Q.The electromagnetic waves used to purify water are ______.
(A) Infrared rays
(B) Ultraviolet rays
(C) X-rays
(D) Gamma rays
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Ultraviolet Radiation
What is Ultraviolet Radiation? — A First Look
Imagine you're standing outside on a sunny day. You feel the warmth of sunlight on your skin — that's infrared radiation. You see the world around you — that's visible light. But there's a third kind of energy coming from the sun that you can neither feel nor see: ultraviolet (UV) radiation.
The name itself gives you the clue: ultra means "beyond," and violet is the highest-energy colour we can see. So ultraviolet means "beyond violet" — it's light that sits just past the violet end of the rainbow, invisible to human eyes but very real.
The Intuition: Energy and Wavelength
Think of light as waves travelling through space. The distance between two consecutive wave crests is called the wavelength. Shorter wavelength means higher energy — like a tight, fast vibration. Longer wavelength means lower energy — like a slow, lazy wave.
Here's the order of the electromagnetic spectrum, from lowest energy to highest:
| Type of radiation | Wavelength (approx.) | What it does |
|---|---|---|
| Radio waves | metres to kilometres | Carry signals |
| Microwaves | centimetres | Heat food |
| Infrared | micrometres | Feel as heat |
| Visible light | 400–700 nanometres | What we see |
| Ultraviolet | 10–400 nanometres | Invisible, high-energy |
| X-rays | 0.01–10 nanometres | Pass through soft tissue |
| Gamma rays | < 0.01 nanometres | Extremely energetic |
UV radiation sits between visible light and X-rays. Its wavelength is shorter than violet light, so it carries more energy per photon than any colour we can see.
The Precise Statement
Ultraviolet radiation is electromagnetic radiation with wavelengths from about 10 nm to 400 nm, lying between visible light and X-rays in the electromagnetic spectrum. It is invisible to the human eye and carries more energy per photon than visible light.
The energy of a UV photon is given by:
E=λhc
where h is Planck's constant, c is the speed of light, and λ is the wavelength. Because λ is small, E is large — large enough to break chemical bonds.
Three Bands of UV — Why It Matters
Scientists split the main UV band into three regions based on how they interact with matter, especially with living tissue (a further slice from about 10-100 nm, called vacuum or extreme UV, is strongly absorbed by air itself and is not usually included in this three-band scheme):
| Band | Wavelength range | Key property |
|---|---|---|
| UVA | 315–400 nm | Reaches deep into skin; causes ageing |
| UVB | 280–315 nm | Mostly absorbed by ozone; causes sunburn |
| UVC | 100–280 nm | Completely absorbed by ozone and air; germicidal |
UVC is the most dangerous — it can kill cells and destroy DNA — but Earth's ozone layer blocks it completely. That's why the ozone hole was such a serious problem: it let more UVB through.
Where Does UV Come From?
The main natural source is the Sun. About 10% of the Sun's output is UV, but most of it is filtered by the atmosphere. Artificial sources include:
- Tanning beds (mostly UVA)
- Mercury-vapour lamps (used for sterilisation — they produce UVC)
- Welding arcs (very intense UV — never look directly)
--- …
The key idea is that ultraviolet (UV) radiation has a wavelength short enough to be absorbed by the DNA of microorganisms, causing damage that kills or inactivates them — this makes it effective for water purification without leaving chemical residues.
Reasoning:
- Infrared rays primarily produce heat and are not germicidal.
- Ultraviolet rays (specifically UV-C, 200–280 nm) are strongly absorbed by nucleic acids, disrupting microbial reproduction. …
The key idea is that ultraviolet (UV) rays have the right photon energy to damage microbial DNA, making them the standard choice for water purification. The correct option is (B).
Why Ultraviolet? The Concept
Water purification isn't about "filtering" with these waves — it's about inactivation. Electromagnetic waves can kill or disable bacteria, viruses, and protozoa by disrupting their genetic material. But not all waves work equally.
The crucial factor is photon energy. A photon needs enough energy to break chemical bonds in DNA (or RNA). If the energy is too low, the wave passes through harmlessly. If it's too high, it becomes dangerous to handle and impractical for routine use.
Ultraviolet radiation sits in the sweet spot: its photons carry just enough energy (around 4–12 eV) to be absorbed by nucleic acids, causing thymine dimers and other lesions that prevent replication. The organism can't reproduce, so it's effectively dead.
A common mistake is to pick X-rays or gamma rays because they are "stronger." But stronger isn't better here — those are ionizing radiation that requires heavy shielding and poses serious health risks. They are used for sterilization of medical equipment, not for drinking water.
Step-by-Step Reasoning
-
Identify the requirement. The question asks which electromagnetic wave is used to purify water. This means a practical, common technology — not a theoretical possibility.
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Check the energy range of each option.
- Infrared (IR) rays: Photon energy ~0.001–1.5 eV. Too low to break DNA bonds. They only heat water (which can kill some microbes, but that's thermal, not electromagnetic purification). Not the primary method. …
- CBSE 2026Set ANNUAL1 markMCQQ.The wavelength range of infra-red waves is :(a) 400 nm to 700 nm(b) 1 nm to 400 nm(c) 1 mm to 0.1 m(d) 700 nm to 1 mm
›Reveal solutionSolution
Infrared spans roughly 700 nm (edge of visible) to 1 mm (edge of microwaves).
In the electromagnetic spectrum, visible light ranges about 400–700 nm. Infrared radiation begins just beyond the red end (about 700 nm) and extends to wavelengths of about 1 mm, where it borders the microwave region.
…
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: The lamps of ______ rays are used to kill germs in water purifiers.
›Reveal solutionSolution
Ultraviolet (UV) lamps kill germs in water purifiers.
Ultraviolet radiation has enough energy to damage the DNA of micro-organisms, killing bacteria, viruses and other germs. This germicidal property is used in UV lamps fitted in water …
- CBSE 2024Set 55/5/11 markMCQQ.The electromagnetic waves used to purify water are ______. (A) Infrared rays (B) Ultraviolet rays (C) X-rays (D) Gamma rays
›Reveal solutionSolution
The key idea is that ultraviolet (UV) rays have the right photon energy to damage microbial DNA, making them the standard choice for water purification. The correct option is (B).
Why Ultraviolet? The Concept
Water purification isn't about "filtering" with these waves — it's about inactivation. Electromagnetic waves can kill or disable bacteria, viruses, and protozoa by disrupting their genetic material. But not all waves work equally.
The crucial factor is photon energy. A photon needs enough energy to break chemical bonds in DNA (or RNA). If the energy is too low, the wave passes through harmlessly. If it's too high, it becomes dangerous to handle and impractical for routine use.
Ultraviolet radiation sits in the sweet spot: its photons carry just enough energy (around 4–12 eV) to be absorbed by nucleic acids, causing thymine dimers and other lesions that prevent replication. The organism can't reproduce, so it's effectively dead.
Watch outA common mistake is to pick X-rays or gamma rays because they are "stronger." But stronger isn't better here — those are ionizing radiation that requires heavy shielding and poses serious health risks. They are used for sterilization of medical equipment, not for drinking water.
Step-by-Step Reasoning
-
Identify the requirement. The question asks which electromagnetic wave is used to purify water. This means a practical, common technology — not a theoretical possibility.
-
Check the energy range of each option.
- Infrared (IR) rays: Photon energy ~0.001–1.5 eV. Too low to break DNA bonds. They only heat water (which can kill some microbes, but that's thermal, not electromagnetic purification). Not the primary method. …
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- CBSE 2024Set ANNUAL1 markQ.Which gas causes the depletion of the ozone layer in the upper atmosphere of the earth?
›Reveal solutionSolution
CFCs released into the upper atmosphere break down ozone molecules.
Chlorofluorocarbons (CFCs), commonly known by trade names such as freons, are synthetic compounds once widely used in refrigerants, aerosols, and foam-blowing agents. When released, they rise to the stratosphere, where ultraviolet radiation breaks them down, releasing chlorine atoms that catalytically destroy ozone (O3) molecules — a single chlorine atom can destroy thousands of ozone molecules before being deactivated …
- CBSE 2023Set 55/3/11 markMCQQ.The electromagnetic radiations used to kill germs in water purifiers are called :(a) Infrared waves(b) X-rays(c) Gamma rays(d) Ultraviolet rays
›Reveal solutionSolution
Germicidal water purification relies on DNA disruption by short-wavelength radiation; ultraviolet rays at 200–280 nm are ideal because they damage microbial DNA without the penetration hazards of X-rays or gamma rays.
Why UV radiation kills germs
The key to understanding germicidal radiation lies in how electromagnetic waves interact with biological molecules. Microorganisms—bacteria, viruses, protozoa—all depend on intact DNA and RNA to reproduce and function. Certain wavelengths of light carry enough energy per photon to break chemical bonds in nucleic acids, but not so much energy that they become dangerous to handle or require heavy shielding.
Ultraviolet light, particularly in the UV-C band (200–280 nm), sits in this sweet spot. When a UV-C photon strikes a DNA molecule, it causes adjacent thymine bases to form abnormal bonds called thymine dimers. These kinks prevent the DNA from replicating correctly, rendering the microbe unable to reproduce or causing it to die outright.
Why the other options don't work
Let's see why each alternative fails the practical test for water purification:
-
Infrared waves (option a) carry too little energy. Their photons vibrate molecules and produce heat, but they cannot break the covalent bonds in DNA. You'd cook the water before sterilizing it—useless for purification.
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X-rays (option b) and gamma rays (option c) do carry enough energy to damage DNA, and both are used in industrial sterilization of medical equipment and food. However, they penetrate matter deeply, require thick lead shielding, pose serious radiation hazards to operators, and are far too expensive and dangerous for household water purifiers. A home device emitting X-rays or gamma rays would be a regulatory and safety nightmare.
-
Ultraviolet rays (option d), by contrast, are absorbed by water and air within centimeters, require only a quartz sleeve around the UV lamp, and can be produced safely with low-pressure mercury vapor lamps. The wavelength around 254 nm (the peak emission of these lamps) coincides almost perfectly with the DNA absorption maximum. …
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- CBSE 2023Set ANNUAL1 markMCQQ.In Earth's atmosphere, ozone layer lies in :(a) Troposphere(b) Stratosphere(c) Mesosphere(d) Ionosphere
›Reveal solutionSolution
The ozone layer lies in the stratosphere.
Ozone (O3) is concentrated in the stratosphere, roughly 15–35 km up, where it absorbs most of the Sun's harmful ultraviolet rays and shields life on Earth. This link between UV and the ozone …
- CBSE 2022Set ANNUAL1 markQ............. waves are produced by hot bodies and molecules.
›Reveal solutionSolution
Waves produced by hot bodies and molecules are infrared waves.
Infrared (heat) waves are electromagnetic waves with wavelengths longer than visible red light (about 700 nm up to 1 mm). They are produced by:
- hot bodies (thermal radiation), and …
- CBSE 2021Set ANNUAL1 markMCQQ.The electromagnetic waves used in LASIK eye surgery is(i) micro waves(ii) ultraviolet rays(iii) infra-red waves(iv) gamma rays
›Reveal solutionSolution
LASIK eye surgery uses an excimer laser that emits ultraviolet light to precisely reshape the cornea.
LASIK (Laser-Assisted in Situ Keratomileusis) surgery uses an excimer laser, which emits highly controlled pulses of ultraviolet radiation (typically around 193 nm). UV light is used because it can be focused very precisely and its photons carry enough energy to break molecular bonds in corneal tissue directly (photoablation) without significantly heating the surrounding tissue, allowing extremely fine, controlled reshaping of the cornea.
…
- CBSE 2020Set 55/3/11 markQ.Write one use of the electromagnetic waves of frequency range from 1016 Hz to 1020 Hz.
›Reveal solutionSolution
The frequency range 1016 Hz to 1020 Hz corresponds mainly to X-rays (extending into gamma rays at the top end). One important use is medical diagnosis — X-ray imaging detects bone fractures and examines internal organs, because X-rays penetrate soft tissue but are absorbed by denser bone.
Why This Frequency Range Matters
The electromagnetic spectrum spans from low-frequency radio waves to high-frequency gamma rays. The range 1016 Hz to 1020 Hz lies beyond the ultraviolet: it is dominated by X-rays (roughly 3×1016 Hz to 3×1019 Hz) and reaches into gamma rays at the highest frequencies. What makes these waves special is their photon energy: E=hf is large enough for the radiation to pass through soft matter and to probe — or damage — structures at the atomic scale.
How X-ray Imaging Works
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Penetration depends on density. X-rays pass readily through skin and muscle but are strongly absorbed by the calcium-rich material of bone. A detector (film or digital sensor) placed behind the body therefore records a shadow image in which bones stand out.
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This makes fractures visible. A crack or break in a bone shows up directly on the image, which is why an X-ray is the standard first investigation for a suspected fracture. The same principle images teeth, chest conditions and swallowed objects.
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Higher-frequency photons treat as well as diagnose. At the top of this range, gamma-ray photons carry enough energy to destroy malignant cells — the basis of radiotherapy for cancer — and to sterilise surgical instruments.
Other Applications in This Range
- Crystallography: X-ray diffraction is used to study the arrangement of atoms in crystals, since X-ray wavelengths are comparable to interatomic spacings. …
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- CBSE 2019Set ANNUAL1 markQ.To which region of the spectrum of electromagnetic wave, the wavelength of 10,000 Å belong?
›Reveal solutionSolution
10,000 Å = 1000 nm, beyond visible red → infrared region.
Convert the wavelength:
10,000 Å = 10⁴ × 10⁻¹⁰ m = 10⁻⁶ m = 1000 nm.
The visible region of the electromagnetic spectrum extends roughly from 400 nm (violet) to 700 nm (red). A wavelength of 1000 nm is longer than the red limit of visible light, so it lies just beyond the visible band. …
- CBSE 2018Set ANNUAL1 markQ.An electromagnetic wave having wavelength 5000 Å will lie in which region of electromagnetic spectrum?
›Reveal solutionSolution
5000A˚=500nm falls in the visible light region.
Concept. The visible region of the electromagnetic spectrum spans roughly 400nm (violet) to 700nm (red). Converting the given wavelength:
λ=5000A˚=5000×10−10m=5×10−7m=500nm
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- CBSE 2018Set ANNUAL1 markMCQQ.Infrared rays lie between :(a) γ-rays and ultraviolet rays(b) micro waves and visible light(c) visible light and ultraviolet rays(d) ultraviolet rays and X-rays
›Reveal solutionSolution
Infrared lies between microwaves and visible light in the EM spectrum.
Arranging the electromagnetic spectrum in order of increasing frequency (decreasing wavelength): radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays.
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