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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.


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

are UV rays dangerous?
Khathutshelo Reply
what's this?
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
what is the wave model of atom ?
Trazy Reply
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
yes that's true
Always know Newton's second law of motion. It appears to be in every question paper
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
how many bones are in the human body?
it's the rate at which the reactants are able to change to products
what is wave lengh
mama Reply
is the de broglie wavelength of the particle
calcium and magnesium, which one can displace aluminum from its compound?
Markia Reply
how is aluminum ion formed?
a substance that has the lower electronegativity which will be in this case the Ca -1.0 this is because the Ca would have a lower ionization energy -needs less energy to fill its outer most shell and therefore will cause a displacement of the Al
an Al ion would form when it has lost electrons and will normally be a cation -positively charged (3+)
molecules with the same molecular formula but different chain
Malaza Reply
what will be the answer
chain isomer
what is structural isomer?
Hey if you define an isomer it will come as an organic molecule with the same number of atoms,same molecule formula ,same molecular mass but different STRUCTURAL FORMULA so what we have is only the chain ,positional and functionally isomer.Never heard of STRUCTURAL isomer
what does the newton's law says?
there are 3 newtons laws which are newton's law of universal gravitation, law of cooling, law of motion
Did the feather or leaf hit the ground first?
Shaloom Reply
both at the same time. air friction ignored
2 to 30 minut long distance race 20 km what's the avarage speed for the race
Jamilla Reply
what is meaning of covalent bonds
Lungani Reply
bonds that exists between non metal atoms. each atom contribute electron(s) which will form a bond joining the two atoms. electrons in the bond now belongs to both atoms
how do we name haloalkanes
Thobeka Reply
how do we name haloalkanes
what are examples of haloalkanes
haloalkanes are compounds that consist of the halogen group chlorine bromine flourine and so on like 2-bromobutan

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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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