Photoacoustic effect a rotating slotted wheel. He noticed that the resulting acoustic signal was dependent on the type of the material and correctly reasoned that the effect was caused by the absorbed light energy, which subsequently heats the sample. Later Bell showed that materials exposed to the non-visible (ultra-violet and infra-red) portions of the solar spectrum can also produce sounds and invented a device, which he called "spectrophone", to apply this effect for spectral identification of materials. Bell himself and later John Tyndall and Wilhelm Röntgen extended these experiments, demonstrating the same effect in liquids and gases. However, the results were too
Invisible ink brightly, in a variety of colors, under UV light. This makes them suitable for use in readmission such as hand stamping. There some invisible ink types that can only be invisible when applied to certain types of surfaces, but are still visible on others. Some vendors now offer invisible ink for use in computer inkjet printers. Such inks are usually visible under ultraviolet light. Typical uses include printing information on business forms for use by the form processor, without cluttering up the visible contents of the form. For example, some United States Postal Service mail sorting stations use UV-visible ink