Introduction
Demonstrate the phenomenon of phosphorescence to help students visualize Einstein’s photoelectric effect. Students often confuse the concepts of intensity of light and energy of light. This demonstration provides a clear way to demonstrate that the intensity, or brightness, of light is NOT the same as the amount of energy a particular color of light possesses.
Concepts
- Phosphorescence
- Energy Levels
- LED (light emitting diode)
Materials
Phosphorescent vinyl sheet
Photoelectric effect board with LEDs
Safety Precautions
Although this activity is considered nonhazardous, please follow all laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory.
Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The materials may be stored for future use or placed in the trash according to Flinn Suggested Disposal Method #26a.
Procedure
- In the dark, remove the phosphorescent vinyl sheet from its package. Ask a volunteer to hold the sheet in front of the class.
- On the photoelectric effect board, switch each LED to the ON position and hold it up in front of the class. Optional: the LEDs are affixed to the board by Velcro™, arrange the LEDs according to ROYGBIV, place the white LED in the last position.
- Provide a brief discussion of phosphorescence and its analogy to the photoelectric effect and ask the class to predict which of the LEDs will produce a glow on the sheet. Most students might predict that the yellow or red LEDs will produce this effect due to their “brightness” or intensities.
- Bring the board close (almost touching) to the top of the sheet and slowly bring it down while the student volunteer is holding the sheet. Allow students to make observations.
- Discuss the difference in wavelength and energy between yellow and red and blue and purple light. Which is higher energy? What is the minimum energy needed for phosphorescence? Why did the white light produce a glow?
Teacher Tips
- Please visit our website (www.flinnsci.com) to see demo videos from Flinn Scientific. Day in the Dark Demonstrations with Jamie Benigna and Phosphorescence both explore the principles of chemiluminescence as well as use LED lights to demonstrate phosphorescence.
- The phosphorescent vinyl sheet has an adhesive backing and can be used as phosphorescent tape. It can also be easily cut into letters, shapes or smaller pieces with scissors.
- Store the phosphorescent vinyl sheet in its flat envelope or some other container that protects it from light. This will lengthen the life of the phosphorescent material in the sheet.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Obtaining, evaluation, and communicating information
Asking questions and defining problems
Disciplinary Core Ideas
MS-PS4.A: Wave Properties
MS-PS4.B: Electromagnetic Radiation
HS-PS4.A: Wave Properties
HS-PS3.A: Definitions of Energy
HS-PS3.C: Relationship between Energy and Forces
HS-PS3.D: Energy in Chemical Processes
Crosscutting Concepts
Energy and matter
Cause and effect
Systems and system models
Patterns
Performance Expectations
MS-PS4-1: Use mathematical representations to describe a simple model for waves that includes how the amplitude of a wave is related to the energy in a wave.
MS-PS4-2: Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
HS-PS4-3: Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
HS-PS4-5: Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.
Discussion
Phosphorescence, also known as “glow-in-the-dark,” is the process of light emission that occurs when electrons that have been promoted to a higher energy level or state return (“relax”) back down to the ground state at a later time. The time interval between when the electrons are excited and when they relax is the primary difference between phosphorescence and other types of luminescence, such as fluorescence. While fluorescent materials return immediately to the ground state following excitation, phosphorescent materials relax at a slower rate. This allows for light to continue to be emitted even after the exciting source has been removed. This is sometimes referred to as the “afterglow.”
In both phosphorescence and fluorescence, a light source is shined on the material, and a photon is absorbed. The energy from the photon is transferred to an electron that makes a transition to an excited electronic state. From this excited state, the electron naturally wants to relax back to its ground state. This relaxation process varies depending on whether the material is fluorescing or phosphorescing. In phosphorescence, the excited electron makes a series of transitions to return to the relaxed ground state. It first makes a slow transition to a second excited state very close in energy to the initial excited state. From this second excited state, the electron makes the transition down to a lower energy level and emits a photon in the process. The characteristic afterglow of phosphorescence is due to the delayed emission that occurs as a result of the slow transition between the first two excited states. A minimum light energy is needed to overcome the energy threshold of a material and initiate phosphorescence. The red, orange, yellow, and green LEDs have lower energy and are unable to cause the material to phosphoresce. In contrast, the blue and purple LEDs have high enough energy that are able to cause phosphorescence.
A Light Emitting Diode (LED) consists of a negatively charged semiconductor bonded to a positively charged semiconductor. The negative semiconductor has an excess of electrons, whereas the positive semiconductor lacks electrons and has holes where those electrons should be. When a current is applied to the diode by connecting the positive side of a battery to the positively charged semiconductor and the negative side of a battery to the negatively charged semiconductor, the electrons move to fill in the holes on the positively charged semiconductor. When these electrons are freely moving around they are in the conduction band, which is outside the valence band and therefore beyond the electric field of the atom. As the excited electrons move through the conduction band they fall into the holes in the positively charged semiconductor. This drop from the excited conduction band to a lower orbital causes the release of a photon; and the larger the drop, the higher the energy of the photon. A higher energy photon emits a higher frequency of light. According to Planck’s Law, the energy of light is directly proportional to the frequency and inversely proportional to the wavelength.
In this demonstration, the blue and purple LED lights are examples of higher energy lights and the yellow, red, orange LED lights are lower in energy. The green LED also does not contain the required energy for phosphorescence. The distance the electron drops is greater in the blue and purple LEDs than in the red, orange, yellow and green LEDs, which in turn means that the photons released are at higher energy levels. These photons excite the electrons on the phosphorescent vinyl sheet, which then relax at a delayed rate causing the glow we call phosphorescence. The white LED contains all of the colors, ROYGBIV, therefore a glow results due to the presence of blue, indigo, and violet. We can relate this demonstration to the photoelectric effect as the photoelectric effect involves the ejection of electrons from a metal surface when light is shined on it; the energy of the electrons ejected depends upon the wavelength of the light, not the intensity. Einstein explained the photoelectric effect by suggesting that light consists of photons, each with energy E = hν. If a photon of light strikes a metal surface with more energy than the energy binding an electron to the surface, the photon will cause an electron to be ejected. The more intense a light source (greater number of photons), the greater the number of electrons ejected. If a photon striking the surface of a metal does not have more energy than the energy binding an electron to the surface, an electron cannot be ejected, no matter how many photons (with this amount of energy) strike the surface.
References
Special thanks to Jamie Benigna and Mike Heinz for providing the idea and the instructions for this activity to Flinn Scientific.
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