Diagrams (1) and (2) are energy-level diagrams for two different LEDs. One LED emits red light, and the other emits blue light. Which one emits red, and which blue? Explain. (1)
(2)

Key Concepts

Energy Levels and Photon Emission

In semiconductors, electrons occupy energy levels within bands, specifically the valence and conduction bands. When an electron transitions from a higher energy level in the conduction band to a lower energy level in the valence band, it emits a photon. The energy of the emitted photon corresponds to the energy difference between these levels, determining the color of the light emitted.

Wavelength and Color of Light

The color of light emitted by an LED is directly related to its wavelength, which is inversely proportional to energy. Red light has a longer wavelength (around 620-750 nm) and lower energy, while blue light has a shorter wavelength (around 450-495 nm) and higher energy. Thus, the energy-level diagram can indicate which LED emits which color based on the energy gap between the bands.

Band Gap Energy

The band gap energy is the energy difference between the top of the valence band and the bottom of the conduction band in a semiconductor. A larger band gap corresponds to higher energy transitions, resulting in the emission of higher-energy (blue) light, while a smaller band gap leads to lower energy transitions, resulting in the emission of lower-energy (red) light. This concept is crucial for identifying the color of light emitted by different LEDs.