Fluorescence is the phenomenon where a substance emits light after absorbing light or other forms of electromagnetic radiation. A notable instance of fluorescence happens when the absorbed radiation falls within the ultraviolet range of the electromagnetic spectrum (invisible to the human eye), and the emitted light falls within the visible range. This imparts a distinct color to the fluorescent substance, visible only when exposed to UV light. Fluorescence finds numerous practical applications in fields such as biology, mineralogy, gemology, and medicine.

In nature, fluorescence commonly occurs in certain minerals and various biological organisms spanning all life kingdoms. In the case of biological entities, it is often termed biofluorescence, denoting that the substance emitting fluorescence is either a part of a living organism or is derived from one. [Source C]

Fluorescence can also be detected in everyday items like banknotes or passports. In this context, it serves as a security feature and can be easily verified using the Lumus Explorer^®. The device includes 4 UV LEDs with a wavelength of 365 nm.

Experiment:

The UV pen provided with the Lumus Explorer^® allows you to write with invisible ink. The ink contains particles that are excited by the device’s UV light and then emit a bright color through fluorescence. The writing is not visible to the naked eye.

However, it’s important to note that not all types of paper are suitable for this experiment. Many papers use optical brighteners, which make the paper appear very white. Titanium dioxide is the white pigment used in paper production, which enhances the paper’s whiteness. Additionally, fluorescent organic substances are added to achieve optical brightening. These additives cause both the writing made with the UV pen and the paper itself to glow brightly under UV light. [Source D]

To address this issue, the Lumus Explorer^® includes a special writing pad made from non-chemically bleached paper. This specific paper is designed for use with the UV pen experiment.

Sources:
[A]: Wikimedia
[B]: Wikimedia
[C]: Universität Jena
[D]: Wikipedia