X-rays are a form of high-energy electromagnetic radiation with wavelengths far shorter than visible light. Discovered by Wilhelm Röntgen in 1895 — earning him the very first Nobel Prize in Physics — they revolutionised medicine and science. For NDA, this chapter tests conceptual understanding of how X-rays are produced, their characteristic properties (especially penetrating power and ionisation), their wide-ranging applications in medicine, security, and defence, and the critical safety precautions required when working with them.
📌 What to expect in NDA (based on 2022–2025 pattern): (1) Discovery — Röntgen, 1895, Nobel Prize 1901; (2) Nature — electromagnetic wave, very short wavelength (0.01–10 nm); (3) Production — Coolidge tube: high-voltage electron acceleration, target metal (tungsten); (4) Properties — penetration, ionisation, effect on photographic film, fluorescence; (5) Types — soft X-rays (long λ, less penetrating) vs hard X-rays (short λ, more penetrating); (6) Applications — medical radiography, CT scan, crystallography, airport security, cancer treatment; (7) Safety — biological hazards, shielding materials (lead, concrete), dose limits, protection.
Topics at a Glance
① Discovery & Nature
Röntgen 1895, EM wave, wavelength range
② Production
Coolidge tube, cathode, anode, target, high voltage
Cell damage, cancer risk, lead shielding, dose limits
⑥ EM Spectrum Comparison
X-ray vs γ-ray, UV, radio; wavelength ordering
1. Discovery & Nature of X-Rays
1.1
Röntgen's Discovery & EM Nature
The accidental discovery that changed medicine — and won the first Nobel Prize in Physics
In November 1895, German physicist Wilhelm Conrad Röntgen accidentally discovered X-rays while experimenting with a cathode ray tube. He noticed that a fluorescent screen across the room glowed even though it was not in the direct beam — the rays were passing through the opaque tube walls and through objects. He named them X-rays because their nature was unknown ("X" for unknown). Röntgen received the first Nobel Prize in Physics in 1901 for this discovery.
⚡ X-Ray — Nature & Wavelength Range
Nature: Electromagnetic radiation (transverse wave)
Travels at speed of light: c = 3 × 10⁸ m/s
No charge, no mass — NOT deflected by electric or magnetic fields
Wavelength: λ = 0.001 nm to 10 nm (i.e., 10⁻¹² m to 10⁻⁸ m)
Shorter than UV, longer than γ-rays
Soft X-rays: λ ≈ 0.1–10 nm (less energetic, less penetrating)
Hard X-rays: λ ≈ 0.001–0.1 nm (highly energetic, deeply penetrating)
Frequency: f = c/λ → f = 3×10¹⁶ to 3×10¹⁹ Hz
Energy of X-ray photon:
E = hf = hc/λ (h = 6.63 × 10⁻³⁴ J·s)
Higher energy → shorter wavelength → harder X-ray → more penetrating
In Coolidge tube: Maximum photon energy = eV
eV = hf_max = hc/λ_min
λ_min = hc/eV (minimum wavelength produced at voltage V)
λ_min (in Å) ≈ 12400/V (V in volts; useful approximation)
Key distinction: X-rays and γ-rays have overlapping wavelength ranges — the difference is their origin, not wavelength. X-rays: produced by electron deceleration or electron shell transitions. γ-rays: produced by nuclear transitions (radioactive decay).
Fig. 1 — Electromagnetic spectrum. X-rays (highlighted) occupy wavelengths from 0.001 to 10 nm — shorter than UV but longer than γ-rays. Higher energy (shorter λ) X-rays are called "hard"; lower energy (longer λ) are called "soft."
📝 TOPIC-WISE PYQ
Discovery & Nature of X-Rays — NDA Pattern Questions
Q1. X-rays were discovered by:
(a) Marie Curie (b) Wilhelm Röntgen (c) Max Planck (d) Ernest Rutherford
Answer: (b) Wilhelm Röntgen
Röntgen discovered X-rays in November 1895 while experimenting with cathode ray tubes. He received the first Nobel Prize in Physics in 1901. He named them X-rays because their nature was initially unknown. Marie Curie (radioactivity), Planck (quantum theory), Rutherford (atomic nucleus) are different discoveries.
Q2. X-rays are:
(a) Positively charged particles (b) High-speed electrons (c) Electromagnetic waves (d) Sound waves of high frequency
Answer: (c) Electromagnetic waves
X-rays are electromagnetic radiation — transverse waves that travel at the speed of light (3×10⁸ m/s). They have no charge, no mass, and are not deflected by electric or magnetic fields. They differ from cathode rays (electrons) and alpha/beta particles.
Q3. Which of the following correctly represents the wavelength range of X-rays?
Answer: (b) 0.001–10 nm
Visible light: 400–700 nm; UV: 10–400 nm; X-rays: 0.001–10 nm; γ-rays: < 0.01 nm. Radio waves have the longest wavelength (mm to km); γ-rays the shortest. X-rays sit between UV and γ-rays.
2. Production of X-Rays — Coolidge Tube
2.1
Coolidge X-Ray Tube — Working Principle
High-voltage electrons slam into a metal target — releasing X-rays
The Coolidge tube (hot cathode X-ray tube) is the standard device for producing X-rays. It was invented by William Coolidge in 1913 and is still the basis of modern X-ray machines. The key principle: rapidly moving electrons are suddenly decelerated when they strike a heavy metal target, converting kinetic energy into X-ray photons.
⚡ Coolidge Tube — Key Relationships
Components:
Cathode: Heated tungsten filament (thermionic emission → electrons)
Anode: Heavy metal target (tungsten, molybdenum)
Vacuum: High vacuum tube (prevents electron-air collisions)
HV supply: 20 kV – 200 kV (accelerates electrons to target)
Energy conversion:
Electrons accelerated: KE = eV (e = 1.6×10⁻¹⁹ C, V = accelerating voltage)
≈ 99% of KE → heat (target must be cooled: rotating anode or water cooling)
≈ 1% of KE → X-ray photons
Maximum X-ray energy: E_max = eV = hf_max = hc/λ_min
Minimum wavelength: λ_min = hc/eV ≈ 12400/V (Å, V in volts)
Controlling X-ray properties:
Intensity (number of X-rays): controlled by filament temperature
higher temperature → more electrons → more X-rays
Hardness (penetrating power): controlled by accelerating voltage
higher voltage → higher energy X-rays → shorter λ → harder
Two independent controls: Filament temperature controls quantity (intensity/dose) of X-rays. Accelerating voltage controls quality (energy/hardness) of X-rays. This independence is crucial for diagnostic imaging — you can adjust dose and penetration separately.
Fig. 2 — Coolidge X-Ray Tube. Electrons emitted from heated filament (cathode, −) are accelerated by high voltage (20–200 kV) toward tungsten target (anode, +). On impact, ≈99% of kinetic energy converts to heat; ≈1% becomes X-ray photons. X-rays exit through a beryllium window.
📌 Why Tungsten as Target?
Very high melting point (3422°C) — withstands intense heat
High atomic number (Z = 74) — efficient X-ray production
Good electrical and thermal conductivity
High density — compact target
Alternative: molybdenum (Z = 42) for mammography (softer X-rays)
🔥 Heat Management
99% energy → heat (major engineering challenge)
Rotating anode: spreads heat over larger area
Water cooling or oil cooling of anode assembly
Thermal limit determines maximum X-ray dose rate
CT scanners: continuous rotation requires superior cooling
📝 TOPIC-WISE PYQ
Production of X-Rays — NDA Pattern Questions
Q1. In a Coolidge X-ray tube, the intensity of X-rays is controlled by:
(a) Accelerating voltage (b) Filament temperature (c) Pressure inside tube (d) Target material
Answer: (b) Filament temperature
Filament temperature controls the number of electrons emitted (thermionic emission) → controls intensity (number of X-ray photons produced). Accelerating voltage controls the energy/hardness of X-rays. These two controls are independent — a common NDA exam question.
Q2. If the accelerating voltage in an X-ray tube is doubled, the minimum wavelength of X-rays produced:
(a) Doubles (b) Halves (c) Remains same (d) Quadruples
Answer: (b) Halves
λ_min = hc/eV. Since λ_min ∝ 1/V, doubling V halves λ_min. Higher voltage → higher energy electrons → higher energy X-rays → shorter minimum wavelength → harder X-rays.
Q3. Tungsten is used as the target material in X-ray tubes because it has:
(a) Low atomic number (b) High melting point and high atomic number (c) Low density (d) Low electrical resistance
Answer: (b) High melting point and high atomic number
Tungsten (W, Z = 74) has the highest melting point of all metals (3422°C) — essential because 99% of electron energy becomes heat. High Z ensures efficient X-ray production (bremsstrahlung and characteristic radiation are both stronger for heavy atoms).
🤔 TRICKY QUESTIONS
X-Ray Production — Conceptual Traps
T1. An X-ray tube operates at 50 kV. Calculate the minimum wavelength of X-rays produced. (h = 6.63×10⁻³⁴ J·s, c = 3×10⁸ m/s, e = 1.6×10⁻¹⁹ C)
T2. Why is a vacuum necessary inside the X-ray tube? What would happen if air were present?
Vacuum prevents electron collisions with air molecules.
If air were present, electrons would collide with gas molecules and lose energy before reaching the target — resulting in little or no X-ray production. The vacuum allows electrons to be accelerated through the full voltage potential without losing energy. The vacuum also prevents electrical breakdown (arcing) between cathode and anode at the very high voltages (tens of kilovolts) used. Early X-ray tubes (Crookes tubes) relied on residual gas for conduction — they were unreliable. The Coolidge tube's sealed vacuum solved this problem.
3. Properties of X-Rays
3.1
Key Physical & Biological Properties
What X-rays do — and why they are both useful and dangerous
X-rays exhibit a unique combination of properties that make them invaluable in medicine and science — but also potentially hazardous without proper precautions. Understanding each property explains both their applications and safety requirements.
Property
Description
Application / Consequence
Penetrating power
Pass through soft tissue but absorbed by dense materials (bone, metal); Hard X-rays penetrate more than soft X-rays
Medical radiography (bone imaging), baggage scanning, NDT of metals
Ionisation
Knock electrons from atoms, creating ions; detected by Geiger counter, ionisation chamber
Biological damage, cancer risk; also used in radiation detectors
Photographic effect
Blacken photographic film (like visible light); pass through to film behind patient
X-ray radiograph — shadows of dense structures on film
Fluorescence
Cause certain substances (barium platino-cyanide, calcium tungstate) to glow
Fluoroscopy; Röntgen's original discovery used fluorescent screen
Not deflected by fields
No charge → not affected by electric or magnetic fields
Distinguishes X-rays from charged particles (α, β); confirms EM nature
Travel at speed of light
c = 3 × 10⁸ m/s in vacuum; refract, reflect (very slightly), diffract
Diffraction by crystal lattice → X-ray crystallography (DNA structure!)
Photoelectric effect
Can eject electrons from metal surfaces
Photoelectric detectors for X-ray dosimetry
Biological effect
Kill or damage cells (especially rapidly dividing cells); mutagenic
Radiation therapy for cancer; also the basis of safety precautions
📌 Soft vs Hard X-Rays
Soft X-rays: longer λ (0.1–10 nm); lower energy; less penetrating
Hard X-rays: shorter λ (0.001–0.1 nm); higher energy; deeply penetrating
Controlled by: accelerating voltage (higher V → harder X-rays)
Strongly absorbed: lead (Pb), barium, thick concrete
Dense/high-Z materials absorb more X-rays
Thickness matters: even Pb allows some passage if thin enough
📝 TOPIC-WISE PYQ
Properties of X-Rays — NDA Pattern Questions
Q1. X-rays are NOT deflected by electric and magnetic fields because:
(a) They have very small mass (b) They travel at the speed of light (c) They carry no electric charge (d) They have very short wavelength
Answer: (c) They carry no electric charge
Only charged particles are deflected by electric and magnetic fields (F = qE and F = qvB). X-rays, being electromagnetic waves with no charge, pass undeflected through both fields. This is a key test to distinguish X-rays from alpha particles (deflected) and beta particles (deflected, but opposite sign).
Q2. Harder X-rays are produced by:
(a) Increasing filament temperature (b) Increasing accelerating voltage (c) Decreasing accelerating voltage (d) Using a lighter target material
Answer: (b) Increasing accelerating voltage
Hard X-rays have shorter wavelength and higher penetrating power. Since E = eV, higher voltage → higher electron kinetic energy → higher energy X-ray photons → shorter wavelength (harder). Filament temperature only changes the number (intensity), not the energy (hardness) of X-rays.
Q3. X-ray diffraction by crystal lattices was first used to determine the crystal structure of DNA by:
(a) Watson & Crick (b) Rosalind Franklin (c) Marie Curie (d) Lawrence Bragg
Answer: (b) Rosalind Franklin
Rosalind Franklin's X-ray diffraction photographs (especially "Photo 51") of DNA in 1952 provided crucial evidence for the double helix structure. Watson and Crick used this data to build their model. Bragg (father and son) developed the foundations of X-ray crystallography. X-ray diffraction is used to determine molecular and crystal structures.
4. Applications of X-Rays
4.1
Medical, Industrial & Defence Applications
From bone fractures to bomb detection — X-rays serve across domains
🏥 Medical Applications
Radiography: plain X-ray of bones (fractures, dislocations)
⚓ Defence Significance — Baggage and Vehicle Scanning: Airport security and military checkpoints use X-ray scanners to examine luggage, packages, and vehicle cargo without physical opening. Different materials absorb X-rays differently — metals appear white/light, organic materials (explosives, drugs) appear orange/brown, inorganic materials blue in colour-coded displays. The scanner operator identifies suspicious shapes and densities. Indian Air Force bases, naval installations, and military cantonments all use X-ray screening at entry points. Portable X-ray systems can inspect suspicious IEDs in the field.
📝 TOPIC-WISE PYQ
Applications of X-Rays — NDA Pattern Questions
Q1. CT scan (Computed Tomography) differs from a conventional X-ray in that it:
(a) Uses sound waves (b) Takes X-rays from multiple angles to create 3D images (c) Uses lower radiation dose (d) Does not use radiation
Answer: (b) Takes X-rays from multiple angles to create 3D images
A CT scan rotates the X-ray source and detector around the patient, taking images from hundreds of angles. Computers reconstruct these into detailed cross-sectional (axial) slices and 3D images, showing soft tissue, tumours, and organs that plain X-rays cannot distinguish. CT uses significantly more radiation than a single X-ray.
Q2. X-ray crystallography is used to:
(a) Detect cancer (b) Determine the arrangement of atoms in crystals and molecules (c) Screen airport luggage (d) Treat tumours
Answer: (b) Determine the arrangement of atoms in crystals and molecules
X-rays are diffracted by crystal lattice planes (spacing ~ same as X-ray wavelength). The diffraction pattern (Bragg's law: 2d sinθ = nλ) reveals the atomic arrangement. This technique determined the structure of DNA, proteins, vitamins, and thousands of complex molecules — one of science's most powerful analytical tools.
Q3. Non-destructive testing (NDT) using X-rays is used to:
(a) Destroy defective metal parts (b) Detect internal flaws in materials without damaging them (c) Clean industrial machinery (d) Measure electrical conductivity
Answer: (b) Detect internal flaws in materials without damaging them
Industrial NDT X-ray inspection can reveal cracks, voids, inclusions, and weld defects inside metal structures without cutting or destroying them. Critical for aircraft fuselage welds, pressure vessel inspection, and pipeline integrity — all essential in defence manufacturing.
🤔 TRICKY QUESTIONS
Applications — Reasoning Traps
T1. Why does bone appear white (opaque) on an X-ray image while surrounding flesh appears dark (transparent)?
Bone absorbs more X-rays (high Ca density); flesh transmits most X-rays.
X-ray attenuation depends on atomic number (Z) and density. Calcium (Z=20) in bone is denser and has higher Z than carbon, hydrogen, oxygen in soft tissue — so bone absorbs significantly more X-rays. Where more X-rays are absorbed, fewer reach the film → that region appears white (unexposed film = white/light). Where X-rays pass through freely (soft tissue), more reach the film → film is darkened → appears dark/transparent on the radiograph. Lead (Z=82) would appear almost completely white.
T2. A patient receives a CT scan for diagnosis. Why is the radiation dose from a CT scan much higher than from a simple chest X-ray?
CT takes hundreds of X-ray exposures from multiple angles; a chest X-ray takes only one.
A single posteroanterior (PA) chest X-ray: radiation dose ≈ 0.02 mSv. A CT scan of the chest: dose ≈ 7–8 mSv — about 350–400× more. The CT scanner rotates around the patient taking images at hundreds of angles to reconstruct 3D cross-sections. Each angular position is an X-ray exposure. The total dose is the sum of all these exposures. This higher dose is medically justified only when the diagnostic benefit outweighs radiation risk — for routine screening, plain X-rays or ultrasound (no radiation) are preferred.
5. Biological Hazards & Safety
5.1
Radiation Hazards, Dose & Protection
X-rays ionise living tissue — understanding risks and protection is essential for every defence professional
X-rays are ionising radiation — they carry enough energy to remove electrons from atoms, breaking chemical bonds and damaging DNA. This can cause cell death, genetic mutations, or cancer. However, at controlled doses, this property is also used therapeutically (radiotherapy). The key principle in radiation safety is ALARA — As Low As Reasonably Achievable.
⚡ Radiation Dose & Units
Absorbed dose: D = Energy absorbed / Mass of tissue
Unit: Gray (Gy) = J/kg
Equivalent dose: H = D × W_R
W_R = radiation weighting factor
(X-rays: W_R = 1; α-particles: W_R = 20)
Unit: Sievert (Sv) = J/kg (for biological effect comparison)
Older unit: rem = roentgen equivalent man
1 Sv = 100 rem
Dose limits (IAEA / AERB guidelines):
Radiation workers: 20 mSv/year averaged over 5 years
General public: 1 mSv/year
Occupational maximum: 50 mSv in any single year
Background radiation (natural): ≈ 2–3 mSv/year (varies by location)
Single chest X-ray: ≈ 0.02 mSv (very low)
CT scan (chest): ≈ 7 mSv
Flight crew (annual): ≈ 3–5 mSv (cosmic rays at altitude)
The Sievert (Sv) accounts for both absorbed dose and biological effectiveness of different radiation types. For X-rays, 1 Gy = 1 Sv (weighting factor = 1). For alpha particles, 1 Gy = 20 Sv (far more biologically damaging per unit absorbed dose).
⚠ Biological Effects of X-Rays
Somatic effects: affect the exposed person (skin burns, cataracts, cancer)
Genetic effects: affect offspring (DNA mutations in reproductive cells)
Most sensitive tissues: bone marrow, gonads, thyroid, lens of eye
Least sensitive: muscle, nerve, bone (in adults)
High acute dose (>1 Sv): Acute Radiation Syndrome (ARS) — nausea, hair loss, death if >6 Sv
Rapidly dividing cells more sensitive (cancer cells, foetus, blood cells)
🛡 Radiation Protection — Three Principles
1. Time: Reduce exposure duration. Less time near source = less dose.
3. Shielding: Dense materials absorb X-rays. Lead (Pb) is most used.
Half-value layer (HVL): thickness reducing intensity by 50%
Lead aprons, thyroid shields, lead glass screens for workers
⚡ Inverse Square Law & Shielding
Inverse Square Law:
Intensity I ∝ 1/r² (r = distance from X-ray source)
I₁/I₂ = r₂²/r₁²
Example: Moving from 1 m to 2 m from source → dose reduces to 1/4
Attenuation (shielding):
I = I₀ × e^(−μx) (exponential attenuation)
μ = linear attenuation coefficient (depends on material and X-ray energy)
x = thickness of shielding material
Half-Value Layer (HVL):
Thickness of material that reduces X-ray intensity to 50%
HVL_lead ≈ 0.6 mm for 100 kV X-rays (much less than other materials)
Shielding materials (decreasing effectiveness, same thickness):
Lead (Pb) > Barium concrete > Concrete > Steel > Aluminium > Water
Lead is used for shielding because of its very high density (11.3 g/cm³) and high atomic number (Z = 82). Just 1 mm of lead provides significant protection against diagnostic X-ray energies. Lead aprons in X-ray rooms typically contain 0.25–0.35 mm Pb equivalent.
Fig. 3 — Left: Inverse square law. Doubling distance reduces intensity to ¼. Right: Lead shielding attenuates X-rays exponentially (I = I₀ e^−μx). Just 0.6 mm of lead halves the intensity of 100 kV X-rays. Workers stand behind lead screens or wear lead aprons.
Q1. Lead is used as a shielding material against X-rays because it has:
(a) Low density and low Z (b) High density and high atomic number (c) High electrical conductivity (d) Low melting point
Answer: (b) High density and high atomic number
Lead (Pb, Z = 82, density = 11.3 g/cm³) is highly effective at absorbing X-rays because high-Z atoms have more electrons and a larger cross-section for photoelectric absorption. High density means more atoms per unit volume. Just a few millimetres of lead provide protection equivalent to centimetres of concrete.
Q2. If the distance from an X-ray source is tripled, the intensity of X-rays received:
(a) Reduces to 1/3 (b) Reduces to 1/6 (c) Reduces to 1/9 (d) Increases 3 times
Answer: (c) Reduces to 1/9
Intensity I ∝ 1/r². Tripling distance: I' = I/3² = I/9. Distance is the simplest and most effective radiation protection measure. Doubling distance → ¼ dose; tripling → 1/9 dose.
Q3. The three cardinal principles of radiation protection are:
(a) Speed, accuracy, cost (b) Time, distance, shielding (c) Dose, frequency, shielding (d) Lead, concrete, water
Answer: (b) Time, distance, shielding
The three cardinal principles — Time (minimise exposure duration), Distance (maximise distance from source; I ∝ 1/r²), and Shielding (use lead/concrete barriers) — form the basis of all radiation safety protocols in hospitals, research labs, and military nuclear facilities.
🤔 TRICKY QUESTIONS
Safety & Properties — Conceptual Questions
T1. Why is a pregnant radiologist advised to take extra precautions (or avoid X-ray work) during her first trimester?
The developing embryo/foetus has rapidly dividing cells — far more sensitive to radiation.
Rapidly dividing cells are most vulnerable to X-ray damage because ionisation disrupts DNA during replication. In the first trimester, organ systems are being formed (organogenesis). Radiation during this period can cause: birth defects, growth retardation, mental disability, or increased cancer risk in the child. Regulatory standards require pregnant workers to limit total gestational dose to 1 mSv to the abdomen. This is 20× lower than the annual occupational limit, reflecting the extreme sensitivity of the developing foetus.
T2. X-rays and gamma rays have overlapping wavelength ranges. How then are they distinguished?
By their origin — NOT by wavelength alone.
X-rays originate from: (a) electron deceleration (bremsstrahlung) or (b) electron shell transitions in atoms — produced in X-ray tubes externally. Gamma rays originate from: nuclear transitions within radioactive nuclei — emitted during radioactive decay. Both are electromagnetic radiation with similar or overlapping wavelengths (0.01 nm X-rays overlap with some γ-rays). The distinction is purely by production mechanism: X-ray = extranuclear origin; γ-ray = intranuclear origin.
⚡ High-Yield Formula Sheet — PN09 X-Rays
📈 X-Ray Nature & Energy
λ range: 0.001–10 nm
Speed = c = 3×10⁸ m/s
E = hf = hc/λ
Not deflected by E or B fields
Shorter λ → harder X-rays → more penetrating
🔌 Coolidge Tube
λ_min = hc/eV ≈ 12400/V (Å, V in volts)
Intensity controlled by: filament temperature
Hardness controlled by: accelerating voltage
Double V → half λ_min (harder X-rays)
Target: W (high mp, high Z)
🔐 Bragg's Law (Crystallography)
2d sinθ = nλ
d = crystal plane spacing; θ = glancing angle
n = order of diffraction (1, 2, 3...)
Used to find crystal structure and molecular geometry
⚡ Dose & Safety
Absorbed dose: Gray (Gy) = J/kg
Equivalent dose: Sievert (Sv) = D × W_R
X-ray weighting factor W_R = 1
Occupational limit: 20 mSv/year
Public limit: 1 mSv/year
🛡 Protection Laws
Inverse square law: I ∝ 1/r²
Triple distance → dose = I/9
Shielding: I = I₀ e^(−μx)
HVL(Pb) ≈ 0.6 mm at 100 kV
3 principles: Time, Distance, Shielding
📌 Key Facts
Discovered: Röntgen, 1895; Nobel 1901
X-ray vs γ-ray: origin, not wavelength
Soft: λ 0.1–10 nm; Hard: 0.001–0.1 nm
CT: multiple angles, 3D; higher dose than plain X-ray
Most sensitive: bone marrow, gonads, thyroid
📌 EM Spectrum (wavelength order, longest to shortest): Radio > Microwave > Infrared > Visible > UV > X-rays > Gamma rays. All travel at c = 3×10⁸ m/s in vacuum.
⚡ Quick Revision Booster — PN09 X-Rays
📈 Nature & Discovery
Röntgen discovered X-rays in 1895 — Nobel Prize 1901
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