Lightness and Brightness and Other Confusions

is length measured in metres, where a metre is given a highly precise and
fixed definition. 57
One important driving force in the development of physics and other natural
sciences has been the urge to find causal connection between physically detectable
forces and human experiences. The famous anecdote about Newton
and the falling apple does not, of course, imply that Newton was the first
person to consciously experience that apples fall. Instead, he started from this
commonly shared experience and managed to formulate a theory about why
apples fall. More recent physical theory, like Einstein’s theory of relativity,
lacks this direct connection to sense experience, but still aims at detecting the
causal relationships inherent in the physical world and consequently making
use of such findings.
The theories of natural science add to understanding the material causes
behind our sense experiences, but they do not distinguish between the experience
and its material cause. One of the most striking examples of this is
the term light, which is used not only for a visual experience but also for the
energy radiation found to be the material precondition for this experience.
Such wordings as the speed of light or light-year exemplify how light has
come to be used for things that cannot be visually experienced. Gradually a
complex physically based terminology on colour and light has been created.
This terminology often uses the same words as those used to describe experience,
but with strictly different definitions.
Physical aspects of ‘light’
From a physical point of view light is defined as electromagnetic radiation, a
form of energy that – depending on the viewpoint – can be described as
waves with different wavelengths or as a shower of small energy packages
called photons. The energy content is expressed in units of Joule (J) or kilowatt-hour
(kWh) and the wavelength in nanometres (nm), one nanometre
being one millionth of a millimetre. The total radiation energy can be measured
by radiometric instruments. A spectrometer is an instrument for dividing
the radiation into different wavelengths, and a spectroradiometer measures
energy in narrow bands of wavelength.
The terms light year and speed of light refer to all such radiation, irrespective
of wavelength. Most often, however, the term light is limited to wavelengths
The metre is one of the bases for the international system of units (SI). Its definition
has, however, been altered several times. Initially the metre was defined as a certain
fraction of the earth’s perimeter, subsequently with reference to a physical metre standard
and after that referring to the spectrum of a specific chemical element. Today the
metre is defined with reference to the path travelled by light in a vacuum.
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