Let There Be Light…

A photograph showing the sun reflecting over the waters at Loch Kathrine through a curtain of trees. Low sun. Image: NaturPhilosophieFiat Lux…

Light.  Most of us take it for granted during the day.  And at night, we have learned to domesticate it.  Light, the natural agent that stimulates our sense of sight and makes things around us visible. 

After graduating from Cambridge Trinity College, in 1665, and developing calculus, a major part of the physicist’s toolbox, Isaac Newtonuses a glass prism to demonstrate how white light is in fact composed of a mixture of colours.

An illustration by Granger, depicting a young Sir Isaac Newton experimenting with the passage of light through a prism. Two diagrams showing the white light experiment. The captions read: "A prism breaks white light into its component colours or spectrum. From violet to red: 400 nm, 500 nm, 600 nm, 700 nm." "First prism breaks light into its spectrum. Screen lets only one colour pass through. Second prism only changes light's direction."

‘Visible’ Light

Light, or ‘visible’ light, is an electromagnetic radiation that ranges from about 380 to 740 nanometres in wavelength.  At least, that is how it goes for visible light.  But there is more to the story of light…

Light is an electromagnetic wave.  And there is light that we can see, and light that we cannot see.


The Optical Window

Visible wavelengths pass through the ‘optical window’, a particular region of the electromagnetic spectrum where the wavelengths pass through the Earth’s atmosphere all the way to the ground largely unaffected.

A photograph showing clear skies and quiet waters at Loch Lomond, Scotland. Image: NaturPhilosophie
The optical window is the optical portion of the electromagnetic spectrum that makes it through the atmosphere all the way to the ground.

The window is said to be ‘optical’ because the wavelengths we can see are all in this range, which stretches from around 300 nanometres (1 nm = 10-9 m) (UV-β) at the short end, up into the range that the human eye can use, that is roughly 400-700 nanometres, and goes through the visual infrared up to around 1100 nanometres, in the near-infrared range.

There are also infrared and radio windows that transmit some infrared and radio waves, respectively.


Why The Sky Is Blue

A cloudless sky appears blue because clean air scatters short wavelengths much more readily than it does with longer wavelengths.  Since blue light is at the short end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light.  The human eye perceives blue when looking towards parts of the sky other than the sun – a monochromatic shade of unsaturated blue light with wavelengths of around 474-476 nanometres.


What We Cannot See

The visible spectrum (380-740 nanometres) is merely defined in terms of human colour perception.  However, many species can see light with frequencies outside this range.

Bees and other insects can see in the ultraviolet (UV) range.  Birds can also see in the UV range (300-400 nanometres), and some have markings on their plumage that are only visible in this range.

A close-up photograph showing the eye-shaped marking on a peacock feather seen under two different wavelengths of light.

From 1864, James Clerk Maxwell manages to derive an accurate value for the speed of light in a vacuum, a constant c=2.998\times10^{8}ms^{-1}, using his famous four equations.

From this point on, the subjects of electricity, magnetism and optics are unified into one branch of physics.  Again, Maxwell predicts the existence of an even wider set of electromagnetic waves.

 A diagram explaining the wavelength ranges and energies of the broad electromagnetic spectrum, and the comparatively small window comprising what we understand to be visible light and most of us perceive as colours in everyday life.

What Else Is Out There?

1800, William Herschel discovers the ‘heat rays’.  And the following year, Johann Wilhelm Ritter stumbles upon the ‘oxidising rays’ at the other end of the invisible spectrum.  Though the terms remain popular throughout the 19th century, they are ultimately dropped in favour of ‘infrared’ and ‘ultraviolet’ radiations.

1888, Hertz‘s experiments confirm the existence of electromagnetic waves with wavelengths much greater than those of visible light: the radio waves, that revolutionised communications.

1895, Röntgen discovers another set of electromagnetic waves with wavelengths much smaller than those of visible light: the X-rays, that revolutionised medicine.

A wordle or "word cloud" gathering different types of radiation and their applied uses in health and medicine.


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