PADI Divemaster Certification Practice Exam

What happens to light speed when it passes from air into water?

1. It increases

2. It decreases

3. It remains constant

4. It stops completely

The correct answer is: It decreases

When light moves from air into water, its speed decreases. This phenomenon is primarily due to the change in medium; light travels at different speeds in different materials based on their refractive indices. Air has a lower refractive index compared to water, which means that as light enters the denser medium of water, it interacts more with the molecules of water causing it to slow down. The speed of light in a vacuum is approximately 299,792 kilometers per second, while in water, it travels at about 225,000 kilometers per second. This reduction in speed is a critical factor in understanding how light behaves as it transitions between different substances. This change in speed also results in the bending of light rays, a principle known as refraction, which is fundamental to various applications in physics, optics, and underwater navigation. Understanding this concept is essential for divers, as it affects visibility and the perception of objects underwater.