London, March 24 (IANS): Obeying the speed limits is the only way to avoid being caught on speed cameras.

Those who wish to do otherwise would have to drive at 119 million miles per hour - one sixth of the speed of light - to make the number plates invisible to speed cameras, research shows.

Given that the fastest man-made object - the Helios Probe - is only capable of travelling at one five thousandth of the speed of light, we clearly have some way to go before we would ever be able to see this effect in action, the researchers said.

“It would be great fun if we could become invisible but unfortunately, our current technology is not advanced enough to do this,” said Dan Worthy, a student at University of Leicester.

The calculation of the researchers is based on the Doppler Effect - the physical effect where the frequencies of light or sound waves emanating from an object increase or decrease when it moves towards or away from you.

This effect is at work when you hear an ambulance - its siren would appear to lower in pitch as it drives past you.

With light, this process creates 'red shift' where the frequency of light from an object travelling away from the observer is shifted towards the red end of the colour spectrum.

The faster an object is travelling, the bigger the shift in frequency.

This means it would theoretically be possible for the light from a fast-moving car number plate to be shifted out of the frequency range which speed cameras are able to detect.

The group assumed the camera would be able to detect a similar frequency range as the human eye - roughly 400 terahertz at the 'red' end of the spectrum to 790 terahertz at the violet end.

Car number plates are generally yellow - which has a frequency of around 515 terahertz.

To work out the necessary speed of the car for the number plate to be "shifted" past the 400 terahertz boundary of the visible spectrum, the group utilised the equation used by astronomers to calculate how fast stars are travelling away from the Earth.

The study appeared in the Journal of Physics Special Topics.