by Shefali Srivastava 12I
Our eyes are sensitive to light which lies in a very small region of the electromagnetic spectrum called ‘visible light’. This light corresponds to a wavelength range of 400-700 nanometres and a colour range of violet through red. The human eye is not capable of seeing waves of wavelengths lesser or more than this range which is why it is called ‘visible’ light. White light is a mixture of all colours whereas black is the total absence of light.
The history behind light
Roger bacon was an English Franciscan friar, philosopher, scientist and scholar of the 13th century who devoted his life to the study of optics and the refraction of light through lenses leading to the development of the spectacles. He said that the passage of light through transparent globes, prisms, crystals etc. was probably related to the production of the rainbow.
Roger Bacon, OFM (c. 1214 – June 1292? scholastic accolade Doctor Mirabilis, meaning “wonderful teacher”), was an English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empirical methods.
In the 17th century Sir Isaac Newton started experimenting with light and he discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. Prior to this people thought that prisms ‘coloured’ light.
He was the first to use the word spectrum (Latin for “appearance” or “apparition”) in this sense in print in 1671 in describing his experiments in Opticks.
Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1726/7) was an English physicist and mathematician (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), first published in 1687, laid the foundations for classical mechanics. Newton made seminal contributions to optics, and he shares credit with Gottfried Leibniz for the development of calculus.
The picture above shows how Newton conducted his experiment. Light entered the prism from the top right and was refracted by the glass. The violet was bent (refracted) more than the yellow and red so the colours separated. Based on the results of this experiment, Newton arranged the colours around the circumference of a circle which proved to be very useful for artists at that time to develop the idea of primary and secondary colours.
Newton’s colour circle, from Opticks of 1704, showing the colours correlated with musical notes. The spectral colours from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton’s circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colours are observed when red and violet light are mixed.
Newton hypothesized light to be made up of “corpuscles” (particles) of different colours, with the different colours of light moving at different speeds in the transparent matter, red light moving more quickly than violet in glass. The result was that red light bent (refracted) less sharply than violet as it passed through the prism, creating a spectrum of colours.
Robert Hooke was an opponent of this theory of colour and produced a scale that went from brilliant red, which was pure light with the least amount of darkness added, to dull blue, the last colour before black which was the complete extinction of light by darkness.
Robert Hooke FRS (28 July [O.S. 18 July] 1635 – 3 March 1703) was an English natural philosopher, architect and polymath.
Modern portrait of Robert Hooke (Rita Greer 2004), based on descriptions by Aubrey and Waller; no contemporary depictions of Hooke are known to survive.
Newton realised that Hooke’s theory was not right. He conducted an experiment where he set up a prism next to his window which projected a beautiful spectrum 22 feet onto the far wall. To prove that the prism does not colour light, he refracted the rainbow of colours back into white light using the prism again.
Newton believed that there were seven colours constituting the spectrum: red, orange, yellow, green, blue, indigo and violet. in consistency with the analogy of the Greek sophists of the age, he chose the seven colours out of the belief that there was a connection between the colours, the musical notes, the days of the week and the then known planets of the solar system.
In the 18th century, Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum (Spektrum) to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colours. Goethe argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them. The spectrum appears only when these edges are close enough to overlap.
Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German writer and statesman. His body of work includes epic and lyric poetry written in a variety of metres and styles; prose and verse dramas; memoirs; an autobiography; literary and aesthetic criticism; treatises on botany, anatomy, and colour; and four novels.
Arthur Schopenhauer (22 February 1788 – 21 September 1860) was a German philosopher best known for his book, The World as Will and Representation.
In the early 19th century the concept of the visible spectrum became more definite as light outside the visible spectrum was discovered and characterized by William Herschel (infrared) and Johann Wilhelm Ritter (ultraviolet), Thomas Young, Thomas Johann Seeback and others. Young Fresnel was the first to discover the wavelengths of the different colours.
The connection between the visible spectrum and colour vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century. Their theory of colour vision correctly proposed that the eye uses three distinct receptors to perceive colour.
Thomas Young (13 June 1773 – 10 May 1829) was an English polymath. Young made notable scientific contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony, and Egyptology.
Thomas Johann Seebeck (9 April 1770 – 10 December 1831) was a physicist who in 1821 discovered the thermoelectric effect.
Hermann Ludwig Ferdinand von Helmholtz (August 31, 1821 – September 8, 1894) was a German physician and physicist who made significant contributions to several widely varied areas of modern science. In physiology and psychology, he is known for his mathematics of the eye, theories of vision, ideas on the visual perception of space, colour vision research, and on the sensation of tone, perception of sound, and empiricism.
Physics principles involved
What is light?
Visible light is the smallest part of the electromagnetic spectrum’. The different colours of the visible spectrum have different wavelengths and carry different amounts of energies as seen in the table below.
The table also shows that as the wavelength of light decreases the energy carried by the wave increases.
Colour and temperature:
As objects grow hotter they radiate energy dominated by shorter wavelengths, changing colour before our eyes. This is because waves with a shorter wavelength have a greater frequency and therefore a higher energy which ties in with the fact that they come from a hotter object (i.e. an object with more energy). A flame on a blow torch from reddish to bluish in colour as it is adjusted to burn hotter. Our sun produces more yellow light than any other colour because its surface temperature is 5500 degrees Celsius if this temperature was lower at 3000 degrees Celsius, it would look reddish and if it was hotter, it would look blue.
As planets and stars move further away from the earth the wavelength of light increases causing the colour of the light to shift to the red end of the spectrum. This is known as the red shift and is one of the most important pieces of evidence for the big bang theory.
How our eyes perceive light:
Cones are the colour-sensing cells of the retina. When light of a given wavelength enters the eye and strikes the cones of the retina, a chemical reaction is activated that results in an electrical impulse being sent along nerves to the brain. It is believed that there are three kinds of cones, each sensitive to its own range of wavelengths within the visible light spectrum. These three kinds of cones are referred to as red cones, green cones, and blue cones because of their respective sensitivity to the wavelengths of light that are associated with red, green and blue. Now suppose that light in the yellow range of wavelengths enters the eye and strikes the retina. Light with these wavelengths would activate both the green and the red cones of the retina. Upon striking the retina, the physiological occurs: electrical messages are sent by both the red and the green cones to the brain. Once received by the brain, the psychological occurs: the brain recognizes that the light has activated both the red and the green cones and somehow interprets this to mean that the object is yellow.
As our eyes can only distinguish red, blue and green wavelengths it could be argued that there are only three wavelengths of visible light. All the colours that we observe are simply due to different combinations of these three colours. It is also amazing to think that we all experience colours differently due to the fact that our brains are different.
Why we see different colours:
Different objects absorb and reflect different wavelengths of light. A leaf appears to be green because it absorbs the wavelengths of all other colours except those for the colour green, which are reflected. As the green wavelength is reflected, we are able to see the leaf as green. Sir Isaac Newton proved using a prism that white light is a mixture of all the other colours. So a white object would reflect wavelengths of all colours in order for us to see it as white light. For an object that is black in colour, it absorbs wavelengths of all colours so that we don’t see anything, in other words we see the colour black. Since black objects absorb all wavelengths of light, they tend to absorb a greater amount of energy than any other coloured object.
Psychological responses to light
Colour is nature’s own powerful signalling system – the universal, nonverbal language. Scientifically, it is the first thing we register when we are assessing anything. For example if there is a fly in our home, we feel more threatened if the fly has yellow stripes rather than just black. In reality if a fly is black or navy blue, we will probably find its bite to be a minor irritation but if it has yellow stripes most of us will recoil. Similarly most people find the colour red related to hunger and the colour green related to positivity. This ‘pre-programmed’ response to some colours is due to the process of natural selection and humans evolved. Many generations ago a random mutation occurred causing some humans to see visible light while others could not. This mutation helped these humans to survive and then it got passed down the generations as the human race expanded. So in a way, the ability to see visible light is like an evolutionary instinct needed to survive.
Modern uses of light (implications of physics – advantages and uses of light)
Light therapy also known as phototherapy uses specific wavelengths of light to help treat medical conditions such as psoriasis and other skin conditions, seasonal affective disorder and jaundice in babies. Light therapy which strikes the retina of the eyes is used to treat circadian rhythm disorders such as delayed sleep phase disorder and can also be used to treat seasonal affective disorder, with some support for its use also with non-seasonal psychiatric disorders.
Telescopes are used for viewing distant objects like stars and planets. Refractive telescopes have an objective lens that collects lots of light from a distant object and brings that light or image to a point or focus. Then an eyepiece lens takes the bright light from the focus and spreads it out in such a way that it takes up a large portion of the retina when the light rays enter our eyes. This makes the small image look bigger.
Another field of use for light is in fibre optic technology. A fibre optic cable is made up of 100 or more incredibly thin strands of glass or plastic known as optical fibres. These cables are used to send data from the source to the receiver. The information is converted to a light signal that is transmitted along the cable by means of total internal reflection and reaches the receiver end of the cable. Using this method to end information is much faster and ensures better quality than any other method of sending information like for example broadband.
Visible wavelengths pass through the “optical window”, the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earth’s atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, and so the midday sky appears blue. The optical window is also referred to as the “visible window” because it overlaps the human visible response spectrum.
Spectroscopy is the study of objects based on the spectrum of colour they emit, absorb or reflect. Spectroscopy is an important investigative tool in astronomy, where scientists use it to analyse the properties of distant objects. Typically, astronomical spectroscopy uses high-dispersion diffraction gratings to observe spectra at very high spectral resolutions. Helium was first detected by analysis of the spectrum of the sun. Chemical elements can be detected in astronomical objects by emission lines and absorption lines.
The shifting of spectral lines can be used to measure the Doppler shift (red shift or blue shift) of distant objects.
Visible light could be considered as one of the reasons why there is life on Earth. Plants use some visible light wavelengths to join carbon dioxide and water together to produce glucose and the very important gas, oxygen.
Too much light can damage the retina in your eye. This can happen when you look at something very bright, such as the Sun. Although the damage can heal, if it’s too bad it’ll be permanent.
 Coffey, Peter (1912). The Science of Logic: An Inquiry Into the Principles of Accurate Thought. Longmans. Page 185
 John C. D. Brand (1995). Lines of light: the sources of dispersive spectroscopy, 1800-1930. CRC Press.. ISBN 978-2-88449-163-1. pp. 30–32.