Which condition guarantees there is no refraction at a boundary?

• A. Speed1 equals Speed2
• B. Speed2 greater than Speed1
• C. Speed2 less than Speed1
• D. Normal incidence

Answer: (A)

When light or any wave crosses a boundary, refraction occurs because the propagation speed changes between the two media. The bending is described by Snell’s law, which ties the transmitted angle to the ratio of speeds (or refractive indices) in the two media. If the speeds are the same in both media, that ratio is one, so sin of the transmitted angle equals the sin of the incident angle, meaning the angle doesn’t change. The wave continues straight through the boundary without bending.

This condition guarantees no refraction for any angle of incidence. While normal incidence also results in no bending, that is limited to a specific angle of incidence rather than a general guarantee across all possible angles.