What phenomenon allows indications to glow under a black light in penetrant testing?

In penetrant testing, the phenomenon that allows indications to glow under a black light is fluorescence. Fluorescence occurs when a substance absorbs light at one wavelength and then emits light at a longer wavelength almost instantaneously. This effect is commonly used in various non-destructive testing methods to enhance the visibility of defects or discontinuities in materials.

When a penetrant with fluorescent properties is used, it becomes excited by the ultraviolet (UV) light from the black light. As the penetrant absorbs the UV light, it re-emits visible light, which makes the defects stand out clearly against the background. This bright fluorescence makes it significantly easier to spot surface flaws that might otherwise be difficult to see.

In contrast, while reflection of light and absorption of light contribute to how light interacts with surfaces, they do not produce the characteristic glowing effect observed in fluorescent materials. Phosphorescence, on the other hand, refers to the longer-lasting emission of light after the excitation source has been removed, which is not typically involved in the immediate visibility of indications in penetrant testing. Thus, fluorescence is the correct and relevant phenomenon in this context.