A ray of light passes from water to air. How does the speed of light change?

Hint: Use the concept of refraction. In refraction, the speed of light is less in the denser medium compared to the rarer medium.

Complete step by step answer:
Refraction is the phenomena of passing of light from one medium to another medium. Light has a fixed velocity in vacuum, but its velocity changes when it enters in some medium. The change in the velocity of light depends on how tightly the molecules are bound in the particular medium.
For refraction, we have two mediums. Both the mediums have different refractive index. That is, both the mediums have different density. The medium which has less refractive index compared to the other medium is called the rarer medium. And the medium which has greater refractive index compared to the other medium is called denser medium.
Since, in the denser medium, the molecules are closely bound, the speed of light in the denser medium is less, compared to the speed of light in the rarer medium.
Now, clearly, water is the denser medium compared to air. So, from the above said concept, we can conclude that the speed of light should be greater in air, compared to the speed of light in water.
Hence, when a ray of light passes from water to air. The speed of light should increase.

Note:In refraction, when light travels from the denser to rarer medium, it deviates away from the normal. And when it travels from the rarer to denser medium, it deviates towards the normal.