (a) Follow a wave crest: If you watch one particular wave crest, you will see that it travels a distance λ (one wavelength) in a time T (one period). Therefore the speed of the crest is v = λ / T (b) Time the arrival of a radiative electric field: One could imagine a different way of measuring the speed of an electromagnetic wave. Suppose that you and a friend synchronize your clocks, then travel to locations that are a distance d apart. Your friend aims a laser at your location, and precisely at time t1, turns on the laser. You record the time t2 at which you first detect the radiative electric field. In the laser light, and knowing the distance between the locations and the elapsed time Δt = t2 - t1, you calculate the speed at which the laser light traveled toward you: v = d / Δt In a vacuum, these two ways of measuring the speed of a sinusoidal electromagnetic wave will give the same answer: 3 x 10^8 m/s. However, this will not necessarily be the case if part or all of the space through which the light wave travels is filled with a medium such as water, glass, or even air. In this case, method 2 (measuring the time required for information about a change in the electromagnetic field to travel a given distance) will still give 3 x 10^8 m/s. However, method 1 (timing the interval between crests in a steady state electromagnetic wave inside the medium) will give a different answer, which will almost always be less than 3 x 10^8 m/s.$

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