Wavelength and Colour
Written by Abin Alex and Anu Joseph.
Abstract
A practical guide to wavelength and colour: photons and the speed of light, frequency and wavelength, how we see objects, why objects have colour and why the sky is blue and the sunset red, with three diagrams and one table.
Keywords: wavelength of light, frequency, visible spectrum, speed of light, photon, why objects have colour, blue sky, red sunset, scattering of light
What is a photon and how fast does light travel?
Short answer. A photon is a particle of electromagnetic radiation that travels at about 300,000 kilometres a second.
- Light, also called visible light, is electromagnetic radiation that our eyes can see. It is made of elementary particles called photons, and photons show the nature of particles and waves at the same time (Hunter and Fuqua, 1997, p. 10).
- A photon is a particle that represents electromagnetic radiation. It has no charge and no resting mass, and it travels at the speed of light.
- Light travels at 299,792,458 metres per second, nearly 300,000 kilometres a second. The circumference of the earth is about 40,000 kilometres, so in one second light could travel around the earth seven and a half times.
- Light travels in straight lines until it hits an object, and shadows are the evidence. Shadows are not totally dark, because some light reflects off the objects around them.
Shine a torch across a dusty room and watch the beam. You see the dust, which reflects the light.
Light is so fast that on any set it arrives at the lens as if instantly. Only the direction and quality matter.
Cham tipYou see a beam only when something in it reflects light.
Cham taskCalculate how many times light could circle the earth in one second.
Check yourself: How fast does light travel?
About 300,000 kilometres a second.
How do wavelength and frequency give light colour?
Short answer. Short wavelengths carry more energy and look violet or blue. Long wavelengths carry less energy and look red.
- Frequency is the number of waves passing a given point per second. Wavelength is the distance between identical points on two adjacent waves. Speed equals wavelength multiplied by frequency.
- Light with short wavelengths has more energy and light with longer wavelengths has less. Our eyes recognise the different energies as colour. Violet or blue light has the shortest wavelength that strikes the eye and red has the longest (Figure 1). The colours in between, orange, yellow and green, have medium wavelengths, and this spread of colours and wavelengths is the spectrum.
- Visible light has a wavelength of 400 to 700 nanometres. Wavelengths below 400 nm or above 700 nm are invisible to human eyes (Hunter and Fuqua, 1997, p. 13). As the wavelength decreases the energy increases, so ultraviolet has more energy than infrared, x-rays have more than ultraviolet, and gamma rays have the most. Microwaves have less energy than infrared, and radio waves have the least. Table 1 lists the order.
| Radiation | Wavelength | Energy |
|---|---|---|
| Radio waves | Longest | Lowest |
| Microwaves | Long | Low |
| Infrared | Above 700 nm | Lower than visible |
| Visible light, red to violet | 700 to 400 nm | Medium |
| Ultraviolet | Below 400 nm | Higher than visible |
| X-rays | Very short | High |
| Gamma rays | Shortest | Highest |
Table 1. Radiation by wavelength and energy.
Look at a rainbow or a prism. Find the end with the longest wavelength.
Warm and cool colour grading is a move along the same wavelength scale.
Cham tipShorter wavelength means more energy.
Cham taskOrder red, green, blue and ultraviolet by wavelength and by energy.
Check yourself: Which visible colour has the longest wavelength?
Red.
How do we see an object?
Short answer. Luminous objects make their own light. Everything else is seen by the light it reflects.
- We cannot see objects in the dark. Objects can be divided into two kinds. Luminous objects, such as the sun, fluorescent bulbs and burning candles, create their own light. Non-luminous objects, such as tables, chairs, walls and paper, cannot create light on their own.
- We see a non-luminous object because it sends light from a luminous source back to our eyes or the camera (Figure 2). The moon is the same: it shines with reflected sunlight. Since most things make no light of their own, everything we see of them is light they reflect (Hunter and Fuqua, 1997, p. 24).
Switch off all the lights and wait. Then switch on one lamp and notice which objects you see first.
A practical light in a scene is a luminous object, and it needs to be exposed differently from the things it lights.
Cham tipEvery picture is a picture of reflected light.
Cham taskList ten luminous and ten non-luminous objects in your house.
Check yourself: Which objects create their own light?
Luminous objects.
Why do objects have colour?
Short answer. An object absorbs some wavelengths and reflects the rest. What it reflects is its colour.
- Colour comes from which wavelengths a surface keeps and which it sends back. A red apple soaks up most of the blue and green in white light and returns mostly red (Grey, 2004, pp. 9-10). This selective absorption is one of the things that sets the colour of any subject.
- An object we see as light in tone reflects efficiently, so much of the light that hits it bounces back into the eyes or the camera. An object we see as dark absorbs more light than it reflects.
- When the wavelengths of all colours are present in equal quantities in the light from a source, whether the sun or a bulb, we see the light as white. When the amounts are unequal, the light is warmer if it has more yellow or red and cooler if it has more blue (Grey, 2004, pp. 10-11). Photographers call light white when it has a roughly even mix of red, green and blue.
Look at a red apple under a red light and under a green light. Note what happens to its colour.
Wardrobe and set colours are chosen knowing that a colour reflects only the wavelengths the light contains.
Cham tipA surface can only reflect colours that the light contains.
Cham taskExplain why a red apple looks black under pure blue light.
Check yourself: Why is an apple red?
It absorbs blue and green and reflects red.
Why is the sky blue and the sunset red?
Short answer. Air scatters short blue waves more than long red waves, and a low sun sends its light through more air.
- Sunlight is a combination of all the colours of the rainbow. When sunlight is refracted and scattered in rain, the white light splits into its colours and a rainbow forms. Red has the longest wavelength and blue the shortest.
- Sunlight reaching the atmosphere is scattered in all directions by the gases and particles of the air. Blue light is scattered more than the other colours because it travels in short waves, so we often see a blue sky (Child and Galer, 2008, p. 16). The amount of scattering is proportional to one divided by the fourth power of the wavelength.
- Near the horizon the sky fades to a lighter blue or white. Light from low in the sky has passed through even more air, and the blue has been scattered and rescattered in many directions. The surface of the earth also reflects and scatters the light, and this mixes the colours again, so we see more white and less blue.
- At sunset the sun is low and its light passes through much more of the atmosphere. More of the blue is scattered away, and the reds and yellows pass straight through to the eye, so the sky near the sun looks red (Child and Galer, 2008, p. 21) (Figure 3).
Photograph the sky overhead at noon and the sky near the sun at sunset. Compare the blue and the red.
Golden hour and blue hour are the same physics. The sun is low, and the air takes out the blue.
Cham tipLow sun, long path, warmer light.
Cham taskExplain in three lines why the sunset is red and the sky is blue.
Check yourself: Which colour is scattered most by the air?
Blue.
Questions beginners ask
How fast does light travel?
About 300,000 kilometres a second, or 299,792,458 metres per second.
What is the wavelength of visible light?
400 to 700 nanometres, from violet or blue at the short end to red at the long end.
Why do objects have colour?
They absorb some wavelengths and reflect the rest, and the reflected wavelengths give the colour.
Why is the sky blue?
Air molecules scatter blue light more than red because blue has a shorter wavelength.
Why is the sunset red?
The low sun sends light through more air, which scatters the blue away and leaves red and yellow.
References
- Child, J. and Galer, M. (2008) Photographic lighting: essential skills. 4th edn. Oxford: Focal Press.
- Grey, C. (2004) Master lighting guide for portrait photographers. Buffalo, NY: Amherst Media.
- Hunter, F. and Fuqua, P. (1997) Light: science and magic: an introduction to photographic lighting. 2nd edn. Boston: Focal Press.
Further reading
- Hirsch, R. (2008) Light and lens: photography in the digital age. Burlington, MA: Focal Press.
- Langford, M. and Bilissi, E. (2011) Langford's advanced photography. 8th edn. Oxford: Focal Press.
How to cite this guide. Alex, A., & Joseph, A. (2026). Wavelength and colour: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/wavelength-and-colour/
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