Cavendish Gravity Experiment: Learn the Law of Reflection in 3 Minutes

YouTube Screenshot (https://www.youtube.com/watch?v=eu_WRsxGnK0)

By Andrew Bennett

What Is the Law of Reflection?

If you have ever played pool, you've probably done a little mental geometry that looks very similar to the Law of Reflection. This law simply states that whenever a wave hits a boundary and reflects, the angle it reflects at is equal to the angle at which it hit the boundary. Lining up a bounce shot in billiards requires the same thinking.

What Is Specular Reflection?

Very flat surfaces (such as calm water, glass, and mirrors) make such clear reflections because every bit of light that hits the surface will encounter a surface at exactly the same angle. In physics, we describe this as regular or specular reflection.

Specular reflection. YouTube Screenshot (https://www.youtube.com/watch?v=eu_WRsxGnK0)

What Is Diffuse Reflection?

Surfaces that aren't as polished (such as walls, clothes, or skin) have tiny bumps covering them. When light hits a surface like that (even laser light that starts out traveling in the same direction), the reflected light will scatter. This is known as diffuse reflection. In this case, the reflected ray still leaves the surface at the same angle as the incident ray hit it, but the surface itself is uneven, so those angles are difficult to predict. This is like trying to catch a ground ball on an infield with a bunch of rocks on it. The ball hits a rock and could bounce off in basically any direction!

Diffuse reflection. YouTube Screenshot (https://www.youtube.com/watch?v=eu_WRsxGnK0)

Applying This Law to the Cavendish Gravity Demo

I'll be using this property in the design of my Cavendish gravity experiment. We'll need to measure the angle through which the lever arm rotates as a result of the gravitational force we add. We can't simply measure this angle with a protractor for two reasons:

Even a slight contact from the protractor would set the arm rotating to a brand-new angle.
We'll need to measure angles much more precisely than you can on a standard protractor.

The Solution: A Giant Laser Protractor

Instead, I'll be building a giant laser protractor. The laser will shine on a mirror attached to the arm, then reflect (leaving the mirror at the same angle it hit the mirror) and hit a large circularly curved surface with angle measurements. The precision of the measurement will depend on the size of the protractor, but there shouldn't be anything that limits that except where the wall of my garage is. We'll be dealing with a 4- to 5-meter-radius protractor for the angle measurement!

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