What you didn't know about cars and light

Our eyes are receptors of light in the visible spectrum, but it's our brains that decipher the images conveyed to each eye's retina by light.

But sometimes our brains can be fooled. Sometimes even the visual media we use to capture images provide us with an incomplete 'picture'.

And among the various everyday items that are frequently viewed – directly or through a video medium – the humble motor car is one of the more surprising.

Here are five examples of ways light transforms unexpectedly during interplay with an object in the form of a car.

Perhaps you've seen old-time westerns, in which the stagecoach's wheels appear to be spinning backwards, relative to the direction of the travel. In the 21st century we don't watch westerns anymore, we watch slow-motion video of fast cars with daytime running lights (DRLs) that appear to blink rather than remain constantly illuminated.

So what's happening?

Normally, our brains smooth out 'strobing' through what is known as 'persistence of vision', as long as the frame rate – the number of distinct images displayed in one second – is higher than 12. Some people can distinguish strobing at a frame rate of 20 FPS (frames per second) or higher. Even modern digital video cameras film action at a rate of 'just' 24 frames per second, but Sir Peter Jackson filmed 'The Hobbit' trilogy at 48FPS.

Why the LEDs (light-emitting diodes) of the DRLs pulse – and often at a rate that's slightly out of synch with the camera's refresh rate – is likely a result of Pulse Width Modulation, which helps the LEDs stay cool and keeps the candlepower low enough to avoid blinding other drivers, but still visible from a distance. There's no after-glow with LEDs as there is with halogen and xenon lights. When the power is cut (briefly), there's no gradual diminution, it goes from on to off immediately. Pulse Width Modulation and the video refresh rate combined leave our persistence of vision all at sea coping with LEDs on video.

Projector-beam (or ellipsoidal) headlights refract light through the thick lens that redirects light from a reflector behind the light source. Whenever light is refracted through a different medium, like the lens, it can be seen at different angles in different wave lengths. The light in these different wavelengths is interpreted by the human eye and brain as light of a different (non-white) colour. This 'dispersion' is often seen when white light passes through a prism. Think of the album cover for Pink Floyd's 'Dark Side of the Moon' as an example.

If you happen to watch a vehicle driving towards you and notice that the headlight beam changes colour intermittently, the vehicle is likely riding over bumps. As it does so, the refracted ray of light from the headlight changes its angle of attack through the lens and hits the retina of your eye in a different wavelength – one moment blue, the next yellow.

Metallic paints that 'flop' (change colour viewed from different angles) are an increasingly common sight on the road, but more often still at custom car shows. The metal flakes within the paint can be oriented in a distinct way when the paint is applied so that they reflect light in a different colour.

Colour, as we perceive it, is light (visible electro-magnetic radiation) in differing wavelengths. The orientation of the metallic flakes in metallic paint can alter the wavelength of the reflected light when seen from various angles, or if the light strikes the bodywork from a certain angle.

It should come as no surprise that yellow paint, as in the Ford colour Citric Acid (from BA-series Falcon vintage), can flop to green and back again to yellow. Yellow and green are very close in terms of wavelength and frequency interval. The wavelength for green ranges from 520 to 560 nanometres and the frequency interval is 540 to 580 terahertz. Yellow is right next door, at 560 to 590 nanometres and 510 to 540 terahertz.

Other 'neighbouring' colours seen in flopping paint include blue and purple, red and orange.

When light reflects off a surface, it becomes 'horizontally polarised'. The sort of reflective surface that creates this type of light can include car bonnets and dashboards.

Unpolarised light is not restricted to a single plane. If you look directly at the sun, a flame or an incandescent light bulb your eyes are registering unpolarised light.

Polarised sunglasses are specifically designed to block 'horizontal light waves', and that unfortunately includes light from the head-up display, which is typically reflected on the windscreen glass from a light source in the top of the dashboard.

You're looking at a white car parked outside with the sun beating down. Your eyes rove over every square centimetre of its surface, taking in the delightful canvas – all the curves, swage lines and other details.

You take a photo with your smartphone and find that all that detail is lost. Everything has 'burnt out' under the glare of the sun, or parts of the car in shadow are too dark to make out clearly from the picture – parts such as the air intake grilles in the front bumper, or the tyre markings, for instance.

What's happening is that your brain is adjusting the dilation of your pupils as your eyes focus on highlights and lowlights of the car. Your eyes are being operated (by the brain) like a video camera – automatically adjusting the 'aperture' to see all the details. The brain retains a composite image that most cameras can't hope to match.

A smartphone (or any automatic camera) will adjust the aperture setting based on an 'average' of the light entering the lens across the whole frame. Any car in a high-contrast colour (contrasting with the background) such as white or black is notoriously hard to photograph on a sunny day. It falls to the photographer to work out the important details to see in the picture and adjust the camera's aperture setting accordingly – or take multiple photos using different aperture settings and then superimpose them together.

If the photographer wants to be able to read the tyre size in the photo, but doesn't care about the sculpture lines on a white car, the aperture must be larger to let more light enter the camera so the dark points of the colour, like the tyres are exposed fully.