Snell's Law relates the ratio of sines to the ratio of speeds as:

• A. Speed of Medium 2/Speed of Medium 1
• B. Wavelengths in the media
• C. Refractive indices
• D. Frequencies of the waves

Answer: (A)

Light changes direction when it crosses a boundary because its speed is different in the two media. Snell’s law ties that directional change to how fast the wave travels in each medium: the ratio of the sines of the angles equals the ratio of the speeds in the two media. In equation form, sin θ1 / sin θ2 = v1 / v2, which can also be written as sin θ2 / sin θ1 = v2 / v1. So when you express the relation in the order of the refracted angle to the incident angle, it matches the speed ratio of medium 2 to medium 1. That’s why the best choice is the speed of medium 2 over the speed of medium 1. Wavelengths, refractive indices, and frequencies don’t directly convey this particular ratio in the same way.