In Christchurch at the moment, as we head into spring, we’re getting some rather amazing sunrises and sunsets. As I drove to work the other day and noticed the beautiful golden light of early morning, low angle sun and thought about what causes this.
Photographers call these times, “the golden hour” for obvious reasons. But why gold? And why red?
Non-technically we tend to think of light as colours rather than frequency, but it’s helpful to see it as the wave that it is (ignoring the particle/wave duality for now, for any physicists reading)
For the sake of smaller numbers, we tend to use wavelength rather than frequency. Now it's counter intuitive, but longer wavelength means lower frequency, and shorter wavelength means higher frequency. If you're as old as me, you might remember household radios marked LW (low wave; 0.1 to 0.5 MHz), MW (medium wave; 0.5 to 1.6 MHz) and SW (short Wave; 2 to 25 MHz)
Visible light goes from a wavelength of 780 nm (380,000 GHz!)
to 400 nm (i.e. 750,000 GHz!)
i.e. a very, very high frequency!
Because the air molecules are so small, they scatter the very high frequency light more than other frequencies so that the sky appears blue (blue and purple, but human eyes see purple poorly). When the path is longer, because the sun is lower, all the blue light is scattered, leaving the lovely shades of red and yellow we see at sunrise and sunset.
Above visible light in frequency, we get to Ultraviolet light (UV). Here at Homershams, we come across UV light often. For curing UV adhesives, for sterilising water and of course in sunburn.
Now, let's drop below visible light in frequency, and we come to Near Infrared (NIR). NIR is the higher frequency portion of the IR band. It is used for chemical analysis, for remote controls and some night vision. NIR covers the range 800 nm to 2.5 µm (2500 nm).
||As we continue to go lower in frequency (and longer in wavelength) we arrive at Infrared. We use infrared in IR cameras and IR thermometers. Standard IR temperature equipment works in the 7-14 µm band (40,000 GHz to 20,000 GHz), but one of our suppliers, Optris make special IR cameras that work at 1µm and 0.5 µm (30,000 GHz and 100,000 GHz) for seeing through glass, looking AT glass or for hot metal.
|As we continue down, we are starting to get into radio waves. For example, 1 mm wavelength is 300 GHz. This is where things become slightly controversial. The new 5G cellular service will use a couple of frequency bands. Up to 6 GHz; which is perfectly safe and has been well studied, and "millimetre wavelength" bands, which have NOT been studied and may or may not be safe. We just don't know (the wireless safety standards are from 1996. Back then, millimetre wavelength radio hadn't been conceived of.
||Lower still, we come to Wi-Fi. Conventional Wi-Fi (802.11n) operates at 2400 MHz (2.4 GHz) where the newer 802.11ac models operate at 5 GHz and the very new "Wi-Fi 6" or 802.11ax at 6 GHz.
|By the way, your microwave also operates at 2.4 GHz (2450 MHz), but at far higher power! 100 mW (0.1 Watts) in the case of the router and around 1000-watts in the case of the oven.