Electromagnetic Radiation

 Introduction

Transmission of energy through a vacuum or using no medium is accomplished by 

electromagnetic waves, caused by the oscillation of electric and magnetic fields. They 

move at a constant speed of 3x108 m/s. often, they are called electromagnetic 

radiation, light, or photons. An electromagnetic radiation, it has both electric and 

magnetic field components, which oscillate in a fixed relationship to one another, 

perpendicular to each other and perpendicular to the direction of propagation.

The two components making up an electromagnetic radiation are the:

a) Electric field B) Magnetic field

The two fields are always perpendicular to each other and both are perpendicular to the 

direction of propagation.

 

Fig. Electromagnetic wave

The notion that electromagnetic radiation contains a quantifiable amount of energy can 

perhaps be better understood if we talk about light as a stream ofparticles, 

called photons, rather than as a wave. (Recall the concept known as ‘wave-particle 

duality’: at the quantum level, wave behavior and particle behavior become 

indistinguishable, and very small particles have an observable ‘wavelength’). If we

describe light as a stream of photons, the energy of a particular wavelength can be 

expressed as:

E = hc / λ

where E is energy in kcal/mol, λ (the Greek letter lambda) is wavelength in 

meters, c is 3.00 x 108 m/s (the speed of light), and h is 9.537 x 10

-14 kcal/s/mol-1

, a 

number known as Planck’s constant.

Because electromagnetic radiation travels at a constant speed, each wavelength 

corresponds to a given frequency, which is the number of times per second that a crest 

passes a given point. Longer waves have lower frequencies, and shorter waves have 

higher frequencies. Frequency is commonly reported in hertz (Hz), meaning ‘cycles 

per second’, or ‘waves per second’. The standard unit for frequency is s-1

.

When talking about electromagnetic waves, we can refer either to wavelength or to 

frequency - the two values are interconverted using the simple expression:

 C 

where ν (the Greek letter ‘nu’) is frequency in s-1

. Visible red light with a wavelength 

of 700 nm, for example, has a frequency of 4.29 x 1014Hz, and an energy of 40.9 kcal 

per mole of photons.

The full range of electromagnetic radiation wavelengths is referred to as 

the electromagnetic spectrum.

1.3 Properties of Electromagnetic radiation 

The radiated EM radiation has certain properties: 

• EM waves travel at the speed of light c

• The electric and magnetic fields are perpendicular to each other. 

• The electric and magnetic fields are in phase (both reach a maximum and minimum 

at the same time). 

• The electric and magnetic fields are perpendicular to the direction of travel 

(transverse waves).

Terms Used 

Wavelength - Is the distance between any two equivalent points on successive waves.

Wavenumber - Is the reciprocal of the wavelength in centimeters.

Frequency - Is the number of oscillations of the field which occur each second.

Velocity- In a vacuum, the velocity of electromagnetic radiation is 2.9979 x 108 m/s

Amplitude - The height of the wave.

Their wavelengths and the corresponding differences in their energies: shorter 

wavelengths correspond to higher energy.

PROBLEM: Calculate wavelength and frequency of waves. 

(a) A local radio station broadcasts at a frequency of 91.7 MHz (91.7 x 106 Hz). What is the 

Wavelength of these radio waves?

b) What is the frequency of blue light with a wavelength of 435 nm?

SOLUTION: 

You are asked to calculate the wavelength or frequency of electromagnetic radiation. 

You are given the frequency or wavelength of the radiation. 

(a) First rearrange Equation 6.1 to solve for wavelength (λ). Then substitute the known values 

into the equation and solve for wavelength.

ʋ = c / λ or λ =c / ʋ

(b) First rearrange Equation 6.1 to solve for frequency (ʋ). Then substitute the known values 

into the equation and solve for frequency. Notice that wavelength must be converted to units 

of meters before using it in Equation (c = λʋ)

435nm43510 9m

= 4.35 x 10-7 m