Question

An air bubble inside water behaves like a ____ lens.

Answer 1

Hint: We need to understand the properties of the air bubble and the convex and concave lenses to analyse the similarities and differences between them. The similar cases to the air bubble will give us the required solution to this problem.

Complete answer:
We know that air bubbles are circular spheres which are made up of thin films of water with air inside them. The surface energy required for this structure is provided by the air pressure inside the bubble.
We are given such a bubble which is present inside the water. We know that the air and water are two different media with different optical densities or refractive indices. The air has a refractive index of 1, whereas the water has the refractive index of 1.33.
\[\begin{align}
  & {{n}_{water}}=1.33 \\
 & {{n}_{air}}=1 \\
 & \therefore {{n}_{water}}>{{n}_{air}} \\
\end{align}\]
This means that the water is the denser medium and the air is the lighter medium.
Now, let us consider a light ray passing through water entering the air bubble. We can understand that the light ray is moving from a denser medium to a rarer medium. Also, we need to keep in mind that the air bubbles have a convex surface. So, we can draw the ray diagram for the light ray entering and exiting the air bubble as shown below.
 

We understand that the concave lens in air and the air bubble in water behaves similarly.
Therefore, the air bubble inside water behaves like a concave lens.
This is the required solution.

Note:
We should keep in mind that the air bubble is exactly behaving like a convex lens in water. A convex lens when kept outside water, in air, converges the light rays and when kept inside water diverges as a concave lens. This is the same way bubbles also behave.