Bob Evans, of Southland Boy's High School, asks :-

How come a video camera sees the invisible signals from a video remote unit?

William Tobin, an astronomer at the University of Canterbury, who uses charged couple detectors in astronomical studies, responded.

The reason is akin to why sheepdogs hear the cockie's dog-whistle but we humans hear nothing.

When you press its buttons, your TV remote sends out coded series of flashes. The flashes are picked up by a hidden receptor on the TV called a photodiode. Electronic circuitry decodes the flashes and changes the channel, etc.

For the TV photodiode to pick up a reliable signal, the LED flashes must outshine the surrounding lighting. Such bright flashes would irritate us, so the remote uses a special lamp that emits brightly at an infrared wavelength of 950 nanometres, well beyond the reddest wavelength of about 700 nanometres to which human eyes are respond. (A nanometer is one millionth of a millimetre.)

Light with wavelengths up to about 1,100 nanometres produces electrical signals when it falls on silicon, which is why the receptor photodiode in the TV is made of silicon. The sensor in your video camera is also made of silicon. It is called a charge-coupled device, or CCD. The silicon in it is arranged into a two-dimensional array of several hundred thousand individual light-sensitive elements, or pixels, from which the picture signal is made up. The CCD acts as a piece of electronic movie film, but one which is sensitive out to 1,100 nanometers. A video camera therefore naturally picks up the flashes from a TV remote at 950 nanometers.

In a nutshell, a dog-whistle is too shrill for our ears but not for dogs' ears, while a TV remote is too bass for our eyes but not for the video camera.