What is a spectrum?
Spreading a beam of light by wavelength reveals the light it contains and how it is distributed. This description is called a spectrum. Wavelength is the distance between neighbouring wave crests.
A prism separates visible sunlight into a continuous band from violet to red. Wavelengths are shorter at the violet end and longer at the red end. Beyond both ends lies light our eyes cannot see. For these concepts, see NASA's explanation of light and spectra.
Three common wavelength bands in sunlight
- Ultraviolet: Wavelengths shorter than visible violet light, invisible to our eyes. Everyday UV protection concerns this part of the spectrum.
- Visible light: The wavelength range our eyes can detect. The colours, sky, and scenery we see relate to this band.
- Infrared: Wavelengths longer than visible red light, also invisible to our eyes. Infrared relates to radiation from objects and warming.
These names refer to different wavelength bands; a spectrum describes their distribution. Before sunlight reaches the ground, it undergoes absorption, scattering, and other processes in the atmosphere.
Why distinguish brightness, warmth, and UV?
The brightness our eyes perceive, the warmth we feel, and environmental UV levels answer different questions. ARPANSA's solar-radiation FAQ explains that skin cannot directly feel UV; cool weather can still have strong UV.
For outdoor protection, check local UV information. For heat, consider temperature and relevant warnings. For everyday light, consider visible light and its timing. One sensation cannot replace every assessment.
Continue with an everyday question
For colour, read Visible light and the sky; for invisible bands, read UV basics and Infrared and warming.
Observing rainbows, object colours, and shadows is a way to explore sunlight without looking directly at the sun.
Use the spectrum to choose information
Distinguishing bands improves a practical decision: brightness concerns visible light, heating calls for temperature and heat conditions, and protection calls for local UVI. Combine the information for your activity rather than choose only one.
For reading in a park at lunch, choose shade and rest options, enough light to read and protection suited to local UV. Move if it is too hot. Observe surrounding colours without looking at the sun.
A coloured band diagram explains concepts, not today’s measured energy distribution. Check wavelength direction and bands, then whether it describes composition, mechanisms or actual conditions. Connect it with a question rather than memorise terms alone.