GED Science TestPhysical ScienceHard
A student is comparing the properties of two unknown substances. Substance X glows brightly when exposed to ultraviolet light, while Substance Y does not. This phenomenon, where a substance absorbs UV light and re-emits it as visible light, is known as:
- APhosphorescence
- BChemiluminescence
- CReflectance
- DFluorescence
Show answer & explanationAnswer & explanation
Correct answer: D. Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence that is typically immediate and ceases almost instantly when the excitation source is removed. Glowing under UV light is a classic example.
Why the other options are wrong
- A. Phosphorescence is similar but involves a delayed emission of light, meaning the glow persists after the UV source is removed.
- B. Chemiluminescence is the emission of light as a result of a chemical reaction, not due to light absorption.
- C. Reflectance is the property of a surface to reflect light, not to emit it after absorption.
Fluorescence
Fluorescence is a type of luminescence where a substance absorbs electromagnetic radiation (typically UV light) and then re-emits light at a longer wavelength (visible light) almost instantaneously.
- Emission stops almost immediately when excitation source is removed.
- Used in fluorescent lights, security features, and biological imaging.
- Different from phosphorescence, which has a delayed emission.
Memory trick: Light can be 'Flashing, Lingering, or Chemical'.