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# Which is optically denser: water or air? Give a reason. Verified
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Hint: If we have to solve this question, we need to understand the nature of the medium through which air or water travels. We know that in the medium having a high refractive index, the speed of the light gets reduced and if the refractive index in a medium is low then the speed of the light is higher. Keeping this concept in mind we will answer this question.

Formula used:
The minimum distance is given by the formula
${d_{\min }} = d\cos \theta$
Where, $d$ is the distance between the projectiles and $\theta$ is the angle difference between them.

Optically denser medium: If the speed of light in a medium is lesser than the speed of light in vacuum than the medium is said to be optically denser medium.
i.e.,${v_{vacuum}} > {v_{medium}}$
We know that if the refractive index of a medium is high then the speed of light in that medium is lower. So, it is a well known fact that the refractive index of the water is higher than that of air so, in the water the speed of light gets reduced.
Hence, we know that if a medium speed of light gets reduced then that medium would be optically denser.
Thus, the refractive index of water is higher than air so the speed of light in water is lesser than that of air so, the water medium is optically denser.
So, water is optically denser than air.

Note: To answer these types of questions we need to keep some points in the mind:
We need to learn some facts related to refractive index in order to identify the optically denser medium.
Absolute Refractive Index: Ratio of speed of light in vacuum to the speed of light in medium.
Relative refractive index: Ratio of speed of light in one medium to the other medium.
The absolute refractive index should never be less than 1.