Question

An air bubble in water will act like a:
A.) Mirror
B.) Convex lens
C.) Reflective mirror
D.) Concave lens

Answer 1

Hint: The air bubble in air can act as a lens put in another medium. So the refractive of the surrounding medium determines which type of a lens it is actually.

We are familiar with the concept of refraction. When light enters from an optically lighter medium to an optically denser medium, the light ray bends towards the normal. If it travels from an optically denser to an optically rarer medium, the light deviates away from the normal. So an air bubble will look like a convex surface as you may imagine and also looks like a lens. But is it a convex lens, No, because air has a smaller refractive index (1.003) compared to water (1.33), the light entering from the water to the air bubble diverges away from the normal and when it strikes the other surface of the bubble it bend towards the normal as it is going from air to water.

So the air bubble was responsible for the divergence of the path taken by the light, so we can say that the air bubble acts as a diverging lens inside water.

So the answer to the question is option (D)- concave lens

Note: If a lens of refractive index n is placed in a liquid whose refractive index is very close to the lens, the lens disappears when we put the lens in the liquid. Since both have similar refractive index, light doesn’t refract instead passes straight through the lens.