2.2: The Physics of Light and Colour

Figure 2.2 Plasma globe by Joshua_Willson via Pixabay, licensed under CC0.

Physics is the science that deals with energy and matter. To understand what colour is and how we see it, we need to explore the physical properties of light as a form of energy and how it interacts with matter – the objects we see.

This section explains the following physics-related topics in an introductory way with videos, links, and learning activities to help you to extend your knowledge of the physics of light and colour.

Light: electromagnetic radiation

 

Light is a form of electromagnetic radiation. We often talk about light as waves because light is made from energy that is radiated in a wave pattern, but light also has properties that are like particles. Sometimes light behaves like waves and sometimes it behaves like particles. This may sound confusing – it is!

Watch this video to learn more about the weird physics of light waves and particles (approximately 10 minutes).

Media attribution: Is Light a Wave or a Particle? – Ask a Spaceman, Paul M. Sutter on YouTube

You can also read the article here.

For Colour Theory, we refer to light as waves to describe how light forms different wavelengths that we see as colours. Wavelength is the distance between each wave of light energy – and as shown in Figure 2.3, it is measured in nanometres. A nanometre is one billionth of a metre, 0.000000001 or 10^–9 metres.

Electromagnetic spectrum

Figure 2.3. Electromagnetic spectrum diagram by RMIT, licensed under CC BY-NC 4.0.

The energy of a light wave is similar to that of a particle moving at the speed of light. However, even the smallest particles would not be able to travel at such high speeds in reality.

The term photon is used to represent the smallest quantity of light from any wavelength. It’s also called a light quantum – an idea that began with Albert Einstein. The energy of a photon depends on the wavelength. Ultraviolet, X-rays and Gamma rays have the most energy and the shortest wavelengths. Radio waves, Microwaves and Infrared have the lowest energy and the longest wavelengths. The visible spectrum sits in between these wavelengths and is only a small part of the whole electromagnetic spectrum.

Colour is how we define specific wavelengths in the visible spectrum. For example, red light has wavelengths from approximately 630 to 700 nanometres. Blue light wavelengths are approximately 450 to 500 nanometres. It’s difficult to say where one colour ends and another colour begins. Our definitions of colour by wavelength are based on our perception of colour with human eyes, and our classification of six main colours in the spectrum.

 

Figure 2.4. The visible light spectrum, by RMIT, licensed under CC BY-NC 4.0.

Interesting Fact: Auroras

Figure 2.5. Aurora Borealis (Northern Lights), Alaska. Image attribution: United States Air Force photo by Senior Airman Joshua Strang, Polarlicht 2, marked as public domain, more details on Wikimedia Commons

Auroras like the Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights) are natural phenomena that occur near the North and South Poles of the Earth. Auroras are shimmering patterns of coloured lights that appear in the sky – usually in winter, when the sky is darkest at the poles.

Auroras are nothing like rainbows (which are caused by refraction), they are made from electromagnetic radiation (light) that is emitted when solar wind particles collide with charged particles in the earth’s outer atmosphere – the magnetosphere plasma. Sun events like coronal ejections can cause more of these disturbances.

Auroras can occur on satellites, comets, and on other planets.

Learn more about the physics of light and colour and the history of theories of light.

Optics 1: lenses and ocular devices

What is Optics?

Optics is a branch of Physics – it’s the science that deals with light. Optics explains how light energy behaves and interacts with matter. Some of the earliest recorded experiments in optics were done by ancient Egyptians, Mesopotamians, and Greeks using glass lenses and reflective materials like obsidian.

During the Middle Ages and Enlightenment periods, the significant work of scientists Ibn al-Haythem (Book of Optics c1011) and Sir Isaac Newton (Opticks, 1704) made contributions to the field of optics with their observations of the visible spectrum and how light moves through different substances, as mentioned in Chapter 1.1 History of colour theories.

Other significant developments, such as Galileo’s refracting telescope (c.1609), also had a great impact on the fields of optics and astronomy. Since that time, many technological developments have expanded the field of optics to include both the visible and non-visible spectrum.

We use optics every day in so many aspects of our lives, including using our eyes to see. Optical glasses, mirrors, devices like cameras and digital devices, remote controls, microwave ovens, X-ray machines, microscopes that can show us cells and viruses up close, telescopes that explore the universe, optical fibre that gives us fast broadband internet, the solar panels we use to generate electricity – all these rely on technological innovations in optics.

 

Learning about optics is very useful for colour theory in science and creative fields such as art and design, because it has impacted many of the technologies we use in these fields. This page covers a selection of topics related to optics, many of which are used in creative and scientific practice:

 

Part 1. Selected technologies and innovations that come from the field of optics

• Lenses
• Camera Obscura and Camera Lucida
• Kaleidoscope
• 3D glasses

Lenses

A lens is an optical device usually made from transparent polished glass or plastic that focuses or disperses light depending on its shape. Light passing through the material of the lens is bent – that is, it changes direction depending on the shape and thickness of the lens. The word lens comes from the Latin for lentil because the biconvex (curving outwards on both sides) shape of a lens looks a lot like a lentil. However, not all lenses are biconvex, some are biconcave (curving inwards on both sides) or combinations of convex and concave. Biconvex lenses focus light, whereas biconcave lenses disperse or spread light (see Figure 2.6).

Figure 2.6. Examples of simple convex and concave lenses which can focus or disperse light. Image attribution: Large convex lens and Concave lens images released under the GNU Free Documentation License.

The most common lenses used in optical eyeglasses are meniscus lenses, which are convex-concave lenses that are customised to suit each individual’s vision requirements.

Lenses are in many devices we use every day, such as eyeglasses, magnifying glasses, cameras, telescopes, microscopes, etc.

Learn more about lenses.

Camera Obscura and Camera Lucida

Figure 2.7. Camera Obscura engraved drawing. Image attribution: James Ayscough, 1755, marked as public domain, on Wikimedia Commons

A Camera Obscura (Latin for dark chamber) is a dark space used to create an image projection. It works with a room, tent or box which has a very small hole or lens on one side that projects an image from the outside of the space to the opposite wall on the inside of the space (Figure 2.7). The projected image is upside-down and back-to-front because light bouncing off objects outside the Camera Obscura enters the internal space through the hole or lens at an angle. Light from an object at the top-left of a scene outside the camera, will project onto the bottom-right of the inner wall of the camera.

The concept behind the Camera Obscura led to the development of the photographic camera – which is where the name comes from. Cameras we use today still work on the same principle as this device, which was created many hundreds of years ago.

Some historians speculate that prehistoric cave paintings may have been inspired by accidental Camera Obscura effects from small holes in tents or animal hides. The Camera Obscura was most notably used as an artist’s device during the Renaissance and later periods of realism in painting to assist in the depiction of realistic scenes and accurate perspective in artworks. There are records of its use in ancient Chinese, Arabic and European cultures as a means of telling the time of day and year by the position of the sun inside the camera. It was also used to safely view phenomena like solar eclipses, which can cause eye damage if viewed directly.

Jump to Activities at the bottom of the page to learn how to create your own Camera Obscura Pinhole Camera box with a cardboard box, or turn your entire room into a Camera Obscura.

Figure 2.8. Camera Lucida diagram. Image attribution: Unknown author, Camera Lucida in use drawing small figurine, marked as public domain, on Wikimedia Commons

A Camera Lucida (Latin for light chamber) is a portable device that was developed after the Camera Obscura. The technology may have existed in the 17th century, but it wasn’t patented until 1806. A Camera Lucida creates an optical illusion, where the scene in front of the artist is superimposed onto paper, which is usually on a flat surface like a table. The artist is able to look down into the device, which uses glass, a one-way mirror, or a prism, and see both the image of the scene in front of them, and the drawing on the paper combined together. See Figure 2.8 which shows a diagram of how a Camera Lucida is used.

The artist can then trace the superimposed image (which is an illusion created by the device) and create an accurate representation of the scene with correct proportions and perspective on the paper below. It was much easier to use than the Camera Obscura because it didn’t require a darkened space, and you could use it anywhere in any light conditions. The technology of the Camera Lucida is similar to that used in microscopes that use mirrors to reflect the contents being viewed up into a lens that magnifies the image.

Artists used both of these technologies to assist in the depiction of realistic scenes in paintings and other art forms. Camera Lucida devices were also used by biologists and naturalists to create scientific illustrations.

Artist David Hockney wrote a controversial book where he claimed that many famous Old Master artworks were, in fact, created with optical devices like these. Some art historians were upset by this claim – they thought that Hockney was accusing the Old Masters of ‘cheating’ because of the suggestion that the artists hadn’t recreated the scenes in the paintings by eye alone. This implied that these Old Masters were less technically skilled than previously thought.

Along with physicist Charles M. Falco, Hockney claimed that many of the developments in realistic painting from the time of the early Renaissance (c.1430) were due to the technological development of optics devices. This is known as the Hockney-Falco Thesis. They didn’t consider using optical devices as ‘cheating’, rather they considered it part of the set of tools and devices artists could use in their work. Artists still require skill in painting techniques and visual style to create great masterpieces.

Hockney, David (2006). Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters. Thames and Hudson. ISBN 9780500286388.

This video explores some of Hockney and Falco’s claims:

Media attribution: A History of how Optics Has Helped Artists Create Better Paintings, Inside Science, on YouTube

Kaleidoscope

Figure 2.9. Example of the view looking through a kaleidoscope with coloured glass beads. Image attribution: Albarubescens, Rotational symmetries in designs produced by a kaleidoscopeDSCN2440, on Wikimedia Commons, CC BY-SA 4.0

How does it work?

Kaleidoscopes were first invented as toys in 1816 by Scottish physicist Sir David Brewster, but they can also be useful devices for designers and artists because they create complex and colourful symmetrical patterns. A kaleidoscope is usually made from a tube with angled mirrors inside of it that reflect light waves. Light usually travels in straight lines, but if it hits a shiny surface like a mirror, the light waves bounce back at a similar angle – like a bouncing ball.

Some kaleidoscopes have bits of coloured glass, beads or shiny objects inside them, so when you look into the tube, you see interesting geometric patterns made from these colourful objects and the reflected light from the mirrors (Figure 2.9). Other kaleidoscopes have lenses in them that bring light from outside objects into the kaleidoscope, and the patterns are made from the image you are viewing through the kaleidoscope tube.

If you rotate the kaleidoscope while looking through it, you can change the coloured pattern by shifting the coloured objects around or changing the light that the mirrors are reflecting.

You can now install mobile device apps that simulate a kaleidoscope effect using your camera or coloured geometric patterns, and you can save these images to use in creative projects. You can also get kaleidoscopic lenses that can be placed over a camera lens which add interesting patterns to a photographic image.

Learn more about kaleidoscopes from this text: The kaleidoscope, its history, theory and construction with its application to the fine and useful arts by Sir David Brewster, 1858 on the Internet Archive.

View Activities below to make your own Kaleidoscope.

3D glasses

Why do we have two eyes?

We have evolved to have two eyes (binocular vision), perhaps partly because there is less risk of total blindness if one eye is damaged. But have you ever thought about how we have two eyes but only see one image? The way our eyes work with our brains allows us to see in 3 dimensions (3D), and this is very useful for judging distances between objects and spatial navigation in the world.

 

More on human vision in 2.3 The human eye – how we see colour.

Stereoscopic photography

 

Binocular vision has also led to technological developments that try to replicate our 3D vision using different devices. The earliest experiments with this were with stereoscopic photography, where two photographic images were captured at a small distance from each other (similar to the distance between human eyes). These two photographs were placed in a viewer so that each eye views a different image (Figure 2.10). The brain interprets this information as a single image, and it appears that we are looking at a 3-dimensional scene.

Figure 2.10. Example of a stereograph image depicting someone viewing a stereograph image through a stereoscope. Image attribution: Underwood & Underwood, Stereograph as an educator, marked as public domain, on Wikimedia Commons. Select image to enlarge.

 

Anaglyph 3D – red/blue 3D glasses

A similar principle is used with the Anaglyph 3D technique, which overlays two images of different colours that are slightly apart. Usually, one image is red, and the other is cyan (Figure 2.11). To view this kind of image in 3 dimensions, you need special glasses with different coloured transparent filters for each eye.

The eye looking through the red filter only sees the cyan image (as a black image), and the eye looking through the cyan filter only sees the red image (as a black image). This is because these two colours are opposites and their light waves do not pass through the filter of the opposite colour.

The effect is the same as the stereoscope because the brain interprets these two images as one, which appears as a 3-dimensional scene.

Figure 2.11. Anaglyph image for red-cyan 3D glasses. Image attribution: John Johnston from Glagow, Scotland, Hammer anaglyph (14656149338), on Wikimedia Commons CC BY-SA 2.0

RealD 3D

RealD 3D cinema technology is a polarised 3D system that uses circularly polarised light to create stereoscopic image projection. The principle for the polarising glasses we wear now for viewing 3D cinema is the same principle as the original stereoscope – the film we are viewing is actually two sets of moving images projected with a special projector that alternates right-eye frames and left-eye frames, switching between them 144 times per second

These separate projections for each eye are made with a special polarising screen that switches the polarisation, so each eye only sees the frames that match the polarisation of the glasses. This is what gives us a realistic 3D effect (Figure 2.12).

Figure 2.12. Two pairs of RealD 3D cinema glasses demonstrating polarising effect. Image attribution: Fritz Jörn at English Wikipedia, TwoRealDglassesblackening, marked as public domain, on Wikimedia Common

Activities

Pinhole camera – make your own Camera Obscura box

You can make one with any cardboard box:

Media attribution: How to make a Pinhole Camera, George Eastman Museum on YouTube

or watch this video to learn how to turn your room into a Camera Obscura:

Media attribution: How to Turn a Room into a Camera Obscura, George Eastman Museum, on YouTube

Media attribution: How It’s Made: Kaleidoscopes, Science Channel, on YouTube

Learn more here about the history of optics.

Test yourself: electromagnetic spectrum, lenses and ocular devices

A multiple choice quiz to test your understanding of the electromagnetic spectrum, lenses and ocular devices

(5 minutes or less)

Optics 2: electronic technologies and spectral analysis

Learning about optics is very useful for colour theory in science and creative fields such as art and design, because it has impacted many of the technologies we use in those fields. This page covers a selection of topics related to optics, many of which are used in creative and scientific practice.

Part 2. Selected technologies and innovations that come from the field of optics

• Laser
• Optical fibre
• Photovoltaic cells
• Spectral analysis in astronomy

Laser

How is it different to normal light?

Albert Einstein’s paper On the Quantum Theory of Radiation (1917) theorised the physics of lasers long before they were first created and used in 1960 by American engineer Theodore Harold Maiman. The word LASER is, in fact, an acronym: Light Amplification by Stimulated Emission of Radiation. A laser light is different from a common light devices because it is a coherent light source – which means that a laser can be a very narrow spectrum of light – only one colour (temporal coherence) that can be focused on a very small spot (spatial coherence). This is unlike other lights, which have a broader spectrum and radiate out in multiple directions. Lasers can be very powerful because of this coherence. An example of this is laser cutting technology, which uses a very powerful beam of light to heat very small areas of materials like metal or wood to melting or burning point – which “cuts” the material very precisely.

Figure 2.13. Example of six different types of laser. Image attribution: 彭家杰, LASER, on Wikimedia Commons CC BY-SA 3.0

How does it work?

A laser is made with a tube that is filled with some type of gas, liquid or crystal material. The tube has mirrors at both ends, but one of the mirrors is partially transparent to allow some of the light to pass through it.

Energy is applied to the material in the tube – this could be a powerful light source, or some other source of energy like gamma rays. This energy also causes the material in the tube to produce light (photons) by exciting the atoms inside it. The photons bounce back and forth between the mirrors at the ends of the tube, which creates more photons. Some of these photons escape the tube through the mirror that is partly transparent, and this is what makes a laser beam (Figure 2.13). Light from a laser travels in a straight line and doesn’t get weaker the further it gets from its source, unlike radiant light. This makes it very useful across long distances.

Lasers re used in lots of ways – in industrial and commercial processes such as in cutting tools, barcode scanners, and laser printers – or at home in optical disc players like DVD and Blu-ray, laser pointing tools, etc.

Learn more about lasers here

Optical fibre

Optical fibre is made from very thin, transparent fibres of glass (silica) or plastic that are a tenth of the thickness of a human hair. These fibres are able to transmit light from one end to another very efficiently by bouncing photons along the length of the fibre.The photons don’t escape from the fibre because of a phenomenon called total internal reflection – the photons bounce off the walls at an angle and keep bouncing along until they reach the end of the fibre. Optical fibres are also covered in a different layer of glass cladding that helps keep the light inside the optical fibre tube until it reaches the end (Figure 2.14).Figure 2.14. Optical fibres. Image attribution: BigRiz, Fibreoptic, on Wikimedia Commons, CC BY-SA 3.0

This technology can be used as a form of lighting, and is very useful in lighting small spaces that are difficult to access, such as in medical procedures. You can view internal organs with optical fibre cables without having to cut a body open (Endoscopy). This method of light transmission is also used for telephone calls and to send data across the Internet (Broadband) through specially developed fibre optic cables that can transmit very large quantities of information very quickly by encoding data in pulses of light.

Optical fibre is a better material than traditional copper wire for transmitting data for a number of reasons:

• there is less signal loss than wire – which means fewer amplifiers are needed to carry data long distances
• there is no electromagnetic interference from other cables – which can happen with copper wire
• it has a higher bandwidth – fibre optic cables can carry a lot more data than equivalent copper wires.

Learn more about optical fibre.

Photovoltaic cells – solar electricity generation

Figure 2.15. A silicon photovoltaic cell. Image attribution: Radiotrefoil, Silicon heterojunction solar cell, on Wikimedia Commons CC BY-SA 4.0

A photovoltaic cell (PV cell) (or solar cell) is a special type of semiconductor that converts visible light into direct current (DC) electricity. Some PV cells can also convert infrared or ultraviolet radiation into DC electricity. Solar electric systems like the panels we put on our roofs for electricity and hot water, or the little grey squares that power small devices like garden lights, use PV cells to generate power that can be used immediately or stored in a battery for later use (Figure 2.15).

In simple terms, a PV cell is made from two layers of crystalline silicon – one layer is positively charged, and the other is negatively charged. When photons enter the layers of the cell, this generates a flow of electrons from the atoms in the cell material, creating an electrical charge of DC electricity. For solar panels in our homes, the DC electricity is changed into alternating current (AC) electricity by an inverter device.

Learn more about photovoltaic cells and how they work.

Spectral analysis in astronomy

Figure 2.16. Anglo-Autralian telescope at Siding Springs Observatory. Image attribution: Ahilan Parameswaran, Anglo-Australian telescope at Siding Springs Observatory, on Wikimedia Commons CC BY-SA 3.0

A spectrometer is a very useful device in astronomy. It splits lightwaves into different parts of the spectrum, which can then be measured and analysed to find out what planets, stars, galaxies and other celestial bodies are made of – even at great distances. It can also detect the speed and direction of their movement. This is called spectral analysis.

In 1868 a spectroscope was used to detect helium for the first time by using a prism to spread out light from the sun. An unusual yellow line was observed in this prism which was determined to be helium.

Learn more about spectral analysis from this CSIRO resource.

Learn more here about the history of optics

Why are things different colours?

Why do different objects we see around us appear to be different colours?

Objects themselves do not create colour (unless they have luminescence – see this section in the Chemistry chapter for more details). What they do is reflect and absorb the spectrum of light waves in different amounts – which includes light waves outside of the human visible spectrum.

For example: a tomato appears to be red because it’s absorbing most of the blue, green and other colour light waves (taking those photons into its atoms) and reflecting red light waves out (Figure 2.17). This is what we detect with our eyes and process with our brains. If it’s night-time and there is no light shining on the tomato, it doesn’t appear red because there are no light waves reflecting off it. The tomato is not generating its own colour. The red colour we see in a tomato comes from a chemical called Lycopene which is a bright red pigment found in other red fruits too. Lycopene absorbs certain light waves and reflects red – this is what we see when we look at a tomato in the light.Figure 2.17. Tomato with light waves reflecting red light, by RMIT, licensed under CC BY-NC 4.0 using tomato by Mostafa Elturkey via Pixabay, CC0.

It’s the structure of molecules that objects are made of that determines which light waves are absorbed and which are reflected. The electrons within those molecules have energy states that determine which types of energy they can absorb and reflect.

See Additive and subtractive colour systems in this resource for more information.

The reason that most plants and tree leaves are green is because of a pigment called chlorophyll which is used for photosynthesis. Photosynthesis requires red and blue light waves that the plant transforms into chemical energy. The plant doesn’t need the green light waves so this is reflected out and that’s what our eyes detect (Figure 2.18).Figure 2.18. Plant with light waves reflecting green light by RMIT, licensed under CC BY-NC 4.0 from plant pot by Satheesh Sankaran via Pixabay, CC0.

Some plants have flowers that reflect lightwaves humans can’t see – like ultraviolet (Figure 2.19). However, certain insects and birds can see these light waves as colours and are attracted to the flowers as a source of food, which also pollinates the plants, or spreads their seeds so they can propagate. The plants have evolved to use this light-reflecting ability to assist their development, just like the insects and birds have evolved the ability to see ultraviolet light waves to assist their growth too.

This website Flowers in Ultraviolet by Bjørn Rørslett, has a very detailed list of flower images showing how they look to humans and how they would look if we could see ultraviolet light waves.

Figure 2.19. Mimulus flower photographed in visible light (left) and ultraviolet light (right) showing a dark nectar guide visible to bees but not to humans. Image attribution: Plantsurfer, Mimulus nectar guide UV VIS, on Wikimedia Commons, CC BY-SA 3.0

See How animals see colour in this resource for more information.

Gemstones

Figure 2.20. Gemstones in a range of colours. Image attribution: Arpingstone, Gem.pebbles.800pix, marked as public domain, more details on Wikimedia Commons

Just like all other objects, gemstones absorb and reflect light waves and we perceive the reflected waves as different colours (Figure 2.20). Rubies are red and emeralds are green because of the atomic structures of their elements, including impurities that are also mixed in.

To some extent, the colours of gemstones seem to correspond with where they lie on the periodic table of elements. This corresponds with the behaviour of electrons in their atoms. For example, diamonds are made of carbon – which is on the first level of the periodic table – and these elements are usually colourless. Coloured diamonds are due to impurities and imperfections in the stone such as traces of nitrogen, sulphur or boron.

Learn more about gemstone colour from the Gem Society’s Gemology page.

Test yourself: electronic technologies, spectral analysis and different colours

A multiple choice quiz to test your understanding of electronic technologies, spectral analysis and different colours

(5 minutes or less)