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You need special optics to focus infra-red.
The longer wavelengths do not focus at the same point (they are bent less by refraction).
Mirror optics are better.
condenser adjustment allow you to focus the optics properly into your eye
think
focus
concentrate
thank you!!
Albert A. Michelson has written:
'Studies in optics'
'Studies in optics' -- subject(s): Optics
Variously: physics, optics, quantum mechanics, etc.