Question

A transparent plastic bag when filled with air forms a concave lens. Now, if this bag is completely immersed in water, it will behave as a ____________.
A) divergent lens
B) convergent lens
C) equilateral prism
D) rectangular slab

Answer 1

Hint:The plastic bag filled with air is said to form a concave lens. A concave lens has diverging properties. When the bag is immersed in water, it is essentially placed in a denser medium. This results in reversing the property of the lens.

Complete step by step answer.
Step 1: Explain the change in the refraction of light as the plastic bag is immersed in the water.
A concave lens diverges the light rays entering it. It is also referred to as the diverging lens.
Here, the plastic bag is said to behave as a concave lens when it is filled with air. This implies that the surrounding medium of the plastic bag is rarer. When this bag becomes completely immersed in water, the surrounding medium changes. The medium outside the bag is now water. We know that water is denser and has a higher refractive index. Thus the nature of the lens reverses.
This suggests that the plastic bag now behaves like a convergent lens.

So, the correct option is B.

Additional information: A convergent lens focuses the refracted light rays to a single point while a diverging lens will cause the light rays coming out of the lens to diverge even more.
The diagram given below shows the refraction of light from a convex lens and a concave lens.

Note: If the refractive index of a material is smaller than the refractive index of its surrounding it behaves as a diverging lens and if the refractive index of the material is greater than the refractive index of its surrounding, it behaves as a converging lens. A denser medium is said to have a higher.