Helium gas is filled in a transparent balloon which is kept in air. The balloon behaves as:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Concave lens
A transparent balloon lets light pass through it, so it acts as a lens, not a mirror. Whether a lens converges or diverges depends on the refractive index of the lens material relative to the surrounding medium.
The balloon is convex in shape, but helium is optically rarer than air (its refractive index is closer to 1). By the lens maker's formula
\( \displaystyle \frac{1}{f} = \left(\frac{n_{\mathrm{He}}}{n_{\mathrm{air}}} - 1\right)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
the first bracket is negative while the second is positive, so \( f \) is negative. The balloon diverges light and behaves as a concave lens.
Option (1) Concave lens — Correct. A convex shape filled with a rarer medium diverges light.
Option (2) Concave Mirror — Incorrect. A transparent balloon transmits light; it does not act as a mirror.
Option (3) Convex lens — Incorrect. A convex shape converges light only when its material is denser than the surroundings.
Option (4) Convex Mirror — Incorrect. The balloon refracts light through it; it is a lens, not a mirror.