OCR 2024 A Level Physics B (Advancing Physics) H557/02 Scientific literacy in
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A Level Physics B (Advancing Physics)
H557/02 Scientific literacy in physics
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Seeing beyond the visible
Although undetectable to the human eye, objects radiate in many wavelengths beyond the visible
spectrum. To better understand the nature of astronomical objects it is necessary to detect all
wavelengths of electromagnetic radiation they emit. Since the middle of the twentieth
5 century, scientists and engineers have been designing telescopes to detect these non‑visible
wavelengths.
The atmosphere is transparent to visible light and radio waves as can be seen in Fig. 1. For the
purposes of this article, the height of the atmosphere can be taken to be 140 km.
Fig. 1
140
UV
120
x-rays IR
100
half 80
absorption visible radio
gamma
altitude 60 rays
/ km
40
20
0
wavelength / nm
10 Observations can be made with ground‑based telescopes in the visible and radio wavelengths, but to
observe in other bands of the spectrum requires telescopes either to be as high up as possible, to
minimise the effects of the atmosphere, or launched into space to completely escape the
atmosphere.
A problem of resolution
15 The angular resolution of an optical instrument is the minimum angle that can be resolved due to the
diameter of the collecting lens (in binoculars, say) or mirror (in large optical and infrared telescopes
and dish radio telescopes) as shown in Fig. 2.
1.2λ
The angular resolution in radians is given by the equation angular resolution = where λ is
d
20 the wavelength of the radiation and d is the diameter of the collecting lens or mirror.
The dependence of resolution on wavelength means that radio receivers, such as the Jodrell Bank
telescope, need very large diameter mirrors. The Jodrell Bank dish shown in Fig. 3 has a diameter of
76.2 m. It collects radio signals of wavelength 0.06 m. The minimum angular separation it can resolve
is about four times that of the unaided eye.
The Chandra X‑ray Observatory was carried into space by the Space Shuttle in July 1999. It was
placed in a highly elliptical orbit, varying its distance from Earth from approximately
1.6 × 107 m (at closest approach) to 1.5 × 108 m (at its furthest from Earth). X‑rays are emitted from highly
energetic objects such as matter spiralling into black holes and the Chandra has made important
30 observations of X‑rays emitted from the region around the super‑massive black hole at the centre of our
galaxy.
The James Webb telescope (JWST), the largest telescope yet taken into space, was launched on
December 25th 2021 from Kourou, French Guiana. It is placed about 1.5 × 109 m beyond
35 Earth in line with Earth and the Sun as illustrated in Fig. 4. At this point, known as the L2 point, the
gravitational forces from the Sun and Earth combine to give sufficient force on the telescope so that its
orbital period around the Sun is the same as that of Earth even though it is further away. In fact, the
telescope is not actually on the L2 point but orbits around it as indicated on the diagram. The radius of
the orbit around the L2 point is about 8 × 106 m.
40 Fig. 4 (not to scale)
orbit of JWST
L2 Earth Sun
The JWST is an infrared telescope which detects electromagnetic radiation at wavelengths from 600 nm
to 29 μm.
Fig. 5
primary mirror
secondary mirror
science instruments
multilayer sunshield
It is important to keep the science instruments as cool as possible. To achieve this, the
45 telescope always faces away from Earth and the Sun. The sunshield reflects much of the radiation
and cooling systems keep the operating temperature of the instruments below 50 K.
One of these instruments is a near infrared camera (NIRCam) which can image in two wavelength ranges;
from 600nm to 2300 nm (short wavelength) and 2400nm to 5000nm (long wavelength).
The short wavelength range uses eight individual sensitive surfaces each of 2040 × 2040 pixels.
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