What is the trend in the frequency when EM waves are arranged in the order of increasing wavelength?

EM radiation is classified into types according to the frequency of the wave: these types include, in order of increasing frequency, radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.

Table 11.1 lists the wavelength and frequency ranges of the divisions of the electromagnetic spectrum.

Category	Range of Wavelengths (nm)	Range of Frequencies (Hz)
gamma rays	\(< 1\)	\(> \text{3} \times \text{10}^{\text{17}}\)
X-rays	\(1–10\)	\(\text{3} \times \text{10}^{\text{16}} – \text{3} \times \text{10}^{\text{17}}\)
ultraviolet light	\(10–400\)	\(\text{7,5} \times \text{10}^{\text{14}} – \text{3} \times \text{10}^{\text{16}}\)
visible light	\(400–700\)	\(\text{4,3} \times \text{10}^{\text{14}} – \text{7,5} \times \text{10}^{\text{14}}\)
infrared	\(700 – \text{10}^{\text{5}}\)	\(\text{3} \times \text{10}^{\text{12}} – \text{4,3} \times \text{10}^{\text{14}}\)
microwave	\(\text{10}^{\text{5}} – \text{10}^{\text{8}}\)	\(\text{3} \times \text{10}^{\text{9}} – \text{3} \times \text{10}^{\text{12}}\)
radio waves	\(> \text{10}^{\text{8}}\)	\(< \text{3} \times \text{10}^{\text{9}}\)

Table 11.1: Electromagnetic spectrum

Examples of some uses of electromagnetic waves are shown in Table 11.2.

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

Table 11.2: Uses of EM waves

Textbook Exercise 11.1

Arrange the following types of EM radiation in order of increasing frequency: infrared, X-rays, ultraviolet, visible, gamma.

Solution not yet available

Calculate the frequency of an EM wave with a wavelength of \(\text{400}\) \(\text{nm}\).

Solution not yet available

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.

Solution not yet available

Figure 11.2: The electromagnetic spectrum as a function of frequency. The different types according to wavelength are shown as well as everyday comparisons.

EM radiation in the visible part of the spectrum is scattered off all of the objects around us. This EM radiation provides the information to our eyes that allows us to see. The frequencies of radiation the human eye is sensitive to constitute only a very small part of all possible frequencies of EM radiation. The full set of EM radiation is called the electromagnetic spectrum. To simplify things the EM spectrum divided into sections (such as radio, microwave, infrared, visible, ultraviolet, X-rays and gamma-rays).

The EM spectrum is continuous (has no gaps) and infinite. Due to technological limitations, we can only use electromagnetic radiation with wavelengths between \(\text{10}^{-\text{14}}\) \(\text{m}\) and \(\text{10}^{\text{15}}\) \(\text{m}\).

Introduction

We have seen that waves can be categorized either electromagnetic or mechanical in nature. Electromagnetic waves are waves resulting from the interaction of oscillating electric and magnetic fields. They include visible light, radio waves, x-rays, infra red, ultraviolet, microwaves and gamma radiations.

When these waves are arranged in a certain pattern e.g in the order of increasing frequency or wavelength then we get an electromagnetic spectrum.

28.2: The electromagnetic spectrum

Increasing frequency

f(Hz) 103 108 1010 10141015 1022

R M IR V UV X G

λ(m) 105 100 10-3 10-6 10-8 10-10- 10-11 10-13

Decreasing wavelength

Where R- Radio waves

M- Microwaves

IR- Infra red

V- Visible light

UV- Ultraviolet

X- X-rays

G- Gamma radiation

[Hint: Roast(R) maize (M) is (IR) a very (V) unusual (UV) x-mass(X) gift (G)].

28.3: Properties of electromagnetic waves

The following properties are common to all electromagnetic waves:

Travel in a vacuum with a speed of 3.0x108m/s.
Do not require material medium for their propagation.
Transverse in nature.
Posses and transfer energy. The amount of energy possessed by an electromagnetic wave of frequency f is expressed as E= hf, where h is Plank’s constant and is equal to 6.63x10-34 The wave equation c=fλalso apply for electromagnetic waves.
Carry no charge (not charged) and are not deflected by a magnetic or electric field.
Undergo reflection, refraction, diffraction, interference and polarization effects.
Can be emitted, transmitted and absorbed by matter.

28.4: Production, detection and applications of electromagnetic radiations

The table below summarizes the production, detection and applications of the various electromagnetic radiations:

Radiation	Production	Detection	Application
Radio waves	From oscillating electrical circuits.	Antennae (aerials), diodes, earphones.	In telecommunication- radio broadcast, TV and satellite communication, cellular telephone, radar and navigation equipments etc.
Microwaves	From special vacuum tubes called magnetrons within microwave ovens.	Crystal detectors, solid state diodes, antennae.	Cooking in microwave cookers. In communication- mobile phones. In speed cameras.
Infra red	From thermal vibration of atoms in very hot bodiese.g the sun.	Thermopile, bolometer, thermometer, photographic film. Heating effect on the skin.	Burglar alarms, in military night vision missiles, cooking, heating and drying of grains, in green housing, in remote controls for TVs and VCD/DVDs, in photography.
Visible light	Sun, hot objects, lamps, laser beams.	The eye, photographic film, photocell.	Vision (sight), photography, photosynthesis and optical fibre. Laser beams used in laser printers, weapon aiming systems, CD players.
Ultraviolet	The sun, sparks, mercury vapour lamps.	Photographic film, photocells, fluorescent materials e.g quinine sulphate.	Detection of forgeries, skin treatment and killing of bacteria, spectroscopy and mineral analysis, making of clothes and a source of vitamin D.
X-rays	In X-ray tubes	Fluorescent screen and photographic film.	Radiography (identification of internal body structures e.g bones), cancer therapy, crystallography (study of crystal structure), pest and germ control and airport security ckecks.
Gamma radiation	Emitted by radioactive substances.	Radiation detectors e.g GM tube.	Sterilizing medical equipment and food. Killing of cancer cells and other malignant growths. Pest control. Detection of flaws in metals.

28.5: Dangers of electromagnetic waves

Some of the electromagnetic waves like X-rays, ultraviolet and gamma radiation posses a lot of energy. Therefore when exposed to the body in large doses, they can damage the body cells, cause skin burns or affect the eyes. Similarly, radio waves can cause cancer, leukemia among other disorders.
Nuclear reactor explosions may lead to losses of lives.

Example 28.1