The word soft in soft X-rays refers to low photon energy, which means these rays are absorbed quickly and do not travel far through matter compared to hard, high energy X-rays.
- Because they are weak and shallow penetrating, soft X-rays are best suited for examining thin, dense objects rather than scanning through thick baggage or bulk material.
- A genuine coin has a precise, standard metal composition set by the mint, while a counterfeit coin, even if it looks identical outside, is usually made of a cheaper or different alloy with a slightly different internal density.
- When soft X-rays pass through both coins, the difference in absorption due to this density and composition mismatch creates a distinguishable image, letting an examiner tell a genuine coin apart from a fake one.
- Detecting contraband, narcotics or bullets hidden in luggage or bodies needs rays that can penetrate much thicker and denser layers, which points to higher energy hard X-rays rather than soft ones.
So the correct application listed here is distinguishing genuine coins from counterfeit coins, option D.