Chapter 11 - Electromagnetic Waves Properties Applications and Biological Effects

Properties of Electromagnetic Waves

  • Electromagnetic (EM) waves are a specific class of waves that transfer energy through a vacuum or through a material medium.
  • An electromagnetic wave contains neither mass nor charge.
  • It consists of a set of varying electric and magnetic fields that are mutually linked with each other at right angles (90∘90^\circ).
  • The electric and magnetic fields oscillate perpendicular to the direction of the transfer of energy (propagation), categorizing them as transverse waves.
  • Sources of electromagnetic radiation include:
    • Cosmic sources, such as the Sun and various stars.
    • Radioactive elements.
    • Manufactured or man-made devices.

General Characteristics of EM Waves

  • Transverse Nature: EM waves are transverse waves, meaning the oscillations of the electric and magnetic fields are perpendicular to the direction of wave travel.
  • Propagation: They can travel through a vacuum and do not require any physical medium for propagation.
  • Speed in Vacuum: In a vacuum, all EM waves travel at the speed of light, denoted by the constant cc. The value is approximately c≈3.0×108 m s−1c \approx 3.0 \times 10^8\,m\,s^{-1}.
  • Energy Transfer: Like all waves, EM waves transfer energy from one location to another.
  • Wave Equation: EM waves obey the universal wave equation:     v=fλv = f\lambda
    • vv = wave speed
    • ff = frequency of the EM wave
    • λ\lambda = wavelength of the EM wave
  • Wave Phenomena: EM waves obey the laws of reflection and the laws of refraction.
  • Effect of Medium Changes: When an EM wave travels from one medium to another (e.g., from vacuum to glass):
    • The frequency remains unchanged.
    • The wave speed changes.
    • The wavelength changes.
  • Energy and Frequency Relationship: The energy of an EM wave is directly related to its frequency. The higher the frequency, the higher the energy carried by the wave.
  • Ionizing Potential: Higher frequency waves with higher energies are classified as ionizing waves. They possess enough energy to eject electrons from atoms and molecules, creating ions. These ions can disrupt cell functions or kill cells.

The Electromagnetic Spectrum

  • The electromagnetic spectrum is a continuous range of all electromagnetic waves, typically arranged according to their frequency and wavelength.
  • Memory Aid for the Spectrum Order (from lowest frequency/longest wavelength to highest frequency/shortest wavelength):
    • Rugby (Radio waves)
    • Match (Microwaves)
    • Is (Infra-red)
    • Very (Visible light)
    • Unlike (Ultra-violet)
    • Xylophone (X-rays)
    • Game (Gamma rays)

Visible Light Sub-Spectrum

  • Visible light occupies a range from approximately 700 nm700\,nm to 400 nm400\,nm.
  • Memory Aid for Colour Order (from longest wavelength to shortest wavelength): Richard Of York Gave Battle In Vain.
    • Red: ∼700 nm\sim 700\,nm
    • Orange: ∼610 nm\sim 610\,nm
    • Yellow: ∼580 nm\sim 580\,nm
    • Green: ∼525 nm\sim 525\,nm
    • Blue: ∼475 nm\sim 475\,nm
    • Indigo: ∼450 nm\sim 450\,nm
    • Violet: ∼400 nm\sim 400\,nm

Applications and Biological Effects of EM Waves

  • Radio Waves (∼104 m\sim 10^4\,m to 10−1 m10^{-1}\,m)

    • Applications: Radio and television signal transmission, radio telescopes (astronomy), Radio Frequency Identification (RFID) tags, and satellite telecommunications.
    • Biological Effects: Can cause heating effects if absorbed by the body in large amounts, potentially leading to burns and body tissue damage.

  • Microwaves (∼10−1 m\sim 10^{-1}\,m to 10−4 m10^{-4}\,m)

    • Applications: Mobile (cell) phone communication, Global Positioning System (GPS) satellite communication, radar, microwave ovens, and satellite television.
    • Biological Effects: Heating of body tissues.

  • Infra-red Radiation (∼10−4 m\sim 10^{-4}\,m to 10−6 m10^{-6}\,m)

    • Applications: Remote controllers, sensors, intruder alarms, electric heaters, camera auto-focusing, thermal imaging, and infra-red thermometers.
    • Biological Effects: Heating effects.

  • Visible Light (∼700 nm\sim 700\,nm to 400 nm400\,nm)

    • Applications: Photography, optical fibres used in medicine (endoscopy) and telecommunications.

  • Ultra-violet Radiation (∼10−6 m\sim 10^{-6}\,m to 10−8 m10^{-8}\,m)

    • Applications: Sun tanning beds, sterilization of equipment (germicidal lamps), bank note authentication, anti-forgery detection, and disinfecting water.
    • Biological Effects: Ionization of cell molecules leading to cell damage and disruption of cell processes.

  • X-rays (∼10−8 m\sim 10^{-8}\,m to 10−10 m10^{-10}\,m)

    • Applications: Medical radiology (X-ray scanners, CT scans), security screening at airports (scanning luggage), and industrial defect detection (scanning metal parts for manufacturing flaws).
    • Biological Effects: Ionization leading to cell death, mutations, and cancer. Also causes heating effects.

  • Gamma (\gamma) Rays (∼10−10 m\sim 10^{-10}\,m to 10−12 m10^{-12}\,m)

    • Applications: Radiotherapy for cancer treatment (e.g., Gamma Knife), cancer detection, and scanners for detecting manufacturing defects in metal parts.
    • Biological Effects: High energy ionizing radiation that leads to cell death, mutations, and cancer.

Effects of EM Waves on Cells and Tissue

  • Over-exposure Hazards: Excessive exposure to EM waves, particularly high-energy ones, results in hazardous heating and ionizing effects on living cells and tissue.
  • Ionization Process:
    • High-energy EM waves (UV, X-rays, Gamma rays) possess the ability to penetrate the human body.
    • When these waves interact with atoms or molecules in the body, they can cause the atom or molecule to lose an electron.
    • This loss results in the formation of a charged ion.
  • Consequences of Ionization:
    • Charged ions can hinder chemical reactions and disrupt vital cell processes.
    • Large-scale disruption leads to cell death or genetic mutations.
    • Mutations can eventually lead to diseases such as cancer or organ failure.
  • Comparison with Radioactive Particles:
    • The term "ionizing radiation" covers both high-energy EM waves and high-energy particles (alpha and beta particles).
    • High-frequency EM waves are often more harmful than radioactive particles because they can penetrate deeper into the body.

Questions & Discussion

  • Example 1: Which of the following options lists the members of the electromagnetic spectrum in order of increasing wavelength?
    • A: Microwaves, ultraviolet, infrared, X-rays
    • B: X-rays, ultraviolet, infrared, microwaves
    • C: Ultraviolet, infrared, microwaves, X-rays
    • D: Infrared, ultraviolet, microwaves, X-rays
    • Answer: B. (Increasing wavelength means starting with the highest frequency/shortest wavelength: X-rays < Ultraviolet < Infrared < Microwaves).
  • Example 2: Do gamma rays or radio waves have a higher speed when travelling in a vacuum?
    • Answer: Both gamma rays and radio waves have the same speed when travelling in a vacuum, which is 3.0×108 m/s3.0 \times 10^8\,m/s. All electromagnetic waves travel at the same speed in a given medium (in this case, a vacuum).