The Electromagnetic Spectrum
Light, Sound and Waves

Seeing red (and beyond)

Stories from Physics for 11-14 14-16 IOP RESOURCES

  • Human vision can be sensitive to infrared radiation. A study has shown that some observers perceive infrared laser radiation of wavelength 1060 nm as a pale greenish colour. The effect occurs due to the simultaneous absorption of two infrared photons by the photopigments in the eye.
  • There is some evidence that aphakic patients (those whose lenses have been removed) have an increased sensitivity to ultraviolet radiation. Following lens removal surgery, some patients have reported being able to perceive ultraviolet radiation. A letter in the journal Nature described the case of one such patient who perceived the world as brighter following the surgery, in particular, blues stood out more intensely.
  • The artist Claude Monet underwent surgery on his right eye to remove a cataract that resulted in the removal of the lens. Following the surgery, Monet produced paintings by covering one eye. An analysis of the canvasses he produced when he was using his left eye suggests warm colours predominated, whereas in work produced using his right eye, cooler colours are dominant.
  • Two groups of snakes (including boa constrictors, rattlesnakes and pythons) have the ability to sense infrared radiation. The snakes have free nerve endings which can respond to temperature differences as small as 0.001°C.
  • Though it had been proposed that polar bear fur fibres act like fibre optics, this claim has turned out to be unsupported by evidence. Polar bear fur is, however, an excellent insulator and opaque to infrared, hence the creatures are almost invisible on infrared cameras.
  • We perceive visible light when a photon hits the retina, triggering a chemical reaction which is reversible. This reaction involves breaking chemical bonds. The energy required to do this ranges from 0.01 eV (Van der Waals bonds) to 5 eV (covalent bonds). We can see visible light as the energy of visible photons runs from 1.6 eV (red) to 3.4 eV (violet) within the energy range required to break these bonds. This means it is difficult for the human eye to detect radiations of  significantly higher frequencies than visible light via this mechanism. Ultraviolet photons can damage human tissues and, though most people have a mechanism to correct such damage, sufferers from the rare genetic condition xeroderma pigmentosum lack the process and must avoid exposure to ultraviolet radiation. The damage caused by ultraviolet radiation occurs through the release of ‘pinball protons’ from the base pairs in DNA that can cause a chain of lesions as they pass through the genetic material.


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