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Chapter 21: Problem 23

(a) In your own words, briefly describe the phenomenon of luminescence. (b) What is the distinction between fluorescence and phosphorescence?

Short Answer

Expert verified
In summary, luminescence is the phenomenon where a substance emits light without being heated. Fluorescence and phosphorescence are both types of luminescence. Fluorescence involves light absorption and immediate re-emission, ceasing when the source of radiation is removed. In contrast, phosphorescence involves delayed light emission, which can continue even after the exciting radiation has been removed, due to the longer-lived metastable states.

Step by step solution

01

(1. Define Luminescence)

Luminescence is a phenomenon in which a substance emits light without being heated. This emission of light occurs when an excited electron releases energy in the form of a photon. Luminescent materials can emit light in different conditions, such as when UV or visible light falls on them, when subjected to electric fields or radiation, or when they experience certain chemical reactions.
02

(2. Describe Fluorescence)

Fluorescence is a type of luminescence that occurs when a substance absorbs light (usually ultraviolet or visible), and the excited electron immediately releases the absorbed energy, emitting light with a longer wavelength than the absorbed light. The key characteristic of fluorescence is that it occurs only while the substance is exposed to the exciting radiation, and it stops almost immediately once the radiation is removed.
03

(3. Describe Phosphorescence)

Phosphorescence is another type of luminescence that occurs when a substance absorbs light and the excited electron releases the absorbed energy more slowly. In this process, the electron undergoes a transition to a metastable state with a longer lifetime. The light is emitted as the electron transitions back to the ground state. Unlike fluorescence, phosphorescence can continue for some time after the removal of the exciting radiation.
04

(4. Distinction Between Fluorescence and Phosphorescence)

The main distinction between fluorescence and phosphorescence lies in the time scale of the emission processes and the transitions that the excited electrons undergo. In fluorescence, the emission of light occurs almost immediately after the absorption of the exciting radiation, and it stops as soon as the radiation is removed. In phosphorescence, the emission of light is delayed and can continue even after the removal of the exciting radiation. Additionally, the electron transitions involved in fluorescence are simpler as compared to those involved in phosphorescence, which involve the longer-lived metastable states.

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