Final answer:
When inspecting closed thermal radiation barrier curtains, only visible light is allowed to pass through as it is the only type of electromagnetic radiation within the visible spectrum used for visual inspection.
Step-by-step explanation:
When inspecting closed thermal radiation barrier curtains, the type of light that is allowed to show through is visible light. This is because visible light makes up a very small portion of the electromagnetic spectrum, which also includes gamma rays, x-rays, ultraviolet light, infrared light, microwaves, and radio waves. The other types of light, such as ultraviolet (UV) and infrared, operate in different bands of the spectrum and are typically not used for visual inspection purposes. Infrared light is most commonly associated with heat radiation, which you can sense as warmth without seeing it. On the other hand, UV light is outside the visible range and can cause damage like sunburn due to its higher energy level.