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Transcript
E X T E N S I O N 3 Chapter 2 The World Communicates: Waves, Energy and Information Transfer Unit 2.3 X-rays, Light, and Radio Waves are all Electromagnetic Waves A Modern View of Electromagnetic Radiation The wave description of light is most useful in describing the way in which electromagnetic disturbances propagate and interact. It can explain the phenomena of reflection, refraction, diffraction and interference. Electromagnetic waves are not usually infinite in length. They have a beginning and end. In some emission processes the accelerating charge may produce electromagnetic waves for a very long time; in others the process may last less than 10–8s. The physical length of a wave train produced is equal to the speed of light multiplied by the emission time. For emission times of 10–8s these lengths are about 3 m. Each wave train of visible light (λ = 500 nm) contains 6 × 106 cycles. We can therefore also think of light as a stream of wave packets. Travelling at the speed of light these wave packets will take about 10–8 s to pass the observer. Each wave train carries an energy, ∆E, which is related to its frequency by ∆E = hf = hc/λ. These wave trains can be likened in their behaviour to particles and are called photons. Figure 1 Light photons are produced by emission from billions of excited atoms. Our eyes average these short wave trains so that we observe continuous emission. Light from a source such as a light globe is composed of emissions from billions of atoms. Each atom emits a short wave train carrying a small amount or quantum of energy. The emission from individual atoms occurs at random. This means that the wave packets are not in step when they arrive at the observer. The eye cannot distinguish individual wave packets or photons. Instead we average the individual wave packets and interpret them as a continuous wave. The penetrating power of electromagnetic radiation We are used to visible light being unable to penetrate solids and metals. However, these materials are transparent to other forms of electromagnetic radiation (for example X-rays). Imagine trying to knock down a brick wall by throwing peas at it. Even if you threw 1000 peas an hour it would be an impossible task. To have any effect on the wall you would need to use bowling balls or objects capable of carrying more energy. The same applies to electromagnetic radiation. It is not the total energy that matters, it is the energy in each wave packet. Electromagnetic waves interact with materials in several different ways depending on the energy of each wave packet or photon. The energy of each wave packet is given by λE = hf = hc/λ where h is Plank’s constant and is 6.63 × 10–34 Js. Energy is usually measured in a unit called the joule. However this is rather a large unit when dealing with atomic physics and it is sometimes easier to talk in terms of energy units of electron volts (eV) rather than joules. 1 electron volt = 1.6 × 10–19 joules Although the energy associated with a wave packet appears small, visible light can kick the electrons in atoms into higher energies. Ultraviolet light with wavelengths of 90 nm has sufficient energy to knock the electrons out of the atom altogether (ionise). Whenever the electromagnetic wave has sufficient energy to either remove electrons or push electrons up into higher energies in atoms in its path, or cause molecules to take up energy through vibration or rotation, then that wave has a high chance of being absorbed by the material. The energy that has been absorbed may reappear at a later stage as light or heat. Table 1 Examples of the photon energy needed for some atomic and subatomic interactions Absorption event Energy (eV) Radiation Knock an electron from the innermost shell of a tungsten atom Kick an electron from the helium atom Kick an electron from the hydrogen atom Excite an electron in a hydrogen atom Be absorbed by silicon Increase the energy of rotation in a diatomic hydrogen molecule Cause vibration in a CO molecule Cause the proton in hydrogen to flip 72k 25 13.6 1.9 1 1 0.001 0.000006 X-rays extreme ultraviolet extreme ultraviolet visible infrared; 2 microns infrared; 2 microns millimetre; mm radio; 21 cm In general it can be said that the penetrating power of high energy electromagnetic radiation such as ultraviolet, X-ray and gamma rays increases with frequency. Gamma rays will penetrate a thin sheet of lead, and X-rays a thin sheet of aluminium, whereas ultraviolet waves will not penetrate a sheet of paper. However, at longer wavelengths we observe more complex effects which are responsible for absorption of radiation. Scattering of light and infrared waves by small grains of dust actually decreases as wavelength increases. Blue light is more scattered than red light. The molecules and dust in the sky scatter the blue light more than the red light of sunlight as it passes through the atmosphere. When we look away from the Sun this scattered light makes the sky its familiar blue. When looking towards the setting or rising sun, the loss of the blue wavelengths makes the Sun appear red. Electromagnetic wave characteristics Light has some properties that are wave-like. Energy is transferred from the source of the light to the observer as electromagnetic waves. These waves move out from the source in all directions as a changing electric field (accompanied by a changing magnetic field). Electromagnetic radiation from a point source can be thought of as series of spherical wave crests, each expanding at the velocity of light, 3 × 108 m s–1. As these wave crests move past charged particles, the particles experience a variable force and begin to oscillate as they take energy from the electromagnetic wave. At any given time we can join up all of the wave crests, which started from the source at the same time, as a wavefront. At any instance the wavefront represents a narrow region in which all of the particles are vibrating in phase. We can represent the direction in which the wavefront is moving by arrows or rays, drawn at right angles to the wavefront. If we follow one set of arrows from the source outward we may join them to represent a ray. Wavelength and frequency Electromagnetic waves, like all waves, can be described by their wavelength, period and frequency. • The wavelength (symbol λ measured in metres) is the distance between adjacent wavefronts. If we take a photograph of charged particles in the path of the wave, the wavelength is the distance between charges that are oscillating in step (e.g. charges at the extreme of their vibrational motion). • The period (symbol T, measured in seconds) is the time that elapses between successive crests of the wave passing an observer. If you imagine you are a small charged particle in the path of an electromagnetic wave this is the time it takes to move through one complete cycle. • The frequency (symbol f, measured in cycles per second or Hz) is the number of complete wavelengths passing an observer in one second. Frequency and period are different ways of looking at the same thing. They are related by: frequency = f= 1 period 1 T