Reflecting on light
By Doug & Dilys Griffiths - 06/04/2009
Picture it, an early Spring morning; the sun, just above the horizon, casts long, bright, white rays across the dew-covered grass. Can you believe your eyes when you see the jewels glinting there? C’mon, you know full well there are no jewels there - it’s only reflections on the dew-drops. Hang on, do you really know how those glints of red, yellow, green, blue and violet have appeared there? “Of course,” you say,” it’s just the sun’s rays”. You are quite right, but there are a lot of things happening inside those dewdrops which you may care to think about.
Let’s take a sliver off a sunray and aim it at a dewdrop. In it goes until it reaches the back of the dew-drop, where there appears to be a mirror-like surface. This bounces our sunray back out and it bursts into all the colours of the rainbow. Some trick, eh?
Here’s another trick of Light. Next time you have a glass of fruit juice, beer, wine or water, put a drinking straw into it and watch the straw develop a bend when you know, full well, that straw is straight. Hold the straw upright then and, bingo, it looks straight!
It’s all an illusion, of course, and scientists have explained it all away by measurements. They use a tool of measurement, which they refer to as the “refractive index”.
Going back to our straw in its glass - under the influence of refraction - how much it appears to bend depends upon the angle it makes with the surface of the liquid. The refractive index varies also with the type of liquid involved.. Our straw will show a bigger bend in beer than in water, because beer is more dense than water. The truth is, the straw does not actually bend – it is the rays of light that allow us to see a bend and produces the illusion.
Any ray of light passing through transparent material, will always bend when it travels from one material to the next. It will bend when passing from air into a solid lump of glass. This is why your spectacles work. Light bends, on entering your spec’s, and is arranged to focus on your retina.
Mirages are well known illusions, thanks to the bends of Light! In the heat of the desert, the air above us develops many different temperatures. The lowest air layer is a darn sight hotter than the final layer at the top - and the higher the layer, the more dense the air. This causes problems for Light as it bends each time it passes through the air layers of different temperatures and densities. When we look towards the sandy horizon, we actually see the sky. Rays of light enter the layers of air and are bent. They then reflect from the air/sand surface. We see the reflected image of the sky on the ground. This image will shimmer in the constantly moving air; resulting in an illusion of water. The unfortunate parched traveller, on his last legs, will crawl toward the “water”, only to see it move away in front of him!
In the Arctic the ground air is cooler than at higher regions, which reverses the light bend. An iceberg, beyond the horizon , which you would expect to be out of sight, can actually appear to be there on the horizon. This illusion was used, in old Musical Hall Days. The audience were shown the image of a girl, standing behind a specially shaped glass screen door– the girl was not really behind the door but to the side of it out of normal sight although well illuminated. When the ‘magician’ opened the door – the girl had disappeared! This illusion is really because we all expect light to travel in straight lines – it does mostly but when it doesn’t it causes confusion.
What makes colourful rainbows? When sunlight falls on a cloud or patch of mist we have a lovely rainbow - providing you’ve got your back to the sun, of course. This colourful display is again due to the bending of light. We are told white light is made up of a range of colours from red to violet .Red light bends least while violet light bends most. When white light enters any transparent medium, it splits up into its component colours, red, yellow, green, blue, violet.
A mist in the sky provides an ideal arrangement for splitting up the colours. A light ray enters a raindrop and each of its component colours becomes separated internally by the bending process. These colours are partially reflected from the back surface of the raindrops and are seen as a rainbow.
Our range of vision tends to be roughly circular and the shape of the rainbow forms part of that circle. A second, weaker rainbow is one that has developed from two internal reflections of light within the raindrop, producing a less intense image but with the colours reversed - violet to red.
When we age it is important to think more about colours. The pupils of our eyes reduce in size, most noticeably from about 40 years onwards and light colours will be pleasing and safer than dark ones. It will also help to consider the positioning of lights. Glare is to be avoided by the use of soft lighting in place of a single bright light.
Here are a few helpful colour hints for this years Spring decorating
That bright red in the paint pot may not be so glorious on the wall. This is because red reflects back only about 6 –16% of the light impinging on it. Deep blue reflects about 11% , dark green about 11%, turquoise 15% and very light grey 31%.
To get a real light and airy feel to a room, cream and primrose reflect about 76% while magnolia reflects about 72%. White comes top of the list, reflecting out 85% although too much of it might give a clinical appearance.
Colours and their reflections have a great influence on our lives. Think of Christmas lights, interior decorations in the home, paintings, pictures, clothes, make-up. The intensity of colour TV as compared to the old black and white. Oh, thank heavens for Light!