An air bubble in water acts as a concave lens because of the difference in refractive indices between air and water.
Refractive Index and Lens Behavior
The behavior of a lens is determined by the relative refractive indices of the lens material and the surrounding medium.
- Refractive Index Defined: The refractive index measures how much light bends when passing from one medium to another. A higher refractive index means more bending.
- Water vs. Air: Water has a higher refractive index (approximately 1.33) than air (approximately 1.00).
Why a Concave Lens?
Since the refractive index of water is greater than that of air, light rays entering the air bubble from the water bend away from the normal (an imaginary line perpendicular to the surface). This divergence of light rays is characteristic of a concave lens. A standard concave lens made of glass in air diverges light for the same refractive index reason.
Focal Length
The air bubble has a negative focal length, further supporting its behavior as a concave, or diverging, lens. This means that parallel light rays passing through the air bubble will appear to originate from a point behind the bubble.
Summary
In summary, an air bubble in water behaves as a concave lens due to the difference in refractive indices between the two materials, causing light to diverge as it passes through the bubble.
