Air bubbles under water act like which type of lens
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Optics: what will an air bubble in water act like?
The standard answer is, a bubble is a sphere, and the source of light is outside the sphere. Light from outside strikes on the convex surface of the sphere, therefore, air bubbles act as a convex lens.
However, the correct answer is just the opposite.
If we compare a convex air bubble and a concave lens made of glass, we can see that the interaction between the water-and-air in the bubble, and the glass-and-air in the lens is identical. The refractive index of air is around 1.0003 whereas the refractive index of water is 1.333 and the average refractive index of glass is around 1.500. We can see that the material with the higher index (water or glass) curves in and the material with the lower index of refraction (air) curves out. The bubble can be seen as two concaves facing each other, whereas in the lens made of glass, the two concaves are in tandem. Therefore, though the bubble is convex it should behave like a concave lens.
It can be seen when a ray of light perpendicular to the plane of the "lens" formed by the bubble enters the air bubble in water, it is refracted away from the center of the bubble and then as it goes out of the bubble, it is refracted towards the original direction. This amounts to divergence of the light ray as if it was going through a concave lens. (A ray of light is bent toward the perpendicular when entering a higher index medium (like water or glass) and away from the perpendicular when exiting it.)
The standard answer is, a bubble is a sphere, and the source of light is outside the sphere. Light from outside strikes on the convex surface of the sphere, therefore, air bubbles act as a convex lens.
However, the correct answer is just the opposite.
If we compare a convex air bubble and a concave lens made of glass, we can see that the interaction between the water-and-air in the bubble, and the glass-and-air in the lens is identical. The refractive index of air is around 1.0003 whereas the refractive index of water is 1.333 and the average refractive index of glass is around 1.500. We can see that the material with the higher index (water or glass) curves in and the material with the lower index of refraction (air) curves out. The bubble can be seen as two concaves facing each other, whereas in the lens made of glass, the two concaves are in tandem. Therefore, though the bubble is convex it should behave like a concave lens.
It can be seen when a ray of light perpendicular to the plane of the "lens" formed by the bubble enters the air bubble in water, it is refracted away from the center of the bubble and then as it goes out of the bubble, it is refracted towards the original direction. This amounts to divergence of the light ray as if it was going through a concave lens. (A ray of light is bent toward the perpendicular when entering a higher index medium (like water or glass) and away from the perpendicular when exiting it.)
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