The Electromagnetic Spectrum by year 12 physics students

Visible light

Sarangan Kathirgamanathan 12V


Visible light is part of the electromagnetic spectrum and like all electromagnetic waves it is a transverse waves which means it moves at right angles to the direction of propagation. It does not need a medium to travel. Visible light is the only part of the electromagnetic spectrum that we can see. A typical human eye will respond to wavelengths from about 390 (blue) to 700 nm (red). In terms of frequency, this corresponds to a band in the vicinity of 430–790 THz.

The higher the frequency of the wave, the more energetic the colour which means that the colour blue, which we normally associate with cold, actually has more energy than red, which we normally associate with hot things.


In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal.


Roger Bacon, O.F.M. (c. 1214–1294) was an English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empirical methods.

In the 17th century, Isaac Newton discovered that prisms could separate white light into its constituent colours and reassemble them back to white light and he described the phenomenon in his book Opticks. 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 optics.

Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1727) was an English physicist and mathematician who is widely regarded as one of the most influential scientists of all time and as a key figure in the scientific revolution.

Newton hypothesized that visible light was made up of “corpuscles” (particles) of different colours, with the different colours of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass. The result is that red light bends (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colours.

Newton divided the spectrum into seven named colours: red, orange, yellow, green, blue, indigo, and violet. He chose seven colours out of a belief, derived from the ancient Greek philosophers, of there being a connection between the colours, the musical notes, the known objects in the solar system, and the days of the week.

In fact the human eye can only perceive three colours, red, blue and green. We see all the other colours due to different quantities of stimulation and how our brain interprets this. This means that each one of us will interpret a colour differently.

Perception of colour begins with specialized retinal cells containing pigments with different spectral sensitivities, known as cone cells. In humans, there are three types of cones sensitive to three different spectra, resulting in trichromatic colour vision.


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 polymath.


In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel (infrared) and Johann Wilhelm Ritter (ultraviolet). Thomas Young, who had used two narrow slits to produce an interference pattern from light which showed that light had wave like properties, was the first to measure the wavelengths of different colours of light, in 1802.


Thomas Young (13 June 1773 – 10 May 1829) was an English polymath.


Each colour of the visible spectrum has its own wavelength. Looking at the diagram we can see that red has the longest wavelength and the shortest frequency. Violet has the shortest wavelength and the longest frequency. When all of the colours of the visible spectrum are seen together they will make a white light. The higher the frequency the shorter the wavelength.


We can repeat Newton’s experiment to see the many colours of the visible spectrum by passing visible light through a prism.


As the white light enters the prism, each of the constituent colours will travel at a different speed and refraction occurs. The light changes direction and exits as the various colours of the spectrum. From the diagram you can see that longer the wavelength the less the wave bends.

By using a second prism you can recombine the spectrum to give white light once again.



We use light to see things! As the Sun sends so much light towards our planet, we’ve evolved to make use of those particular wavelengths in order to sense our environment. It allows us to communicate with one another through books, hand signals and video, for example. The use of visible light needs the transmitter and receiver to be in the line of sight. But it is more secure against eavesdroppers than radio waves.

Cameras let us record still pictures and movies, and photography is an important use of visible light.

Visible light can be used to treat various medical conditions such as eczema.

Colour can dramatically affect moods, feelings, and emotions. It is a powerful communication tool and can be used to signal action, influence mood, and cause physiological reactions. Certain colours have been associated with increased blood pressure, increased metabolism, and eyestrain.

Plants, like animals, have evolved to utilize and respond to parts of the electromagnetic spectrum they are embedded in. Plants (and many bacteria) convert the light energy captured from the Sun into chemical energy that can be used to fuel the organism’s activities. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for all aerobic life on Earth (such as humans and animals).

Very bright light can damage our eyes – you should never look directly into the Sun.

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