ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC SPECTRUM

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

  1. 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.
  2. Nuclear reactor explosions may lead to losses of lives.

Example 28.1

  • A radio transmitter produces waves of frequency 1.0x108 Calculate the wavelength of the signal.

c=fλ

3.0x108m/s= 1.0x108xλ

λ=3.0m

  • An X-ray machine produces a radiation of wavelength 1.0x10-11 calculate:
  1. The frequency of the radiation.

c=fλ

3.0x108m/s=fx1.0x10-11m

f=3.0x1019Hz

  1. The energy content of the radiation. Take h=6.63x10-34

E=hf=6.63x10-34Js x3.0x1019Hz

=1.989x10-14J.

  • Arrange the following radiations in order of increasing wavelength: infra red, blue light, UV light, radio waves and X-rays.

X-rays, UV light, blue light, infra red and radio waves.