• In vacuum light always travels at the same speed:

  c = 3.0 x 10⁸  m/s .

• Until the middle of the 1800's, the generally accepted theory of light was the particle picture. In this viewpoint, advocated by Newton, light was considered to be a stream of tiny particles. However, in the late 1800's, the particle picture was replaced by the wave theory of light. This was because certain phenomena associated with light, namely refraction, diffraction and interference, could only be explained using the wave picture.
• Visible light is just one particular type of electromagnetic radiation. Other types of electromagnetic radiation include radio waves, infrared radiation (heat), ultraviolet radiation, x-rays and -rays. The different types of radiation are distinguished by their wavelength, or frequency, as shown in Fig. (22.1).
  
    

  Figure 22.1: The Electromagnetic Spectrum

  
  For example blue light has a wavelength (in vacuum) of 434 x 10^{- 9}m = 434 nanometers (nm), while red light has a wavelength of 768 nm. Radiation outside the visible spectrum with wavelengths longer than red light is called infrared, while radiation with wavelength shorter than blue is called ultraviolet. The theory which accurately describes the wave-like properties of all types of electromagnetic radiation is called Maxwell's Theory of Electromagnetism.
• In the early 20th century, experiments revealed that there were some phenomena associated with light that could only be explained by a particle picture. Thus, light as it is now understood, has attributes of both particles and waves. In this Chapter we will deal mainly with the wave attributes of light. The particle-like behaviour of light is described by the modern theory of quantum mechanics, which will be described in Chapter 28.

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