Chapter 2
The Nature of Light
Abstract Our understanding of the universe at large comes almost exclusively from
our understanding of light and how quantum mechanics explains the physical features
we see in the spectra of astronomical objects. Astronomers measure the brightness
of astronomical objects in terms of flux and magnitude. The use of coloured filters
enables us to examine specific characteristics of the object being observed.
2.1 Introduction
The physics of light is central to our understanding of astrophysics. For the
vast bulk of history, astronomy was confined to the so-called visible spectrum,
that wavelength region that can be seen by the human eye. In the year 1800,
William Herschel—famous for the discovery of Uranus, the first new planetary
discovery since prehistory—attempted to measure the temperatures of individual
colours in the rainbow. Those efforts led to an inadvertent discovery. Herschel’s
thermometer continued to detect warmth even after it was moved outside the visible
spectrum. Herschel had accidentally discovered the existence of infrared light.
Infrared (IR) astronomy began to develop as a quantitative science in the
mid-1800s (the first actual object detected in the IR was a cow, as it happens).
Radio astronomy began as a discipline in the 1940s, having been made possible
by advances in electronics and electrical technology. Subsequent to this, ultraviolet
(UV), X-ray, millimetre and submillimetre observations also became possible. Many
of these depend on space-based or balloon-borne instruments, due to the opacity of
Earth’s atmosphere at many wavelengths. (This is also why observatories tend to be
built at high altitude—the more of the air you’re above, the better. Water vapour in
particular can bedevil infrared astronomy, due to its deep absorption features, and
mid-infrared astronomy is effectively impossible at sea level.)
Classical physics presents light as a travelling wave consisting of an oscillating
electric field with a perpendicularly oscillating magnetic field. Both the electric and
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_2
9
The Nature of Light
Abstract Our understanding of the universe at large comes almost exclusively from
our understanding of light and how quantum mechanics explains the physical features
we see in the spectra of astronomical objects. Astronomers measure the brightness
of astronomical objects in terms of flux and magnitude. The use of coloured filters
enables us to examine specific characteristics of the object being observed.
2.1 Introduction
The physics of light is central to our understanding of astrophysics. For the
vast bulk of history, astronomy was confined to the so-called visible spectrum,
that wavelength region that can be seen by the human eye. In the year 1800,
William Herschel—famous for the discovery of Uranus, the first new planetary
discovery since prehistory—attempted to measure the temperatures of individual
colours in the rainbow. Those efforts led to an inadvertent discovery. Herschel’s
thermometer continued to detect warmth even after it was moved outside the visible
spectrum. Herschel had accidentally discovered the existence of infrared light.
Infrared (IR) astronomy began to develop as a quantitative science in the
mid-1800s (the first actual object detected in the IR was a cow, as it happens).
Radio astronomy began as a discipline in the 1940s, having been made possible
by advances in electronics and electrical technology. Subsequent to this, ultraviolet
(UV), X-ray, millimetre and submillimetre observations also became possible. Many
of these depend on space-based or balloon-borne instruments, due to the opacity of
Earth’s atmosphere at many wavelengths. (This is also why observatories tend to be
built at high altitude—the more of the air you’re above, the better. Water vapour in
particular can bedevil infrared astronomy, due to its deep absorption features, and
mid-infrared astronomy is effectively impossible at sea level.)
Classical physics presents light as a travelling wave consisting of an oscillating
electric field with a perpendicularly oscillating magnetic field. Both the electric and
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_2
9