Why does a bubble look different from the water around it?
Water and air interact with light very differently. When light traveling through water meets a bubble, it crosses into a medium with a much lower optical density, and the boundary between the two bends and reflects the light. This scattering is what makes suspended bubbles alter the appearance of the water and the light field beneath the surface [1]. The same scattering process is what makes bubbles a source of fading in underwater optical signals, because they redirect light away from its original path [2].
The practical result is contrast. Where the water is relatively clear, a bubble's scattered light stands out against the darker background, so you notice it. The bubble itself is not glowing; it is simply sending light toward your eye from angles the surrounding water does not.
Does bubble size or water clarity change how visible they are?
Bubble size matters. Work on underwater optical channels distinguishes the statistical behavior of small and large air bubbles, indicating that bubbles of different sizes affect light scattering in different ways [2]. That fits the everyday observation that a fine mist of tiny bubbles can look like a faint haze while a larger bubble reads as a distinct, sharply outlined sphere.
Water clarity also shapes what you see. In turbid, murky water, particles already scatter light, and added bubbles contribute further scattering and fading to the optical channel [2]. In clearer water the bubble's own scattering is more isolated, which is part of why bubbles are often easier to pick out against a clean background.
Why does this matter beyond just looking at bubbles?
The same scattering that makes bubbles visible has practical consequences. In the ocean, bubbles entrained by breaking wind waves change how the surface reflects light and how light is distributed underwater [1]. That means bubble visibility is not just a curiosity; it is part of how light behaves in natural waters.
For engineered systems, the effect is a nuisance. Bubbles in the water path scatter and fade optical signals, which is why researchers model bubble-induced scattering when designing underwater optical communication links [2]. The visibility that helps your eye spot a bubble is the same phenomenon that degrades a light-based signal.
What are you actually seeing when you look at a bubble?
You are seeing scattered light, not the gas itself. The bubble's surface redirects light from its surroundings toward your eye, and the brightness and shape you perceive come from that redirected light. This is consistent with the finding that injecting air bubbles changes the optical characteristics of water and its reflectance [1]. So when a bubble catches your eye, you are watching light take a detour around a pocket of air.
Sources used in this answer
Effects of intermittent entrainment of air bubbles by breaking wind waves on ocean reflectance and underwater light field
Light scattering by suspended air bubbles alters ocean reflectance and the underwater light field, which helps explain why bubbles are optically visible in water.
On the effect of air bubbles-induced scattering on turbid waters: an experimental UOWC channel modeling approach
Air bubbles scatter light and cause fading in underwater optical channels, with small and large bubbles showing different statistical scattering behavior in turbid water.