Which one of the following waves is used for detecting forgery in currency notes ?
- (a)Ultraviolet waves
- (b)Infrared waves
- (c)Radio waves
- (d)Microwaves
Correct — A, ultraviolet waves. Ultraviolet photons carry enough energy to lift molecules into an excited state, from which they drop back and re-emit the surplus as visible light — the effect called fluorescence. Currency printing exploits this in two opposite directions at once. Genuine banknote paper deliberately leaves out the optical brightening agents that make ordinary commercial paper glow faintly under a UV lamp, so a note run off on ordinary paper lights up all over and gives itself away. At the same time, the genuine note's security thread and ink carry fluorescent elements that are meant to glow in a fixed pattern, so a copy without them stays dark where it should shine. That is why the small purple-glowing box at a bank counter is an ultraviolet lamp.
- (b)Infrared waves — Infrared lies just beyond the red end of the visible band and its photons carry less energy than visible light, far too little to excite the fluorescence on which the UV test depends. Infrared is the heat-carrying radiation used in remote controls, thermal imaging and night-vision equipment.
- (c)Radio waves — Radio waves have the longest wavelengths and the lowest photon energies in the electromagnetic spectrum. They pass through paper and ink without exciting anything, which is why they are used for broadcasting and communication rather than for inspecting a document.
- (d)Microwaves — Microwaves are used in radar, in satellite and mobile links and in ovens, where they heat food by making water molecules rotate. Heating a note tells you nothing about its printing, and microwave photons are far too weak to make ink fluoresce.
Fluorescence is the absorption of a high-energy photon by a molecule followed by the prompt emission of a lower-energy, visible one. Because the absorbed photon must be more energetic than the emitted one, the exciting radiation has to sit on the short-wavelength side of the visible band — that is, in the ultraviolet. Arranged by increasing wavelength the electromagnetic spectrum runs gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves, and energy falls as wavelength rises.
Reason from the energy ladder rather than from familiarity. Of the four options only ultraviolet lies on the high-energy side of visible light; infrared, microwaves and radio waves are all on the low-energy side and cannot make anything glow visibly. A second cue is that only ultraviolet is routinely used to inspect documents, passports and hospital wristbands, all of which rely on the same fluorescent inks.
- Banknote paper is deliberately made without the optical brighteners present in ordinary paper, so genuine notes do not glow all over under ultraviolet light.
- A banknote's security thread carries fluorescent, magnetic, metallic and microprint elements, and the fluorescent part is what should light up under a UV lamp.
- Ultraviolet occupies roughly 10 to 400 nanometres, between X-rays and visible light.
- Fluorescence requires the exciting photon to be more energetic than the emitted one, so it cannot be produced by infrared, microwaves or radio waves.
- The same ultraviolet fluorescence test is used on passports, visas, cheques and identity documents.
Only the ultraviolet band sits above visible light in energy, so only it can drive fluorescence — option (a).
- Picking infrared because it also sounds like an 'invisible ray' — it is on the wrong side of the visible band in energy.
- Assuming a longer wavelength means more energy; energy rises as wavelength falls.
- Thinking the genuine note is the one that glows everywhere — it is ordinary paper that glows all over, while the genuine note glows only in the designed places.
Both NDA and UPSC set spectrum questions as either an ordering of bands or an application item like this one; knowing one use for each band answers most of them.
What is the role of ultraviolet (UV) radiation in the water purification systems? 1. It inactivates/kills the harmful microorganisms in water. 2. It removes all the undesirable odours from the water. 3. It quickens the sedimentation of solid particles, removes turbidity and improves the clarity of water. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer(a) 1 only
The same band tested through a different application — ultraviolet works because its photons are energetic enough to damage microbial DNA, just as they are energetic enough to excite fluorescent ink.
Consider the following statements about visible light, UV light and X-rays : 1. The wavelength of visible light is more than that of X-rays. 2. The energy of X-ray photons is higher than that of UV light photons. 3. The energy of UV light photons is less than that of visible light photons. Which of the statements given above is/are correct?
- (a) 1, 2 and 3
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1 only
Answer(b) 1 and 2 only
Tests the very energy ordering this question turns on — statement 3 is wrong because ultraviolet photons carry more energy than visible ones, which is exactly why ultraviolet can make ink fluoresce and visible light cannot.
- practice — not a real PYQ
Which of the following has the shortest wavelength?
- (a)Infrared radiation
- (b)Ultraviolet radiation
- (c)Microwaves
- (d)Radio waves
Answer(b) Ultraviolet radiation — it lies between X-rays and visible light, so it is the shortest of the four.
- practice — not a real PYQ
The radiation used in a television remote control to send signals to the set is
- (a)ultraviolet
- (b)infrared
- (c)X-rays
- (d)gamma rays
Answer(b) infrared — a small infrared LED in the remote sends coded pulses to a detector on the set.