Question

Which one is optically denser: water or air? Give a reason.

Answer 1

Hint: To find the answer to the above question we should know that a medium is said to be denser if the speed of light gets reduced when light travels through that medium. So, we will compare the speed of light in both water and air so as to know which of these two is optically denser.

Complete step-by-step solution:
In order to solve the above question let us first discuss what is optically denser medium?
We know that a medium will be optically denser if the speed of light gets reduced when light enters into that medium. That means we have to compare the speed of light in both the mediums that is water and air.
 We know that the speed of light in air is $$3\times {10}^8\frac{m}{s}$$ and
And also, the speed of light in water is $$2.25\times {10}^8\frac{m}{s}$$
So here, we can clearly see that the speed of light in water is less than that of the air, which means water is an optically denser medium.
OR
We can say that the speed of light in air is more than the speed of light in water, which means water is optically denser than air.
Now, let us try and find why the speed of light is less in liquids as compared to air. The reason for this is the less intermolecular spaces present in liquids as compared to air, that is the molecules of liquids are closely packed because the rays of light entering in water gets bent towards the normal which leads to large change in angle of incidence and angle of refraction. The intermolecular spaces present in air are less as the molecules are loosely packed as compared to water so, the bending of light rays of light is also less resulting in making it less dense medium as compared to water. This is also called the phenomenon of total internal refraction.

Note: While solving above question we got to know that water is optically denser medium as compared to air which is mainly because the critical angle of water and air interface which is equals to $${49}^o$$because of the phenomenon of total internal refraction acting on air and water.