Most of us are familiar with ‘Richard Of York Gave Battle In Vain’ to help us recall the order of colours in a rainbow. We also probably know that there are colours beyond this visible spectrum, being ultraviolet and infra-red. We can feel the warmth of infra-red and we know that ultraviolet can give us sunburn but our eyes cannot detect these wavelengths. Digital and smartphone cameras can actually detect ultraviolet and infra-red light but manufacturers cover the sensors with special coatings in these devices to filter out the ‘extra' wavelengths. What if you take your camera apart (definitely not recommended!) and remove these filters? What does the camera then see? Better still, what can you ‘see’ if you include a special filter that blocks out all of the visible and ultra-violet light and just allow the infra-red light to get through? The answer to that last question is shown in the images.
Look carefully at the image of the Soar Bridge and compare it with the cover of your Autumn 2015 edition of ‘Barrow Voice’. The immediate differences are that vegetation shows up as white and blue skies are very dark. That picture and the one showing Jerusalem Island were taken last September, though it looks as if there has been a heavy frost.
The wavelengths of the light our eyes detect lie within the region of 400 to 700 nanometres. A nanometre is one millionth of a millimetre - very small indeed! (Each one of our red blood cells is around 7000 nanometres in diameter.) The camera and special filter used to take these pictures allows wavelengths through in the range of 750 to 900 nanometres, which is referred to as ‘near’ infra-red. So, seeing light at these wavelengths is as extrasensory to us as animals, such as cats and dogs, hearing very high frequency sounds beyond our range of hearing. Having said that, it is well known that bees and many other insects can see ultraviolet light, though they can’t see anything at all at the red end of the visible spectrum.
For the record, the wavelengths of the colours we can see are: blue - 475nm; green - 510nm, yellow - 570nm and red - 650nm