Page 199 - Physics Coursebook 2015 (A level)

Chapter 13: Waves
The observed frequency of light from a moving source can be calculated using the same equation as for sound,
f = f × c but there is an important condition. The obs (c±vs)
speed of the source vs must be small compared to the speed of light c. For speeds approaching c, the equation must be altered to take account of the theory of relativity.
QUESTIONS
11 Red light of wavelength 700 nm in a vacuum travels into glass, where its speed decreases to 2.0 × 108 m s−1. Determine:
a the frequency of the light in a vacuum
b its frequency and wavelength in the glass.
12 An astronomer observes light from a distant star. A particular line in its spectrum has a wavelength of 550 nm. When measured in the laboratory, the same spectral line has a wavelength of 535 nm. Determine:
a the change in wavelength of the spectral line
b the speed of the star
c the direction of movement of the star (towards or away from the observer).
(Speed of light in free space = 3.0 × 108 m s−1)
Orders of magnitude
Table 13.3 shows the approximate ranges of wavelengths in a vacuum of the principal bands which make up the electromagnetic spectrum. This information is shown as a diagram in Figure 13.15.
Here are some points to note:
■■ There are no clear divisions between the different ranges or bands in the spectrum. The divisions shown in Table 13.3 are somewhat arbitrary.
■■ The naming of subdivisions is also arbitrary. For example, microwaves are sometimes regarded as a subdivision of radio waves.
■■ The ranges of X-rays and γ-rays overlap. The distinction is that X-rays are produced when electrons decelerate rapidly
or when they hit a target metal at high speeds. γ-rays are produced by nuclear reactions such as radioactive decay. There is no difference whatsoever in the radiation between an X-ray and a γ-ray of wavelength, say, 10−11 m.
Radiation
Wavelength range / m
radio waves
microwaves
infrared
visible
ultraviolet
X-rays
γ-rays
Table 13.3
spectrum.
>106 to 10−1
10−1 to 10−3
10−3 to 7 × 10−7
7 × 10−7 (red) to 4 × 10−7 (violet)
4 × 10−7 to 10−8
10−8 to 10−13
10−10 to 10−16
Wavelengths (in a vacuum) of the electromagnetic
X-rays
10–10
visible
ultraviolet
10–8 10–6 Wavelength / m
a 200kHz b 100 MHz
radio waves 102 104
c 5×1014Hz d 1018 Hz
Figure 13.15 Wavelengths of the electromagnetic spectrum. The boundaries between some regions are fuzzy.
microwaves
QUESTIONS
13 Copy Table 13.3. Add a third column showing the range of frequencies of each type of radiation.
14 Study Table 13.3 and answer the questions.
a Which type of radiation has the narrowest
range of wavelengths?
b Which has the second narrowest range?
c What is the range of wavelengths of microwaves, in millimetres?
d What is the range of wavelengths of visible light, in nanometres?
e What is the frequency range of visible light?
15 For each of the following wavelengths measured in a vacuum, state the type of electromagnetic radiation to which it corresponds.
a 1km c 5000nm b 3cm d 500nm
e 50nm f 10−12m
16 For each of the following frequencies, state the type of electromagnetic radiation to which it corresponds.
γ-rays 10–14 10–12
infrared
10–4 10–2 1
106
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