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6.8 Electromagnetic Waves

Introduction

Electromagnetic waves are transverse waves ,consisting of oscillating electric fields and magnetic fields .

Energy resulting from the acceleration of electric charge and the associated electric fields and magnetic fields .The energy can be regarded through space involving oscillating electric fields and magnetic fields at right angle to each other to the direction of propagation. In a vacuum the waves travel with a constant speed i.e. speed of light , 3 x 108 m s-1.

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The electromagnetic spectrum

The electromagnetic spectrum is a continuous spectrum , with no gaps in it and no frequencies anywhere in the range which do not exist. Like the spectrum of light , the different kinds of radiation gradually change from one to another as their properties gradually change. So there is no sharp boundary between one type of radiation and the next. There is often a large overlap at the boundaries.

The following diagram shows all the members of the electromagnetic family of waves.

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The common properties of the electromagnetic waves

  1. They all transfer energy from one place to another.
  2. They all transverse, electromagnetic waves
  3. They all can travel through a vacuum
  4. They all travel at a speed of approximately3 x 108 m s-1
  5. They all show the wave properties : reflection, refraction, diffraction and interference
  6. The all obey the wave equation v = fλ
  7. They all carry no charge
  8. They all can be emitted and absorbed by matter

Detecting and using the electromagnetic spectrum

(1) Gamma rays

Sources : nuclei of radioactive atoms and cosmic rays

Detector : photographic film, Geiger-Muller Tube

Special Properties : High penetrating power, very dangerous can causes cancer and mutation

Uses : to kill cancerous growth to find flaws in metal castings to sterilise equipment to control pest in agriculture

(2) X-ray

Sources : X-ray tubes

Detector : photographic film, Fluorescent screen

Special Properties : High penetrating power,very dangerous , damage to cells

Uses : to take X-ray picture;Radiography to treat skin disorders to study crystal structures,to scan baggage,to scan cracks in building structures,to detect art forgeries

(3) Ultra-violet (UV)

Sources : the sun, very hot objects , arcs,and sparks mercury vapour lamps, discharge tube

Detector : photographic film, photo cells, fluorescent chemicals

Special Properties : absorbed by glass , cause many chemical reactions, damages and kills living cells,causes sun burn,causes blindness

Uses : treatment of skin complaints,killing bacteria,fluorescent lighting,burglar alarms automatic counting in industry washing powders ,to identify counterfeit notes Production of vitamin D in the skin

(4) Visible light

Sources : the sun, hot objects , lamps, lasers , lightening, flames

Detector : photographic film, eye , photo cells

Special Properties : refracted by glass and focused by the eye increased rates of premature skin aging and skin cancer

Uses : observation of visible world, essential for photosynthesis and plant growth laser and optical fibres flame test to identify elements in chemistry flash in camera

(5) Infra-red light (IR)

Sources : the sun, warm and hot objects such as fires and people,

Detector : special photographic film, thermopile , photo transistor eye , photo cells

Special Properties : causes heating when absorbed, makes skin feel warm, skin burns

Uses : night vision,thermal imaging and physiotherapy,photocopiers(thermal), cooking,heating,remote control for TV / VCR,photography through haze and fog computer and hand phone to connect

(6) Microwave

Sources : microwave transmitter , microwave ovens

Detector : wave guide tube

Special Properties : spread round hills and building by diffraction internal heating of body tissues

Uses : cooking,radar system to detect objects,(size ,form and position),satellite transmissions

(7) Radio waves

Sources : electrical oscillating circuit

Detector : aerials connected to TV or radio

Special Properties : harm body cells, prevalence of migraine, headache disorders

Uses : telecommunications (telephone),broadcasting (TV and radio),communication in aircrafts, ships and missiles,used in radio astronomy

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