Electromagnetic Waves. Light

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11.2Electromagnetic Waves. Light


In these revision notes for Electromagnetic Waves. Light, we cover the following key points:

  • What are electromagnetic waves?
  • How are produced the EM waves?
  • What is the propagation speed of EM waves?
  • What is the electromagnetic spectrum?
  • How do we classify the EM waves?
  • How are the EM waves used in technology?
  • Which kind of EM waves should we avoid?
  • What are the penetrating abilities of EM waves?

Electromagnetic Waves. Light Revision Notes

Electromagnetic (in short EM) waves are transverse waves emitted by hot objects when they release some of their energy to the surroundings during the cooling process. These waves have different frequencies, depending on their temperature.

The entirety of EM waves produced by a hot source is otherwise known as Electromagnetic Radiation. Sun is our main source of EM radiation.

EM waves do not need a material medium to propagate. They propagate at a very high speed. In vacuum the speed of EM waves reach a value of c = 300 000 km/s. This is the highest known speed in the universe.

We are unable to see all phenomena occurring around us because of our limited ability to detect all kinds of EM waves since visible light is only a small portion of EM radiation produced by the Sun.

The range of EM waves from the shortest to the longest is known as "electromagnetic spectrum". We can classify EM waves from the least powerful to the most powerful (i.e. from the least energetic to the most energetic) based on two criteria:

a)According to the wavelength. In this classification, the first (the least powerful) waves are those with the longest wavelength because it is frequency the quantity which varies directly with energy.

b)According to the frequency. You can find the corresponding range of frequencies for each category of EM waves based on the equation of waves

c = λ × f

where c = constant = 3 × 108 m/s. In this way, you find the following values:

  1. Radio waves. Since the range of wavelengths for these waves is 10-1 m ≤ λ ≤ 106 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 102 Hz ≤ f ≤ 109 Hz.
  2. Microwaves. Since the range of wavelengths for these waves is 10-5 m ≤ λ ≤ 10-1 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 109 Hz ≤ f ≤ 1013 Hz.
  3. Infrared Radiation. Since the range of wavelengths for these waves is 7 × 10-7 m ≤ λ ≤ 10-5 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 1013 Hz ≤ f ≤ 4.5 × 1014 Hz.
  4. Visible Light. Since the range of wavelengths for these waves is 4 × 10-7 m ≤ λ ≤ 7 × 10-7 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 4.5 × 1014 Hz ≤ f ≤ 7.5 × 1014 Hz.
  5. Ultraviolet radiation. Since the range of wavelengths for these waves is 4 × 10-7 m ≤ λ ≤ 10-10 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 7.5 × 1014 Hz ≤ f ≤ 1018 Hz.
  6. X-rays. Since the range of wavelengths for these waves is 10-10 m ≤ λ ≤ 10-12 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 1018 Hz ≤ f ≤ 1020 Hz.
  7. Gamma rays. Since the range of wavelengths for these waves is 10-12 m ≤ λ ≤ 10-16 m, we obtain for the range of corresponding frequencies (neglecting the constants before the powers of ten): 1020 Hz ≤ f ≤ 1024 Hz.

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