What is the speed of visible light per second in CR-39?

The correct answer focuses on the context of light speed in a specific medium, in this case, CR-39, which is a type of plastic commonly used in eyeglass lenses. The speed of light in a vacuum is commonly referenced as approximately 186,000 miles per second. However, when light travels through a medium like CR-39, its speed is reduced due to the refractive index of the material.

CR-39 has a refractive index around 1.5, which causes the speed of light to be slower compared to its speed in a vacuum. The speed of light in CR-39 is approximately 124,000 miles per second, accounting for the effects of this medium. This reduction is consistent with the principles of optics where light's velocity changes based on the density and refractive properties of the material through which it travels.

Understanding these principles is crucial for opticians, as it affects how lenses bend light, impacting vision correction.