Final answer:
Ultraviolet (UV) light rays are both chemical and germicidal because they cause thymine dimers to form in DNA, leading to mutations that kill microorganisms.
Step-by-step explanation:
The light rays that are both chemical and germicidal are ultraviolet (UV) rays.
UV light can be effective against certain bacteria by causing thymine dimers to form in DNA, leading to mutations that kill microorganisms.
Germicidal lamps emitting UV light at a wavelength of 260 nm are commonly used in surgical suites and biological safety cabinets.