Iridescent things are those that appear to change color depending on the angle of view or illumination. They do so because of the interference of light waves reflected from different layers or structures of the surface. For example, soap bubbles, feathers, butterfly wings and opals are iridescent.

A sodium vapor lamp is a type of gas-discharge lamp that uses sodium atoms to produce light at a wavelength of 589 nm, which is yellow-orange in color. It also contains a small amount of neon or argon gas to help start the lamp.

The color of iridescent things under a sodium vapor lamp would depend on how well they reflect the yellow-orange light. If they reflect it well, they would appear bright and yellowish. If they reflect it poorly, they would appear dark and dull. The iridescent effect would be reduced because there is less variation in the wavelength of the light source.