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
Ultraviolet (Light My Way) it. "Ultraviolet" played a featured role during the encores of the group's 1992–1993 Zoo TV Tour, 2009–2011 U2 360° Tour, and the Joshua Tree Tour 2017. "Ultraviolet (Light My Way)" began as two different demos, one variously called "Ultraviolet" and "69" (which eventually evolved into the B-side "Lady with the Spinning Head") and an alternately arranged demo called "Light My Way". Over the course of the recording sessions, U2 added various overdubs to the song, but producer Brian Eno believed these additions negatively impacted the track. Eno aided the group in editing down the song, and he explained his assistance