This article is motivated by the physics exam papers in Singapore :
- the 2016 A level Paper 3 Question 8, and
- the 2020 O level Paper 2 Question 6
which have questions asking for wavelengths of certain colours.
It is unusual for physics exams to have questions asking for specific numbers. For example, can you imagine a physics exam question asking you for the value of g, the acceleration due to gravity? Or the value of c, the speed of light?
You might think that this is not possible, since the value of g is always given in the exam. But what if they decide not to provide the value of g any more? Then you may think that it is ok. You already know that it is 9.81 m/s2. But what if they decide to leave out the values of other constants like charge of electron, mass of electron, Planck's constant, Boltzmann constant, ...
It is going to be a nightmare.
This nightmare would probably not happen. But something close is happening. In recent years, O and A level exams papers have started asking for the wavelengths of colours of light. Like the two examples I have given in the first paragraph above.
Just to be clear - yes, I do mean that O and A level students in Singapore have to memorise the wavelengths of the colours of light now. Like this :
Wait - isn't it supposed to be 7 colours of the rainbow? That was what I heard and learnt growing up through the schools in Singapore. The "missing" one on the list above is the colour indigo, which is supposed to be between blue and violet. We were taught to memorise them by pronoucing the word that is made by putting the initial letters together : ROYGBIV.
ROYGBIV : roy - jee - beef
When I googled for the colours, however, I found that the list of colours given on different websites may not always agree, and indigo is often not included.
So much for lessons in school. Welcome to the real world !
Colours of Light in Physics and Everyday Life
Light, one of the fundamental aspects of nature, plays a central role in our understanding of the universe. It enables us to see the world around us, gives life its vibrancy, and has been the subject of human fascination for centuries. The colours of light, in particular, are an essential part of both the physics of light and its impact on our daily experiences. From the way light interacts with matter to its use in technology, art, and communication, colours of light are integral to our perception of reality.
This article explores the science of colours in light, examining how they arise from the interactions of light with matter, how we perceive them, and their significance in various aspects of life. We will delve into the physics behind the colours of light and explore how they manifest in our everyday environment.
1. Understanding Light: The Spectrum of Colours
Light is a form of electromagnetic radiation, and like all electromagnetic waves, it consists of oscillating electric and magnetic fields. The key characteristic that differentiates one type of light from another is its wavelength, or the distance between successive peaks of the electromagnetic wave. Light travels through space at a constant speed of approximately 299,792 kilometers per second (the speed of light in a vacuum).
The human eye is capable of detecting only a small portion of the electromagnetic spectrum, called the visible spectrum. This spectrum encompasses light waves with wavelengths roughly between 380 nanometers (nm) and 750 nm. The visible spectrum is divided into a range of colours, each corresponding to a specific wavelength of light. The primary colours of light, as seen in a rainbow, are red, orange, yellow, green, blue, indigo, and violet, arranged in increasing order of wavelength.
• Red light has the longest wavelength, approximately 620–750 nm.
• Orange light falls between 590–620 nm.
• Yellow light occupies the 570–590 nm range.
• Green light spans from 495–570 nm.
• Blue light lies between 450–495 nm.
• Indigo light is found in the 425–450 nm range. (may be included as part of violet, like in some exam questions!)
• Violet light has the shortest wavelength, around 380–425 nm.
Beyond the visible spectrum, there are other forms of electromagnetic radiation, such as ultraviolet (UV) light, which has shorter wavelengths than violet, and infrared (IR) light, which has longer wavelengths than red. However, the visible spectrum is where light truly reveals its colourful diversity to us.
2. The Physics Behind Colours of Light
The colours of light are determined by their wavelengths, but there is more to the story when it comes to their interactions with matter. When light strikes an object, several things can happen: the light can be reflected, refracted, transmitted, or absorbed. These interactions explain how we see different colours and why some objects appear coloured while others do not.
• Reflection and Absorption: When light hits an object, the object may absorb some wavelengths and reflect others. The colour we see is determined by the wavelengths of light that are reflected. For example, a red apple absorbs most wavelengths of light except for red, which it reflects. Our eyes then perceive the apple as red.
• Transmission: Some materials, like clear glass or water, allow light to pass through them. In these cases, the material may selectively absorb certain wavelengths while transmitting others. This can result in coloured light filtering through, as seen in stained glass windows.
• Refraction: When light passes from one medium to another, such as from air into water, it bends. This bending of light, called refraction, can cause different colours to spread out. This is how a prism works to separate light into its constituent colours, creating a spectrum.
The wavelength of light affects its energy as well. Light is made up of packets of energy called photons. Shorter wavelengths (such as violet or blue) have higher photon energy, while longer wavelengths (like red) have lower photon energy. This difference in energy has important consequences, particularly in fields like astronomy and chemistry.
3. The Role of the Eye in Colour Perception
The human eye is an amazing organ designed to detect light and enable us to see the world. Within the eye, the retina contains specialised cells called photoreceptors that are responsible for capturing light. These cells come in two types: rods and cones.
• Rods are responsible for vision in low-light conditions, but they cannot detect colour.
• Cones, on the other hand, are sensitive to different wavelengths of light, allowing us to perceive colours. There are three types of cones, each sensitive to different parts of the visible spectrum:
◦ S-cones are most sensitive to short wavelengths (blue light),
◦ M-cones are sensitive to medium wavelengths (green light),
◦ L-cones are sensitive to long wavelengths (red light).
When light enters the eye, the cones send electrical signals to the brain that correspond to the wavelengths of light they detect. The brain processes these signals to create the perception of colour. If the cones detect a mixture of wavelengths, the brain perceives a colour that is a combination of those wavelengths. This is why, for example, when red and blue light combine, we perceive the colour purple.
4. Colour Mixing and the Theory of Additive and Subtractive Colour
The colours we perceive can result from the mixing of different wavelengths of light. This can be understood through two primary models of colour mixing: additive and subtractive colour theory.
Additive Colour Mixing
Additive colour mixing occurs when different colours of light are combined. This is how colours appear on screens, such as televisions, computer monitors, or projectors. The three primary colours in additive mixing are red, green, and blue (RGB). When these colours are mixed in different proportions, they create a broad range of colours. For example:
• Red + Green = Yellow
• Red + Blue = Magenta
• Green + Blue = Cyan
• Red + Green + Blue = White
This additive colour model is based on the fact that light is a form of energy, and when different wavelengths of light combine, the energy is added together. This is why screens use red, green, and blue light to create the full spectrum of colours.
Subtractive Colour Mixing
Subtractive colour mixing, on the other hand, is typically used in painting, printing, and other pigment-based media. When pigments or dyes are mixed, they absorb (subtract) certain wavelengths of light and reflect others. The primary colours for subtractive mixing are cyan, magenta, and yellow (CMY). When these pigments are combined, they subtract different portions of the spectrum:
• Cyan + Magenta = Blue
• Cyan + Yellow = Green
• Magenta + Yellow = Red
• Cyan + Magenta + Yellow = Black (or a very dark brown)
Subtractive mixing works by removing (or absorbing) certain wavelengths of light. For instance, a yellow pigment absorbs blue light and reflects red and green light, giving the impression of yellow.
5. The Significance of Colours in Everyday Life
In daily life, colours of light are not just an abstract concept. They have profound implications for our emotional states, health, and even productivity. Here are some ways in which the colours of light impact our daily existence:
Aesthetic and Psychological Impact of Colours
Colours have a significant psychological effect on individuals. Different colours can evoke specific emotions, which is why colour plays a crucial role in art, design, and advertising. For instance:
• Red often evokes feelings of passion, excitement, or urgency, and is used in warning signs or promotional materials to attract attention.
• Blue tends to have a calming effect and is commonly used in spaces designed for relaxation, such as bedrooms or spas.
• Yellow is associated with happiness, energy, and optimism, making it a popular choice in kitchens and creative workspaces.
• Green is often linked with nature, tranquility, and balance, making it a popular choice for hospitals or eco-friendly designs.
Colours and Health
The colours of light can also influence our health. Natural sunlight, which contains a full spectrum of light, is essential for the production of vitamin D in the skin. Exposure to sunlight also helps regulate our circadian rhythms, the internal body clock that controls sleep patterns. The artificial lighting in homes and offices can also impact mood and productivity. Blue light, which is prevalent in screens and LED lighting, can disrupt sleep by interfering with the production of the sleep hormone melatonin. On the other hand, warm light, like that from incandescent bulbs, is less disruptive to sleep.
Colours in Technology and Communication
In technology, the manipulation of colours of light has far-reaching applications. From digital screens to fibre-optic communication, understanding and controlling the colours of light are key to many modern devices. For instance, LEDs (light-emitting diodes) use the principles of additive colour mixing to create vivid displays on everything from smartphones to televisions. In fibre-optic communications, light is used to transmit data across long distances. The ability to modulate the colour of light allows for different channels of data to be transmitted simultaneously, greatly increasing the capacity of communication networks.
6. Conclusion
The colours of light are much more than just an aesthetic experience. They are deeply embedded in the physics of light, our biological makeup, and the way we interact with the world. The science behind light colours, from their wavelengths to the way our eyes perceive them, forms the foundation for a wide array of applications in technology, art, and medicine. Understanding light and colour not only enhances our perception of the world but also opens doors to innovations that improve our daily lives. Whether in the gentle glow of a sunset or the complex circuitry of a computer, the colours of light are an essential part of the fabric of life.
You can learn these concepts and more at Dr Hock's maths and physics tuition.