Electromagnetic Waves and the Spectrum: Exam Notes
Electromagnetic waves and the spectrum explained for exams: properties, speed, the order of the seven bands, wavelength ranges, their discoverers and their uses.
By GK24 Editorial Team· Published · 4 min read
An electromagnetic wave is a wave of electric and magnetic fields that carries energy without carrying any matter. James Clerk Maxwell showed in the eighteen sixties that a changing electric field produces a magnetic field and a changing magnetic field produces an electric field, so the two can sustain each other and travel outward; Heinrich Hertz produced and detected such waves in the laboratory about twenty years later. Everything from the radio in a bus to the X-ray plate in a hospital is one band of the same family, and that is exactly how examiners test the chapter: the order of the bands, the number attached to each, who found it and what it is used for.
How electromagnetic waves are produced
An electric charge at rest has only an electric field and a charge moving steadily gives a steady current, but an accelerating or oscillating charge radiates. That is why an alternating current in an aerial sends out radio waves, why an electron braking suddenly in a metal target gives X-rays, and why a hot body glows. Gamma rays are different in origin: they come from inside the nucleus, from radioactive decay, which is why they are the most penetrating of all.
Properties worth memorising
- They are transverse waves: the electric field, the magnetic field and the direction of travel are mutually perpendicular.
- They need no material medium and travel through vacuum, which is how sunlight reaches the earth while sound cannot.
- In vacuum every band travels at the same speed, about 3 lakh kilometres per second, that is 3 into 10 to the power 8 metres per second; Maxwell showed this speed to be fixed by the electric and magnetic constants of free space.
- For all of them the speed equals frequency multiplied by wavelength, so a higher frequency always means a shorter wavelength.
- They carry energy and momentum and so exert a small radiation pressure, the principle behind the solar sail.
- Being uncharged, they are not deflected by electric or magnetic fields, unlike a beam of electrons or alpha particles.
The electromagnetic spectrum in order
| Band | Approximate wavelength | Discovered by | Chief uses |
|---|---|---|---|
| Radio waves | Longer than about a tenth of a metre | Produced by Heinrich Hertz, 1887 | Radio and television broadcasting, mobile telephony, astronomy |
| Microwaves | About a millimetre to a tenth of a metre | Early work by Heinrich Hertz and J. C. Bose | Radar, satellite links, microwave ovens |
| Infrared | About 700 nanometres to a millimetre | William Herschel, 1800 | Remote controls, night vision and thermal imaging, physiotherapy |
| Visible light | About 400 to 700 nanometres | Dispersed into colours by Isaac Newton | Vision, photography, photosynthesis |
| Ultraviolet | About 10 to 400 nanometres | Johann Wilhelm Ritter, 1801 | Sterilising water and instruments, vitamin D in the skin, detecting forged notes |
| X-rays | About 0.01 to 10 nanometres | Wilhelm Conrad Roentgen, 1895 | Medical radiography, crystal structure study, security scanning |
| Gamma rays | Shorter than about 0.01 nanometre | Paul Villard, 1900 | Radiotherapy for cancer, sterilising medical goods, food preservation |
Read the table downwards and wavelength falls while frequency, energy and penetrating power rise. Read it upwards and the opposite happens. Within visible light the same rule orders the colours: violet has the shortest wavelength and the highest frequency, red the longest wavelength and the lowest frequency, and the seven colours in between are remembered as VIBGYOR.
Where each band matters in daily life
Radio waves of the longer sort bend round the curve of the earth and are reflected by the ionosphere, which is why short wave broadcasts travel between continents. Microwaves are strongly absorbed by water molecules, so a microwave oven heats the water in food from within rather than the plate around it, and the same short wavelength makes microwaves the natural choice for radar and for satellite communication. Infrared is the heat radiation of warm bodies; greenhouse gases in the atmosphere absorb the infrared sent back by the earth surface, which is the greenhouse effect, and a thermal camera sees a human body by the same radiation. Ultraviolet from the sun is largely stopped by the ozone layer of the stratosphere, and the part that reaches us makes vitamin D in the skin but also causes sunburn and damages the eyes, so welders wear filtering goggles. X-rays pass through flesh but not through bone and metal, which makes the radiograph possible, and gamma rays, the most energetic of all, are used to kill tumour cells and to sterilise surgical instruments.
Light year and the distances of astronomy
Because electromagnetic waves travel at a fixed speed, astronomers measure distance by time of travel. A light year is the distance light covers in one year in vacuum, about 9.46 into 10 to the power 12 kilometres; it is a unit of distance and not of time, which is a favourite trap. The astronomical unit is the mean distance of the earth from the sun, and a parsec is about 3.26 light years.
Exam Point of View
Three kinds of question come up. First, the order of the spectrum: arrange the bands by increasing wavelength, frequency or energy, or pick the one with the longest wavelength or the highest penetrating power. Second, the single fact: who discovered X-rays, infrared or ultraviolet, what is the speed of electromagnetic waves in vacuum, which band is used in radar, in a television remote, in sterilising water or in cancer therapy, and which is stopped by the ozone layer. Third, the property: whether electromagnetic waves are transverse or longitudinal, whether they need a medium, whether they are deflected by a magnetic field, and whether all bands travel at the same speed. The standard traps are calling a light year a unit of time, saying that gamma rays travel faster than radio waves because they carry more energy, and placing ultraviolet on the longer wavelength side of visible light instead of the shorter.
Important Facts
| Predicted by | James Clerk Maxwell, in the eighteen sixties |
|---|---|
| Produced experimentally by | Heinrich Hertz, 1887 |
| Speed in vacuum | About 3 into 10 to the power 8 metres per second for every band |
| Nature of the wave | Transverse; electric field, magnetic field and direction of travel mutually perpendicular |
| Longest wavelength band | Radio waves |
| Shortest wavelength and highest energy | Gamma rays |
| Visible light range | About 400 to 700 nanometres, violet to red |
| Infrared | Discovered by William Herschel in 1800; used in remote controls and thermal imaging |
| Ultraviolet | Discovered by Johann Wilhelm Ritter in 1801; absorbed by the ozone layer |
| X-rays | Discovered by Wilhelm Conrad Roentgen in 1895; used in radiography |
| Gamma rays | Discovered by Paul Villard in 1900; emitted by radioactive nuclei |
| Microwave oven | Works by the absorption of microwaves by water molecules in food |
| Light year | A unit of distance, about 9.46 into 10 to the power 12 kilometres |
| Parsec | About 3.26 light years |
Practice MCQs on this topic
Electromagnetic waves were first produced and detected in the laboratory by which scientist?
- A.James Clerk Maxwell
- B.Heinrich Hertz
- C.Michael Faraday
- D.Guglielmo Marconi
Show answer
In 1887 he used a spark gap oscillator to generate waves and a loop with a gap to detect them, and he measured their wavelength and showed that they behaved like light in reflection and refraction, which confirmed the theory.
Read the full explanation →Which of the following statements about electromagnetic waves is correct?
- A.They are longitudinal waves and need a material medium
- B.They are transverse waves and can travel through vacuum
- C.They are deflected by a strong magnetic field
- D.Their speed in vacuum depends on their frequency
Show answer
In an electromagnetic wave the electric field, the magnetic field and the direction of propagation are mutually perpendicular, and because the wave is made of fields and not of vibrating particles it needs no medium, which is how sunlight crosses empty space while sound cannot.
Read the full explanation →Which part of the electromagnetic spectrum has the longest wavelength?
- A.Radio waves
- B.Infrared rays
- C.Ultraviolet rays
- D.Gamma rays
Show answer
Their wavelength runs from about a tenth of a metre up to many kilometres, which makes them the longest and therefore the lowest in frequency and energy of the whole spectrum; they are used in broadcasting, mobile telephony and radio astronomy.
Read the full explanation →Which electromagnetic radiation has the highest frequency and the greatest penetrating power?
- A.X-rays
- B.Gamma rays
- C.Microwaves
- D.Visible light
Show answer
They have the shortest wavelength of the spectrum and therefore the highest frequency and energy, and being uncharged and very energetic they pass through several centimetres of lead, which is why thick shielding is needed in radiotherapy rooms.
Read the full explanation →Which of the following arranges the radiations in order of increasing wavelength?
- A.Gamma rays, X-rays, ultraviolet, infrared, radio waves
- B.Radio waves, infrared, ultraviolet, X-rays, gamma rays
- C.X-rays, gamma rays, infrared, ultraviolet, radio waves
- D.Infrared, ultraviolet, radio waves, X-rays, gamma rays
Show answer
The spectrum in order of rising wavelength runs gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves, and the option follows that sequence with the visible and microwave bands left out.
Read the full explanation →X-rays were discovered by which scientist?
- A.Wilhelm Conrad Roentgen
- B.Henri Becquerel
- C.William Herschel
- D.Paul Villard
Show answer
In 1895 he noticed that a screen coated with a fluorescent salt glowed when a discharge tube was working nearby even though the tube was covered, and he called the unknown radiation X-rays; he received the first Nobel Prize in Physics in 1901 for the discovery.
Read the full explanation →Infrared radiation was discovered by which scientist?
- A.Johann Wilhelm Ritter
- B.William Herschel
- C.Isaac Newton
- D.J. C. Bose
Show answer
In 1800 he passed sunlight through a prism and moved a thermometer across the coloured band; the reading went on rising past the red end, which showed the presence of an invisible heating radiation now called infrared.
Read the full explanation →Which radiation is used in the remote control of a television set?
- A.Ultraviolet rays
- B.Infrared rays
- C.X-rays
- D.Gamma rays
Show answer
A remote control carries a small light emitting diode that sends coded pulses of near infrared, which a sensor in the set decodes; the radiation is invisible, harmless at that power and does not pass through walls, so remotes in neighbouring rooms do not interfere.
Read the full explanation →Which radiation coming from the sun is largely absorbed by the ozone layer of the atmosphere?
- A.Infrared radiation
- B.Ultraviolet radiation
- C.Radio waves
- D.Visible light
Show answer
Ozone in the stratosphere absorbs most of the harmful shorter ultraviolet, and that is why the thinning of the layer by chlorofluorocarbons raises the risk of skin cancer, cataract and damage to crops; the small part of ultraviolet that does reach the surface makes vitamin D in human skin.
Read the full explanation →The speed of electromagnetic waves in vacuum is about
- A.3 into 10 to the power 8 metres per second
- B.3 into 10 to the power 6 metres per second
- C.340 metres per second
- D.1,500 metres per second
Show answer
Maxwell showed that this speed is fixed by the electric permittivity and magnetic permeability of free space, and its agreement with the measured speed of light was the proof that light is an electromagnetic wave; every band of the spectrum shares the same value in vacuum.
Read the full explanation →In the visible spectrum, which colour has the shortest wavelength?
- A.Red
- B.Green
- C.Violet
- D.Yellow
Show answer
Visible light runs from about 400 nanometres at the violet end to about 700 nanometres at the red end, so violet has the shortest wavelength, the highest frequency and the greatest energy of the seven colours remembered as VIBGYOR.
Read the full explanation →Light year is a measure of
- A.Time
- B.Distance
- C.Total amount of light falling on the earth in a year
- D.Average intensity of light falling on the earth in a year
Show answer
A light year is the distance that light travels in vacuum in one year at about 3 into 10 to the power 8 metres per second, which works out to roughly 9.46 into 10 to the power 12 kilometres; astronomers use it because distances between stars are awkward to write…
Read the full explanation →Frequently Asked Questions
Who discovered electromagnetic waves?
James Clerk Maxwell predicted them mathematically in the eighteen sixties by showing that changing electric and magnetic fields sustain each other, and Heinrich Hertz produced and detected them in the laboratory in 1887, which is why the unit of frequency is named after him.
Do all electromagnetic waves travel at the same speed?
In vacuum, yes. Radio waves and gamma rays both travel at about 3 into 10 to the power 8 metres per second; they differ in wavelength and frequency, not in speed. In a material medium the speed falls and depends on the medium and the wavelength.
Which electromagnetic radiation has the shortest wavelength?
Gamma rays, shorter than about 0.01 nanometre. They come from the nucleus in radioactive decay, carry the highest energy of the spectrum and have the greatest penetrating power, which is why thick lead shielding is used against them.
Why does a microwave oven heat food quickly?
Because microwaves are strongly absorbed by the water molecules present in food. The absorbed energy sets the molecules in rapid motion, so heat is generated throughout the food instead of being conducted inwards from the surface as in an ordinary oven.
Is a light year a unit of time or of distance?
Of distance. It is the distance that light travels in vacuum in one year, about 9.46 into 10 to the power 12 kilometres. The word year in the name refers to the time of travel, which is why the question is set so often.
Which radiation is blocked by the ozone layer?
Ultraviolet radiation from the sun. Ozone in the stratosphere absorbs most of the harmful shorter ultraviolet, and the thinning of this layer by chlorofluorocarbons therefore raises the risk of skin cancer, cataract and damage to crops.
Sources
- Physics Part I (Class XII), chapter on Electromagnetic Waves — NCERT
- Science (Class X), chapter on Light: Reflection and Refraction — NCERT
