P5.2 - The Electromagnetic Spectrum

• The spectrum of white light is continuous.
• We say that there are different waves in the spectrum, but we mean that there are different bands of frequencies that we call waves.
• Our eyes are sensitive to a narrow range of frequencies called visible light, which is not absorbed by the atmosphere.
• electromagnetic spectrum (increasing freq, decreasing wavelength):

  1. Radio waves.
  2. Microwaves.
  3. Infrared.
  4. Visible light.
  5. Ultraviolet.
  6. X-rays.
  7. Gamma rays.

  
• Electromagnetic waves consist of oscillating electric and magnetic fields. The fields are oscillating in a direction at 90 degrees to the direction of the wave:

  
• All electromagnetic waves travel through a vacuum.
• Sources, such as the sun, emit electromagnetic waves.
• Electromagnetic waves transfer energy from sources to absorbers. Some waves, such as microwaves to satellites, transfer information.
• Wave speed / wave length = frequency
• Your phone communicates with transmitters using microwaves.
• Microwaves are used to communicate with satellites, and for WiFi and Bluetooth systems.
• TV and radio stations uses radio signals by adding sound and picture information to radiowaves.
• Remote controls uses pulses of infrared to communicate with televisions. You can also send information at high speeds using infrared pulses down optical fibres.
• Ships sometimes communicates with flashing visible light and Morse code.
• In a microwave oven, the water and fat in food absorb the microwaves, which heats up the outside of the food. Conductions transfers energy to the middle.
• Infrared radiation cooks food in a grill or oven. Infrared from a radiator can heat you by transferring energy to a thermal store.
• Lasers in CDs, DVDs, and Blu-ray Discs use visible light.
• Ultraviolet radiation helps produce vitamin D, needed for strong bones.
• Forensic scientists use the fact that bodily fluids glow in ultraviolet light, and you can use ultraviolet light to detect forged bank notes.
• Ultraviolet, x-rays, and gamma rays can damage or kill cells, this is useful because:

  • ultraviolet radiation kills bacteria in water.
  • x-rays can kill cancerous cells.
  • gamma rays can kill cancer cells, and also kill bacteria in food.
• Ultraviolet radiation can damage DNA in cells. The cells can go rapidly and cause cancer. If you expose your eyes to ultraviolet, you can develop cataracts, which makes your corneas cloudy.
• X-rays can cause damage to cells and cancer. A radiographer would stand behind a lead screen in another room while the machine is wrong.
• A thermal imaging camera produces an image called a thermogram, that shows different regions of temperatures. The colours are added by a computer. Your skin will emit more infrared radiation if it is hot due to injury or infection.
• Pixels inside a charged-coupled device (CCD) such as your phone camera, absorb infrared and produce an image.
• Thermograms can show problems in blood flow in blood vessels.
• An x-ray can show if your arm is broken. Bones absorb many x-rays but soft tissues such as skin and muscles do not. Photographic film darkens when it absorbs x-rays and shows the details of the internal structure of a person.
• A CCD can also detect x-rays. The colours show differences in intensity due to the different densities of the materials that the x-rays have travelled through. A higher density material absorbs more x-rays.
• Dentists use x-rays to see problems with teeth.
• A computer can use x-rays to make an image look like a slice through your body. This is called a computerised tomography, and it produces CT scans. Scientists can also produce 3D images of organs.
• Gamma rays are used as tracers to treat problems with organs such as kidneys. A doctor injects a patient with a radioactive substance that emits gamma rays, called a tracer. The patients kidney absorbs the tracer.
• Tracers can also be used to find leaks in underground pipes. You will detect more gamma rays from a point above a leak than in the rest of the pipe.