Practical Activity for 11-14
Some digital cameras are sensitive to wavelengths of light that we cannot see with our eyes. This simple demonstration shows that an ordinary camera on a mobile phone can “see” the invisible infrared light emitted by a television (or other) remote control.
Apparatus and Materials
- Infared remote control
- Camera phone
Health & Safety and Technical Notes
- Expensive cameras often contain an infrared blocking filter that stops infrared light from reaching the CCD (charge coupled device) that forms the light sensitive part of the camera. However, many cheap cameras and cameras on mobile phones do not contain these filters.
- Light from a remote control can be detected by many cameras, and it usually appears as white light rather than red.
- Colour CCDs that form the light sensitive part of a camera work by using an array of coloured filters over the pixels (picture elements) to allow some pixels to see red, some green and others blue. In the camera software the imaging system is then told which pixels saw which colour and a colour image can be produced. If the camera sees something that is white, it sees equal amounts of red, green and blue. Infrared light can penetrate these coloured filters and so all the pixels (whether they are filtered red, green or blue) pick up the infrared. The software then interprets that information as being white.
- Apart from getting students to think about why you would want to block infrared light in a camera (blocking it improves picture quality), you could also ask why infrared light appears as white in the images.
- Many webcams can be easily modified to remove the infrared filter (if they have one) and you can carry out a range of experiments with these.