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Chapter 15: Problem 2

What is luminescence? Describe in detail the photoluminescence

Short Answer

Expert verified
Luminescence is light emission unrelated to heat. Photoluminescence occurs when a material absorbs photons and then re-emits light. It is categorized into fluorescence and phosphorescence.

Step by step solution

01

Define Luminescence

Luminescence is the emission of light by a substance not resulting from heat. It is typically caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal.
02

Introduce Photoluminescence

The term photoluminescence refers to the emission of light from a material after it absorbs photons. This is a specific type of luminescence induced by photo-excitation.
03

Explain the Mechanism of Photoluminescence

The process begins when a material absorbs photons, causing electrons to move to higher energy states (or excitation states). As the electrons return to their ground states, they release energy in the form of light, which is observed as photoluminescence.
04

Discuss Types of Photoluminescence

Photoluminescence can be further classified into two main types: fluorescence and phosphorescence. Fluorescence occurs almost instantaneously after excitation, while phosphorescence involves a delay between excitation and emission due to electrons being trapped in a higher energy state before returning to ground state.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

What is Luminescence?
Luminescence is the emission of light by a substance that does not result from heat. This fascinating phenomenon happens due to various sources of energy, not thermal energy.
Whenever a material emits light because of chemical reactions, electrical energy, subatomic motions, or stress on a crystal, it is displaying luminescence.
Under the broad umbrella of luminescence, multiple sub-types arise, each classified by its excitation source or the process leading to light emission.
Understanding Photo-excitation
Photo-excitation is the core process leading to photoluminescence. It starts when a material absorbs photons, the tiny packets of light energy.
These absorbed photons energize the material's electrons, pushing them from their ground state to a higher energy level, also known as the excited state.
  • Ground State: The electron's normal, low-energy position.
  • Excited State: The elevated energy state after absorbing photons.
Once in the excited state, electrons are unstable and seek to return to their lower energy, ground state. This return trip creates the opportunity for photoluminescence.
Exploring Fluorescence
Fluorescence, a sub-type of photoluminescence, occurs almost immediately after photo-excitation. When the excited electrons drop back down to their ground state, they release energy in the form of light.
The key characteristics of fluorescence include:
  • Instantaneous Emission: Light is emitted nearly instantaneously (within nanoseconds) after excitation.
  • Short-lived: The emitted light lasts only as long as the excitation source is present.
  • Common in Nature: Many biological substances, like proteins and vitamins, exhibit fluorescence under UV light.
Practical applications often utilize fluorescence for imaging, sensors, and illuminating displays.
Decoding Phosphorescence
Phosphorescence is another fascinating form of photoluminescence, but it comes with a twist compared to fluorescence.
In phosphorescence, excited electrons get trapped in a meta-stable state between the excited and ground states. This creates a delay before they emit light and return to the ground state.
Key points of phosphorescence:
  • Delayed Emission: The light emission can last from milliseconds to several hours after photo-excitation.
  • Glow-in-the-dark: Common in glow-in-the-dark materials, where the light continues even after the excitation source is removed.
  • Energy Traps: Electrons are temporarily 'trapped' in energy states that require some time to overcome.
Phosphorescent materials are widely used in safety signs, watches, and novelty items, providing a persistent glow that is both useful and captivating.

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Most popular questions from this chapter

A metallic surface, when illuminated with light of wavelength \(3333 \mathrm{~A}\) emits electrons with energies upto \(0.6 \mathrm{eV}\) and when illuminated with light of wavelength \(2400 \mathrm{~A}\), it emits electrons with energies upto 204 \(\mathrm{eV}\). Calculate Planck's constant and the workfunction of the metal. Ans. \(6.58 \times 10^{-34}\) Js, \(3.1 \mathrm{eV}\).

Explain the term (i) stimulated emission, (ii) population inversion as applied to Laser. Discuss the working of ruby laser.
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