An iridescent substance appears to change color based upon the angle of observation, and I assume this is because the substance reflects different frequencies of light at different angles.

My question is whether there are substances that actually alter the frequency of the incoming light?

I suppose this could look the same as an ordinary iridescent surface, but this is mechanically different than a substance that simply divides an incoming white light by frequency. This would instead actually change the frequency of the incoming light.

  • $\begingroup$ Iridescence you refer to is based on thin film filters which do have a high dependency on angle of observation. Flouresence is where incoming blue could be changed to green yellow or red. $\endgroup$ – PhysicsDave Jul 11 at 18:20
  • $\begingroup$ Thank you, that's helpful. What is believed to happen to the energy lost in the process? That is, a shift from blue down to another lower frequency color necessarily implies a shift down in energy. What do people assume happens to the energy given up by the incoming light? $\endgroup$ – Feynmanfan85 Jul 12 at 0:15
  • $\begingroup$ Wavelengths are either reflected ( to your eye ) or transmitted and absorbed ( like the inner stone of an opal or the water of an oily puddle). $\endgroup$ – PhysicsDave Jul 12 at 1:12
  • $\begingroup$ Right, but in this case, part of the energy is being absorbed, and part of it is being reflected, and if I understand you correctly, the portion that is being reflected is also monochromatic, but a different color than the incident light. My question is what's the mechanic that causes this? Or is it not understood, and rather, we just know that certain surfaces have this property? $\endgroup$ – Feynmanfan85 Jul 12 at 3:35
  • $\begingroup$ The incident light is most commonly made of many (all color) wavelengths (sunlight, lamp, etc) and the reflected portion can be more than one color ( remember that blue + red = purple for example). The opal for example can be made of tens or hundreds of layers, all of which in addition may change thickness over the surface. The concept is thin film interference, check wikipedia, there are older classical "interference explanations" and more modern but less published photon wave function based explanations. $\endgroup$ – PhysicsDave Jul 13 at 1:03

Only nonlinear optical phenomenon are capable of changing the color of incident light. Most materials do not exhibit this property. Information about materials that do can be found here. This site also provides links to various tupes of frequency conversion phenomena (frequency doubling, parametric oscillators, 4-wave mixing, etc).

All of these phenomena are based on the interaction of light with light, something that does not happen in a vacuum, but is possible in the presences of a transparent medium. To be observable these devices require high intensity sources of light (usually lasers).

  • $\begingroup$ Thank you, do you know of any examples of materials with this property? I'm interested in this topic because it suggests the material itself is absorbing energy from the incoming light, but not all of the energy, without destroying the incoming light (i.e., turning into a mixed frequency, white light). $\endgroup$ – Feynmanfan85 Jul 12 at 0:17
  • $\begingroup$ @Feynmanfan85 I will edit my answer to respond to your commnt. $\endgroup$ – Lewis Miller Jul 15 at 13:19

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