9.1 Electromagnetic spectrum

Electromagnetic spectrum

Electromagnetic radiation allows us to observe the world around us. Some materials and objects emit electromagnetic radiation and some reflect the electromagnetic radiation emitted by other objects (such as the Sun, a light bulb or a fire). When electromagnetic radiation comes from an object (whether the radiation is emitted or reflected by the object) and enters the eye, we see that object. Everything you see around you either emits or reflects electromagnetic radiation or both.

Electromagnetic radiation comes in a wide range of frequencies (or wavelengths) and the frequencies of radiation the human eye is sensitive to is only a very small part of it. The collection of all possible frequencies of electromagnetic radiation is called the electromagnetic spectrum, which (for convenience) is divided into sections (such as radio, microwave, infrared, visible, ultraviolet, X-rays and gamma-rays).

The electromagnetic spectrum is continuous (has no gaps) and infinite. In practice, we can only use electromagnetic radiation with wavelengths between (very roughly) ${10}^{-14}\mathrm{m}$ (very high energy gamma rays) and ${10}^{15}\mathrm{m}$ (very long wavelength radio waves) due to technological limitations in the detectors used to receive electromagnetic radiation and in the devices used to produce or emit electromagnetic radiation.

The various frequencies (or wavelengths) of electromagnetic radiation coming from a particular object or material depends on how the object or material reflects and/or emits electromagnetic radiation.

Wave nature of em radiation

1. List one source of electromagnetic waves. Hint: consider the spectrum diagram and look at the names we give to different wavelengths.
   
2. Explain how an EM wave propagates, with the aid of a diagram.
   
3. What is the speed of light? What symbol is used to refer to the speed of light? Does the speed of light change?
   
4. Do EM waves need a medium to travel through?
   

[link] lists the wavelength- and frequency ranges of the divisions of the electromagnetic spectrum.

Examples of some uses of electromagnetic waves are shown in [link] .

Em radiation

1. Arrange the following types of EM radiation in order of increasing frequency: infrared, X-rays, ultraviolet, visible, gamma.
   
2. Calculate the frequency of an EM wave with a wavelength of 400 nm.
   
3. Give an example of the use of each type of EM radiation, i.e. gamma rays, X-rays, ultraviolet light, visible light, infrared, microwave and radio and TV waves.
   

The particle nature of electromagnetic radiation

When we talk of electromagnetic radiation as a particle, we refer to photons, which are packets of energy. The energy of the photon is related to the wavelength of electromagnetic radiation according to:

Planck's constant

Planck's constant is a physical constant named after Max Planck.

$h=6,626\times {10}^{-34}$ J $·$ s

The energy of a photon can be calculated using the formula: $E=hf$ or $E=h\frac{c}{\lambda }$ . Where E is the energy of the photon in joules (J), h is planck's constant, c is the speed of light, f is the frequency in hertz (Hz) and $\lambda$ is the wavelength in metres (m).

Exercise - particle nature of em waves

1. How is the energy of a photon related to its frequency and wavelength?
   
2. Calculate the energy of a photon of EM radiation with a frequency of ${10}^{12}$  Hz.
   
3. Determine the energy of a photon of EM radiation with a wavelength of 600 nm.