Glowing Wonders: The Magic of Phosphorescence and Illuminating The Difference Between Fluorescence Vs Phosphorescence

Glowing Wonders: The Magic of Phosphorescence and Illuminating The Difference Between Fluorescence Vs Phosphorescence

 

What is Phosphorescence?

Phosphorescence is a fascinating optical phenomenon exhibited by certain materials that can absorb and store energy from light or other radiation sources and later emit it in the form of light over an extended period of time. This emission of light persists even after the initial light source is removed. Phosphorescence is often confused with fluorescence, but they are distinct processes. 

Here are the key basic features of phosphorescence:

• Absorption of Energy: When certain materials are exposed to radiation, such as ultraviolet (UV) light, X-rays, or other high-energy particles, some of their electrons get excited to higher energy levels. These excited electrons are temporarily promoted to a higher energy state.

• Temporary Storage: Unlike fluorescence, where the emission of light occurs almost immediately after excitation, in phosphorescence, the excited electrons get trapped in a higher energy state for an extended period of time, sometimes ranging from seconds to hours.

• Emission of Light: After a delay, the trapped electrons undergo a transition back to their original, lower energy state. During this transition, they release the excess energy they had gained as light. This emitted light is often of lower energy (longer wavelength) than the initial excitation source, typically in the visible range.

• Glow in the Dark: Phosphorescent materials are often referred to as "glow-in-the-dark" materials because they can emit light in the absence of any external light source. The emitted light is usually dim and can be observed in a darkened environment.

• Examples of Phosphorescent Materials: Common examples of phosphorescent materials include certain types of minerals (e.g., willemite, sphalerite), certain chemical compounds (e.g., zinc sulfide, strontium aluminate).

Phosphorescence has various practical applications. For instance, it is used in glow-in-the-dark toys, emergency signs, exit signs, and safety equipment to provide illumination during power outages or low-light situations without the need for an external power source.

 

What is the difference between fluorescence vs phosphorescence?

Fluorescence and phosphorescence are both involve the emission of light by certain materials after they have been exposed to external radiation. However, they differ in several key aspects, including the duration of light emission and the mechanism responsible for light emission. Let's explore the main differences between fluorescence and phosphorescence:

1. Duration of Light Emission:

• Fluorescence: In fluorescence, the emission of light occurs immediately when the material is exposed to external radiation (e.g., ultraviolet light). Once the external light source is removed, the fluorescence stops almost instantly. The excited electrons return to their ground state quickly, and the light emission ceases.
• Phosphorescence: Phosphorescence, on the other hand, involves a delayed emission of light. After the material is exposed to external radiation, the emission of light persists even after the excitation source is removed. The excited electrons remain trapped at higher energy states for a longer period before they gradually transition back to their ground state, releasing the stored energy as light. This can result in a glow-in-the-dark effect.

2. More About The Energy States of Electrons:

• Fluorescence: During fluorescence, the excited electrons are promoted to higher energy states, but they typically return to their ground state very quickly (nanoseconds to microseconds). The transition between energy states is usually accompanied by the emission of light. 

• Phosphorescence: In phosphorescence, the excited electrons become trapped at higher energy states due to certain defects or impurities in the material's crystal structure. The return of the electrons to their ground state occurs through a slower process (milliseconds to hours), and this transition is usually accompanied by the emission of light.

3. Common Examples:

• Fluorescence: Fluorescence is common in various everyday objects, such as fluorescent lights, highlighter pens, and certain minerals or gemstones (e.g., fluorescent minerals under UV light).

• Phosphorescence: Phosphorescence is often observed in "glow-in-the-dark" materials, such as glow-in-the-dark toys, exit signs, and certain minerals like zinc sulfide or willemite.

In summary, fluorescence is characterized by immediate light emission upon excitation and short-lived afterglow, while phosphorescence exhibits delayed light emission and a more prolonged glow-in-the-dark effect after the excitation source is removed.

 

What causes minerals to phosphoresce?

The phosphorescence of minerals is caused by specific impurities or defects in their crystal structures. These impurities or defects create energy levels within the mineral that allow electrons to become temporarily trapped at higher energy states when exposed to certain types of radiation, such as ultraviolet (UV) light. When the electrons eventually return to their original energy levels, they emit light as a result of the energy release.

The key factors contributing to phosphorescence in minerals are:

1. Crystal Structure: The crystal lattice of the mineral must have specific defects or imperfections that can trap the excited electrons. These defects can be vacancies (missing atoms) in the lattice or the presence of certain elements that do not fit perfectly into the crystal structure.

2. Activator Ions: In some cases, the phosphorescence is caused by impurities in the form of activator ions, which are atoms of different elements present within the mineral's crystal structure. These activator ions absorb the energy from the external radiation and transfer it to the host mineral lattice, leading to the excitation of electrons.

3. Energy Levels: The crystal lattice must have energy levels within its electronic structure that allow electrons to be promoted to higher energy states when they absorb energy from radiation. These energy levels are associated with the impurities or defects in the crystal structure. 

4. Long-lasting Traps: The trapped electrons should have relatively long lifetimes in their higher energy states, allowing for a delay in the emission of light during the phosphorescence process. 

Zinc sulfide (ZnS) is one of the most common phosphorescent materials responsible for the phosphorescence in minerals such as willemite. It contains defects in its crystal structure, and when it absorbs energy from UV light, electrons become trapped in higher energy states. As these trapped electrons return to their lower energy states, they emit light in the visible spectrum, causing the characteristic glow-in-the-dark effect.

It's worth noting that not all minerals exhibit phosphorescence. The ability to phosphoresce depends on the specific combination of crystal structure, impurities, and energy levels within the mineral. As a result, phosphorescence is a relatively rare and special property observed in only certain types of minerals.

 

What are some examples of phosphorescent minerals?

There are several examples of minerals that exhibit phosphorescence, producing a glow-in-the-dark effect after exposure to ultraviolet (UV) light or other sources of radiation. Here are some common examples of phosphorescent minerals:

 

 

 

Aragonite (CaCO3): Aragonite is a carbonate mineral and many times strongly phosphorescent. The combination of beautiful crystal structures and phosphorescence often makes for very attractive mineral specimens.

 

 

 

 

 

 

 

 

Willemite (Zn2SiO4): Willemite is a zinc silicate mineral that is known for its strong green phosphorescence. It is often found in association with other minerals, such as franklinite and calcite, and is a key component in the famous Franklin and Sterling Hill mining districts in New Jersey, USA. Minerals from Puttapa Mine in Australia are also famous for their phosphorescence.

 

 

 

Sphalerite (ZnS): Sphalerite is a zinc sulfide mineral that can display phosphorescence when exposed to shortwave UV light. The emitted light is usually blue or green in color, and sphalerite's phosphorescence is often associated with its impurities or defects.

 

 

Sodalite and Hackmanite (Na8Al6Si6O24S2): Some sodalites, most notably from Afghanistan and Greenland can be very phosphorescent. Hackmanite is a variety of sodalite mineral that shows a unique property called tenebrescence, which is the ability to change color when exposed to light. In addition to tenebrescence, hackmanite can also phosphoresce with a whitish glow after UV exposure. Finding phosphorescence in hackmanite or sodalite is usually pretty rare, so if you come across a piece of phosphorescent sodalite, consider it a fantastic find!

 

 

Wernerite (a variety of scapolite): Wernerite is a rare variety of scapolite, a complex silicate mineral. Some forms of wernerite can phosphoresce very briefly with a bright yellow to orange glow when exposed to UV light. This type of brief phosphorescence is called “Brief Intense Phosphorescence” or “BIP”. When you see this BIP happen, it’s a bright, short-lived flash of color usually lasting less than ½ a second. It’s very difficult to capture this in the photos. BIP is commonly found in calcite.

And our personal favorite, Tugtupite: Tugtupite displays a striking blue phosphorescence when exposed to UV light. 

It's important to note that not all specimens of these minerals will necessarily phosphoresce. The ability to phosphoresce depends on specific factors, such as the presence of certain impurities, the crystal structure, and the conditions under which the mineral formed. When collecting or observing minerals for phosphorescence, it's recommended to use a UV light source, such as a shortwave UV flashlight, to excite the specimens to witness the glow-in-the-dark effect.  This is best observed with eyes “dark adapted”, e.g. 10 minutes in a dark room.

 

Diamond Phosphorescence

Certain diamonds will glow in the dark for a short time after they absorb high-energy UV light. The glow can last a few seconds or minutes. What’s causing the phosphorescence in these diamonds is the UV light interacting with trace elements such as nitrogen and boron within the diamonds. Some scientists around the world use the color of this glow to identify individual gems.

 

More helpful links on explaining phosphorescence and various luminescent properties, as well as photos to browse through: 

https://uvminerals.org/science/luminescence/

https://www.naturesrainbows.com/photo-archive/categories/phosphorescent