A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

When a ray of light travels from one medium to another, such as from air to water, it undergoes a change in speed, and this change in speed causes the light ray to change direction. This phenomenon is known as refraction.

The general rule for the behavior of light when it undergoes refraction is described by Snell’s Law, which states that the ratio of the sine of the angle of incidence (the angle between the incident ray and the normal) to the sine of the angle of refraction (the angle between the refracted ray and the normal) is constant for a given pair of media.

Mathematically, Snell’s Law is expressed as:

sinθ₁/sinθ₂ = v₁/v₂
​
where:

» θ₁ is the angle of incidence,
» θ₂ is the angle of refraction,
» v₁ is the velocity of light in the first medium,
» v₂ is the velocity of light in the second medium.

In this case, when light travels from air (a less optically dense medium) to water (a more optically dense medium), the speed of light decreases in water. According to Snell’s Law, for light traveling from a less dense medium to a more dense medium, the refracted ray bends toward the normal.

So, in the scenario you described, the light ray would bend toward the normal as it enters water.