An air bubble inside water behaves optically like which one of the following objects?

Introduction / Context:
This question deals with the behaviour of light at the boundary between two media of different optical densities. An air bubble in water is a classic example used in optics to test whether students understand how curved surfaces refract light. It requires you to visualise the bubble as a region of lower refractive index surrounded by a higher refractive index medium and then decide whether it will converge or diverge light rays, just like a particular type of lens or mirror.

Given Data / Assumptions:

• There is an air bubble inside water.
• Water has a higher refractive index than air.
• The bubble boundary is approximately spherical and smooth.
• We are comparing the behaviour of the bubble to that of lenses or mirrors.

Concept / Approach:
A lens works by refracting light at curved surfaces. A convex lens in air converges parallel rays because the glass has a higher refractive index than the surrounding air. A concave lens in air diverges parallel rays. When the roles are reversed and the surrounding medium is denser (water) while the enclosed region is rarer (air), the same shape behaves in the opposite way. An air bubble in water has the shape of a converging lens but consists of lower refractive index material inside, so its net effect is diverging. Therefore it behaves like a concave lens, which diverges rays passing through it.

Step-by-Step Solution:
Step 1: Recognise that water has a higher refractive index than air, so water is optically denser. Step 2: Visualise the air bubble as a roughly spherical region of air surrounded by water. Step 3: Recall that in air, a convex glass lens converges light because glass is denser than air. Step 4: Note that if we keep the same shape but swap which region is denser, the behaviour reverses. Step 5: For an air bubble in water, the inside (air) is rarer and outside (water) is denser, so the net effect is to diverge rays coming through the bubble. Step 6: A device that diverges parallel rays is optically equivalent to a concave lens.

Verification / Alternative check:
In ray diagrams for a spherical bubble, tracing incident parallel rays shows that after refraction at both surfaces, the emergent rays spread out rather than converging to a point. This is the signature of a diverging lens. Many textbooks explicitly state that an air bubble in water behaves like a concave lens, and this result is widely used in qualitative problems in geometrical optics. The same idea applies to cavities filled with lower index material inside a higher index medium.

Why Other Options Are Wrong:
Convex lens: A convex lens in a rarer outside medium converges light, which is not what an air bubble in water does.
Convex mirror: Mirrors reflect light rather than refract it, and an air bubble is a transparent region, not a reflecting surface.
Concave mirror: Again, a mirror reflects light, whereas the bubble refracts light at its surfaces, so mirror options are not appropriate here.

Common Pitfalls:
Students often associate convex shape mechanically with convergence and concave shape with divergence without considering refractive index. Another pitfall is to forget that the surrounding medium has changed here. To correctly answer such questions, you must always consider the relative refractive indices of the media and whether the shape is in a denser or rarer surrounding. Thinking only in terms of shape can lead to incorrect conclusions.

Final Answer:
An air bubble in water behaves optically like a Concave lens because it diverges light rays passing through it.