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Electromagnetic radiation allows us to observe the world around us. It is this radiation which reflects off of the objects around you and into your eye. The radiation your eye is sensitive to is only a small fraction of the total radiation emitted in the physical universe. All of the different fractions taped together make up the electromagnetic spectrum.

Dispersion

When white light is split into its component colours by a prism, you are looking at a portion of the electromagnetic spectrum.

The wavelength of a particular electromagnetic radiation will depend on how it was created.

Wave nature of em radiation

  1. List one source of electromagnetic waves. Hint: consider the spectrum diagram and look at the names we give to different wavelengths.
  2. Explain how an EM wave propagates, with the aid of a diagram.
  3. What is the speed of light? What symbol is used to refer to the speed of light? Does the speed of light change?
  4. Do EM waves need a medium to travel through?

The radiation can take on any wavelength, which means that the spectrum is continuous. Physicists broke down this continuous band into sections. Each section is defined by how the radiation is created, not the wavelength of the radiation. But each category is continuous within the min and max wavelength of that category, meaning there are no wavelengths excluded within some range.

The spectrum is in order of wavelength, with the shortest wavelength at one end and the longest wavelength at the other. The spectrum is then broken down into categories as detailed in [link] .

Electromagnetic spectrum
Category Range of Wavelengths (nm) Range of Frequencies (Hz)
gamma rays < 1 > 3 × 10 19
X-rays 1-10 3 × 10 17 - 3 × 10 19
ultraviolet light 10-400 7 , 5 × 10 14 - 3 × 10 17
visible light 400-700 4 , 3 × 10 14 - 7 , 5 × 10 14
infrared 700- 10 5 3 × 10 12 - 4 , 3 × 10 19
microwave 10 5 - 10 8 3 × 10 9 - 3 × 10 12
radio waves > 10 8 < 3 × 10 9

Since an electromagnetic wave is still a wave, the following equation that you learnt in Grade 10 still applies:

c = f · λ

Calculate the frequency of red light with a wavelength of 4 , 2 × 10 - 7  m

  1. We use the formula: c = f λ to calculate frequency. The speed of light is a constant 3 × 10 8 m/s.

    c = f λ 3 × 10 8 = f × 4 , 2 × 10 - 7 f = 7 , 14 × 10 14 Hz
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Ultraviolet radiation has a wavelength of 200 nm . What is the frequency of the radiation?

  1. Recall that all radiation travels at the speed of light ( c ) in vacuum. Since the question does not specify through what type of material the Ultraviolet radiationis traveling, one can assume that it is traveling through a vacuum. We can identify two properties of the radiation - w a v e l e n g t h ( 200 nm ) and speed ( c ).

  2. c = f λ 3 × 10 8 = f × 200 × 10 - 9 f = 1 . 5 × 10 15 Hz
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Examples of some uses of electromagnetic waves are shown in [link] .

Uses of EM waves
Category Uses
gamma rays used to kill the bacteria in marshmallows and to sterilise medical equipment
X-rays used to image bone structures
ultraviolet light bees can see into the ultraviolet because flowers stand out more clearly at this frequency
visible light used by humans to observe the world
infrared night vision, heat sensors, laser metal cutting
microwave microwave ovens, radar
radio waves radio, television broadcasts

In theory the spectrum is infinite, although realistically we can only observe wavelengths from a few hundred kilometers to those of gamma rays due to experimental limitations.

Humans experience electromagnetic waves differently depending on their wavelength. Our eyes are sensitive to visible light while our skin is sensitive to infrared, and many wavelengths we do not detect at all.

Em radiation

  1. Arrange the following types of EM radiation in order of increasing frequency: infrared, X-rays, ultraviolet, visible, gamma.
  2. Calculate the frequency of an EM wave with a wavelength of 400 nm.
  3. Give an example of the use of each type of EM radiation, i.e. gamma rays, X-rays, ultraviolet light, visible light, infrared, microwave and radio and TV waves.

The particle nature of electromagnetic radiation

When we talk of electromagnetic radiation as a particle, we refer to photons, which are packets of energy. The energy of the photon is related to the wavelength of electromagnetic radiation according to:

Planck's constant

Planck's constant is a physical constant named after Max Planck.

h = 6 , 626 × 10 - 34 J · s

The energy of a photon can be calculated using the formula: E = h f or E = h c λ . Where E is the energy of the photon in joules (J), h is planck's constant, c is the speed of light, f is the frequency in hertz (Hz) and λ is the wavelength in metres (m).

Calculate the energy of a photon with a frequency of 3 × 10 18  Hz

  1. E = h f = 6 , 6 × 10 - 34 × 3 × 10 18 = 2 × 10 - 15 J
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What is the energy of an ultraviolet photon with a wavelength of 200 nm?

  1. We are required to calculate the energy associated with a photon of ultraviolet light with a wavelength of 200 nm.

    We can use:

    E = h c λ
  2. E = h c λ = ( 6 , 626 × 10 - 34 ) 3 × 10 8 200 × 10 - 9 = 9 , 939 × 10 - 10 J
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Exercise - particle nature of em waves

  1. How is the energy of a photon related to its frequency and wavelength?
  2. Calculate the energy of a photon of EM radiation with a frequency of 10 12  Hz.
  3. Determine the energy of a photon of EM radiation with a wavelength of 600 nm.

Questions & Answers

what is an isomer?
Kiba Reply
organic compounds with the same molecular formula but different structural formula
liphelo
how is frequency directly proportional to kinetic energy max. and what is the relationship between the wavelength and the energy of a photon. eg if a photon with a greater wavelength and a photon with less frequency. what would be the energies. as in which one would be greater and why
Pearl Reply
relationship wave depend n enrgy greadt reflect of energy n the form of tex shape
thulare
what is Cell capacity?
liphelo Reply
what are hydrocarbons
Nyiko Reply
compounds that are made up of hydrogen and carbon only
Patience
They are compounds that consist only with hydrogen &carbon elements.
Duncan
how do i calculate the work done on an object by a frictional force
Emmelda Reply
@ Emmelda it's W = FxCos180 Where 180° is the angle between Frictional force and where the object is going, it's always 180° because it opposes the motion. Then F is the magnitude of the force and x is the displacement
Vukona
Ohm's law state that provide the temperature remains constant, that potential difference is directly proportional to the current flowing in the circuit
Ezile Reply
state ohms law
Booi
what does ohm's law state ?
Gugu Reply
ohm's law state that the potential difference across a conductor is directly proportional to the current in the conductor at constant temperature
Nqobile
ohm'law in metalic conductor is directly proportional to the potential difference provided the temparature
Nandise
what is the principle on which generator is based
Nqobile Reply
electromagnetic induction
Sethu
Explain it😌
Nelisa
mechanical​ energy is used to rotate turbines inside the generator where a coil placed between two fixed magnets.as the coil rotates it cuts the magnetic flux due to the magnets. the magnetic flux will then induce an electric current into the coil which will be transmitted to the brushes
Ruva
may you Please instruct me on how to do Flemmings Right hand dynamo Rule
Siphe
Fleming's right-hand rule shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field. It can be used to determine the direction of current in a generator's windings
Ruva
 hold out the right hand with the first finger, second finger and thumb at the right angle to each other.forefinger represents the direction of the line of Force, the thuMb points in the direction of Motion or applied force, then seCond finger points in the direction of the induced Current
Ruva
what is spectrum
Mpeh Reply
It is a different combinations of frequency of light or any types of waves.
Duncan
Heii guys what's the topic? Am neww
Vannessa Reply
In nuclear reactions, is it necessary that there must only be one element on the product side and a radioactive particle or you can have a product that are both radioactive particles?
Jobena Reply
conservative force is a force when the work done in moving the object between the two points that does not depend on the path way
Bongiwe Reply
what is conservative force?
Mh Reply
It is a force , that does not have any external features affecting it
Duncan
what does the alternating current do
Kwanga Reply
what is definition of hydrocarbon
jessha Reply
it is the atoms that Contain of C atoms and H atoms only
Xolani
You mean molecules
Jc
yea 💯
Xolani
what's the definition of le chatelier?
Karabo
what is electromagnetic
Daniel
differentiate between DC motor and Ac generator
Kwanga
dc motor converts electrical energy into mechanical energy while ac generator converts mechanical into electical
Ophola
DC has a Split-ring but then Ac has a slip-rings.
Joyous
what's the other name of galvanic cell?
Cwenga
galvanic cell is also called Voltanic cell
Bongiwe
is the chain whereby there is only Hydrogen and Carbon atoms only.
Hlonny
what is the difference between AC and DC moto
Mihle
please check the answer from Ophola and Joyous
Nomsa
AC motor consists of 2 slip rings and a DC motor has a single split ring commutator
Vukona
ac have slip rings n DC has split rings
mudodzwa

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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