I heard that when sunlight enters house through windows, it gets converted into longer wavelengths and gets trapped inside.

So does the glass have anything to do with this or is this about heated materials radiation, and why does the light get trapped?


3 Answers 3


Just for completeness, there are some materials that do actually convert the frequency of light. They're called optical frequency converters. These tend to work only on paired photons, and thus are transparent when you look at them, but when you shine a whole lot of one frequency on them it converts up. That allows you to do some interesting things:

enter image description here

In this case the green is being upconverted from the monochromatic red light.

This isn't "glass" so it's not what you're asking, but one could use it in this fashion to, say convert red photons to green on the front of a solar panel and then capture the green photons at higher energy. In theory this is more efficient than capturing the two original red ones. You could, in theory, make a window that upconverted IR to visible and thus better light the room.

  • $\begingroup$ Thank you for enriching the subject, it really interesting. $\endgroup$ Dec 10, 2018 at 16:23

Here's how it works. Passage through a piece of glass does not change the wavelength of light. The change occurs when the light strikes objects in the room. Absorbing light energy causes those objects to warm up, and then they begin re-radiating that heat energy at wavelengths which are in the infrared range- which do not pass easily through glass.

  • $\begingroup$ Thank you, but it seem that the glass also change the wavelengths as explained in @adesh mishra answer $\endgroup$ Dec 5, 2018 at 18:12
  • $\begingroup$ it can subtract specific wavelengths from the light passing through it, but this is not what causes the room to heat up. $\endgroup$ Dec 5, 2018 at 18:18
  • $\begingroup$ Yeah you're right it seems that light return to its speed after leaving the glass. $\endgroup$ Dec 5, 2018 at 18:37

When light enters a medium of different refractive index (different than through which it was travelling before entering into it, in your example from air to glass) it’s speed changes but frequency doesn’t change as no new light is being created and since

Speed = frequency x wavelength

And frequency is constant, therefore, wavelength must change.

Hope it helps.

  • $\begingroup$ You're right but I think OP also wanted to know how the glass traps/maybe reflects the light away. $\endgroup$ Dec 5, 2018 at 17:44
  • $\begingroup$ @TausifHossain I’m not understanding what he means by trapped. $\endgroup$
    – user214211
    Dec 5, 2018 at 17:46
  • $\begingroup$ Same here, I'm not an expert at this so I'm not attempting to answer. What he means maybe is: "how does glass insulate us from heat" $\endgroup$ Dec 5, 2018 at 17:47
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    $\begingroup$ @TausifHossain Actually you’re a good person, people in this site don’t answer the simple questions. Feynman once said that questions are questions no matter how simple they are and everyone should clear out his own. $\endgroup$
    – user214211
    Dec 5, 2018 at 17:50
  • $\begingroup$ Thank you, what i mean by trapped is that it keep been trapped inside the house making the interior warmer. $\endgroup$ Dec 5, 2018 at 18:08

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