Page 198 - Physics Coursebook 2015 (A level)

Cambridge International AS Level Physics
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Faraday’s studies were extended by James Clerk Maxwell. He produced mathematical equations that predicted that a changing electric or magnetic field would give rise to waves travelling through space. When he calculated the speed of these waves, it turned out to be the known speed of light. He concluded that light is a wave, known as an electromagnetic wave, that can travel through space (including a vacuum) as a disturbance of electric and magnetic fields.
Faraday had unified electricity and magnetism; now Maxwell had unified electromagnetism and light. In the 20th century, Abdus Salam (Figure 13.14) managed to unify electromagnetic forces with the weak nuclear force, responsible for radioactive decay. Physicists continue
to strive to unify the big ideas of physics; you may occasionally hear talk of a theory of everything. This would not truly explain everything, but it would explain all known forces, as well as the existence of the various fundamental particles of matter.
Electromagnetic radiation
By the end of the 19th century, several types of electromagnetic wave had been discovered:
■■ radio waves – these were discovered by Heinrich Hertz when he was investigating electrical sparks
■■ infrared and ultraviolet waves – these lie beyond either end of the visible spectrum
■■ X-rays – these were discovered by Wilhelm Röntgen and were produced when a beam of electrons collided with a metal target such as tungsten
■■ γ-rays – these were discovered by Henri Becquerel when he was investigating radioactive substances.
We now regard all of these types of radiation as parts of the same electromagnetic spectrum, and we know that they can be produced in a variety of different ways.
The speed of light
James Clerk Maxwell showed that the speed c of electromagnetic radiation in a vacuum (free space) was independent of the frequency of the waves. In other words, all types of electromagnetic wave travel at the same speed in a vacuum. In the SI system of units, c has the value:
c = 299792458ms−1
The approximate value for the speed of light in a vacuum (often used in calculations) is 3.0 × 108 m s−1.
The wavelength λ and frequency f of the radiation are related by the equation:
c = fλ
When light travels from a vacuum into a material medium such as glass, its speed decreases but its frequency remains the same, and so we conclude that its wavelength must decrease. We often think of different forms of electromagnetic radiation as being characterised by
their different wavelengths, but it is better to think of their different frequencies as being their fundamental characteristic, since their wavelengths depend on the medium through which they are travelling.
Light waves show the Doppler effect in the same way that sound waves do. So, for example, if an astronomer looks at the light from a distant star which is receding from Earth at speed vs, its wavelength will be increased and its frequency will be decreased. The change in wavelength Δλ is simply given by Δλ/λ = vs/c.
Since longer wavelengths are towards the red end of the visible spectrum, the light from the star will look redder than if it were stationary. This is the origin of the ‘red shift’ which allows astronomers to determine the speed at which stars and galaxies are moving away from us, and which first provided evidence that the Universe is expanding.
Figure 13.13 These telecommunications masts are situated 4500 metres above sea level in Ecuador. They transmit microwaves, a form of electromagnetic radiation, across the mountain range of the Andes.
Figure 13.14 Abdus Salam, the Pakistani physicist, won the 1979 Nobel Prize for Physics for his work on unification of the fundamental forces.