SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/reptile-uvb-glass-plastic

Why doesn't sunlight through a closed window give a reptile the UV it needs?

Most glass or plastic blocks UV, so window light does not count.

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  1. Most glass or plastic blocks almost all UV ✔
  2. Window glass makes UV too strong
  3. UV only reaches reptiles at night

Why: Merck says sunlight filtered through most glass or plastic has almost no UV. Unfiltered sunlight is best; indoors, use a UV bulb near the basking spot.

Huh, didn't know that: Merck says typical UV bulbs should be within 18 inches of resting or basking areas.

The dad joke
Why did the turtle open the window? It wanted the real sun, not a glass imitation.
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References

Answer: Most glass or plastic blocks almost all UV. Merck says sunlight filtered through most glass or plastic has almost no UV. Unfiltered sunlight is best; indoors, use a UV bulb near the basking spot.

  1. Merck Veterinary Manual (pet owner): Providing a Home for a Reptile www.merckvetmanual.com/all-other-pets/reptiles/providing-a-home-for-a-reptile
    sunlight filtered through most glass or plastic contains almost no ultraviolet rays.
    Checked 2026-10-11.

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