When a small air bubble rises up through water, it appears to glitter or shine. Which of the following optical phenomena is mainly responsible for this sparkling appearance of the bubble?

Introduction / Context:
When you observe small air bubbles rising through water in a glass or aquarium, you may notice that they often appear bright and glittering. This sparkling effect is not just a matter of simple reflection but involves deeper optical behaviour at the boundary between air and water. The question tests your understanding of total internal reflection, refraction, and scattering in the context of everyday phenomena, which is a common topic in optics based general knowledge questions.

Given Data / Assumptions:

• There is a small air bubble inside water, rising upward.
• Light from the surroundings falls on the bubble and its surface.
• The refractive index of water is higher than that of air.
• The bubble surface forms a curved air water interface.

Concept / Approach:
The boundary between water and the air inside the bubble acts like a spherical interface between a denser medium (water) and a rarer medium (air). When light travels from water (higher refractive index) toward air (lower refractive index) and hits the boundary at angles greater than a certain critical angle, it undergoes total internal reflection. Instead of passing into the air inside the bubble, the light is reflected back into the water side, but the curved geometry and multiple reflections make portions of the bubble appear bright to an outside observer. Simple reflection or refraction alone cannot fully account for the intense, sparkling effect seen at certain angles, whereas total internal reflection explains the high brightness and directional nature of the glittering.

Step-by-Step Solution:
Step 1: Recognise that the air bubble has a curved surface separating water and air.Step 2: Light travelling in water toward the bubble meets the boundary from the denser side going to a rarer medium.Step 3: For incidence angles greater than the critical angle, the light rays cannot escape into the air; instead, they are totally internally reflected.Step 4: These internally reflected rays are redirected within the water and bubble interface, making parts of the bubble appear very bright from specific viewing directions.Step 5: Therefore, the glittering appearance is mainly due to total internal reflection of light inside and at the surface of the bubble.

Verification / Alternative check:
Similar shimmering effects are seen in many situations where total internal reflection occurs, such as the sparkling of cut gemstones, the bright edges of glass, and the guiding of light in optical fibres. In all these cases, light is trapped and reflected inside a medium with higher refractive index, emerging in intensified directions. The air bubble in water is a small, natural example of this effect. If the phenomenon were due only to simple reflection or refraction, the brightness would be more uniform and less dependent on viewing angle. The concentration of brightness at particular positions supports the idea of total internal reflection.

Why Other Options Are Wrong:
Simple reflection at the flat water surface, option A, does not explain why the bubble itself appears to glitter as it rises below the surface. Refraction at the curved interface, option B, does bend light rays but typically reduces brightness by transmitting light into the air rather than reflecting it strongly. Scattering by water molecules, option D, creates diffuse effects like the bluish colour of water but does not produce sharp sparkling points on the bubble. Only option C, total internal reflection of light inside the bubble interface, accounts for the intense, directional glittering observed.

Common Pitfalls:
Students often select refraction because they know that light bends at a curved interface. However, refraction alone does not usually lead to strong sparkling effects. Another pitfall is to associate any bright appearance in a medium with scattering, while ignoring the role of internal reflections. To avoid such errors, remember that total internal reflection occurs only when light travels from a denser to a rarer medium and strikes the boundary at angles greater than the critical angle, producing bright and often shimmering effects. This key concept explains many optical phenomena, including the glittering of rising air bubbles in water.

Final Answer:
Total internal reflection of light inside the bubble