American Board of Ophthalmology (ABO) Practice Exam

What principle explains why light refracts as it moves between different media?

1. Snell's Law

2. Newton's Law

3. Maxwell's Equations

4. Huygens' Principle

The correct answer is: Snell's Law

The principle that explains why light refracts as it moves between different media is Snell's Law. This fundamental law of optics describes how the angle of incidence and the angle of refraction are related when light travels from one medium to another with different refractive indices. Snell's Law states that the ratio of the sine of the angles of incidence and refraction is equal to the ratio of the velocities of light in the two media, which is also equivalent to the inverse ratio of their refractive indices. This relationship explains the bending of light rays at the interface between different materials, such as air and water, resulting in phenomena such as the apparent bending of a straw when it is placed in a glass of water. Understanding Snell's Law allows one to predict how much a light ray will bend based on the refractive indices of the materials involved, which is critical in designing lenses and understanding visual optics.