6.2 - Electromagnetic Waves

• Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber
• Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air
• The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency
• Going from long to short wavelength (or from low to high frequency) the groups of waves are:

  A) Radio Waves

  B) Microwaves

  C) Infrared

  D) Visible Light

  E) Ultraviolet

  F) X-rays

  G) Gamma rays

  
• Our eyes only detect visible light and so detect a limited range of electromagnetic waves
• Different substances may do these 4 things to electromagnetic waves in ways that vary with wavelength:

  • Absorb
  • Transmit
  • Refract
  • Reflect

  Multiple of these can happen at the same time
• Some effects, for example refraction, are due to the difference in velocity of the waves in different substances
• Radio waves can be produced by oscillations in electrical circuits, such as a radio or a television.
• When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit
• Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range. Gamma rays originate from changes in the nucleus of an atom.
• Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue. The effects depend on the type of radiation and the size of the dose. Radiation dose (in sieverts) is a measure of the risk of harm resulting from an exposure of the body to the radiation.
• 1000 millisieverts (mSv) = 1 sievert (Sv)
• Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer. X-rays and gamma rays are ionising radiation that can cause the mutation of genes and cancer.
• Electromagnetic waves have many practical applications. For example:

  • Radio waves – television and radio
  • Microwaves – satellite communications, cooking food
  • Infrared – electrical heaters, cooking food, infrared cameras
  • Visible light – fibre optic communications
  • Ultraviolet – energy efficient lamps, sun tanning
  • X-rays and Gamma rays – medical imaging and treatments
• A lens forms an image by refracting light. In a convex lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length. Ray diagrams are used to show the formation of images by convex and concave lenses.
• The image produced by a convex lens can be either real or virtual.
• The image produced by a concave lens is always virtual.
• magnification = image height / object height
• In ray diagrams a convex lens will be represented by:

  
• A concave lens will be represented by:

  
• Each colour within the visible light spectrum has its own narrow band of wavelength and frequency.
• Reflection from a smooth surface in a single direction is called specular reflection. Reflection from a rough surface causes scattering: this is called diffuse reflection.
• Colour filters work by absorbing certain wavelengths (and colour) and transmitting other wavelengths (and colour).
• The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object appears white. If all wavelengths are absorbed the objects appears black.
• Objects that transmit light are either transparent or translucent.
• White light is a mixture of all the colours in the spectrum
• More infrared radiation is emitted by hotter objects
• An oscilloscope lets us see the frequency of the alternating current, allowing us to determine the frequency of the wave.
• Lights can be:

  • Opaque - Reflect/absorb all light
  • Translucent - Transmit and refract light
  • Transparent - Transmit all light
• Concave (aka diverging) lenses make light spread out
• Convex (converging) lenses focus light
• The principal focus (focal point, mostly referred to as F in images) is the point where parallel rays are focused
• The focal length is the distance from the centre of the lens to the principal focus