# What will happen to a ray of light when it moves from water to air? Give reason for the answer

electreto05 | Certified Educator

When the light beam passes from water to air occurs a refraction . The refraction of light is the deflection of light rays as they cross the boundary between two media with different indices of refraction.

When the refractive index of the first medium is less than the index of the second medium the angle of refraction (θ2), is less than the angle of incidence (θ1), that is, the refracted ray approaches the normal line to the surface. If the index of the first medium is larger than the second, the angle of refraction is greater than the angle of incidence that is, the beam refracted moves away from the normal line.

In this case, the refractive index of water and air are:

n(H2O) = 1.33;     n(air) = 1

We will apply the law of refraction, to the given situation:

n1 sin (θ1) = n2 sin (θ2)

sin (θ1)/sin (θ2) = n2/n1

In our case, n1 = 1.33 and  n2 = 1

sin (θ1)/sin (θ2) = 1/(1.33)

sin (θ1)/sin (θ2) = 0.75 < 1

sin (θ1) < sin (θ2)

θ1 < θ2

As we can see, the relationship between the sine of the angle (θ1) and the angle (θ2), is less than the unity; therefore we can conclude that the angle of incidence is less than the angle of refraction, that is, the beam refracted moves away from the normal.