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==== Light ====
**Light**, as a core carrier spanning the fields of optical engineering and electronic information, is a key medium for energy transfer and information transmission. From basic illumination and high-precision imaging to fiber-optic communication, laser precision manufacturing, and cutting-edge quantum technologies, the applications of light are deeply embedded in the lifeline of modern technology. In terms of its physical nature, **light is a form of electromagnetic radiation** that exhibits significant wave-particle duality. In practical applications and theoretical research, its characteristics can be interpreted through three core models: the **ray model** analyzes macroscopic propagation paths, the **wave model** reveals microscopic interference and diffraction laws, and the **photon model** defines its quantized energy nature. This article will systematically analyze the characteristics and principles of light based on these three core models.\\
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^  Figure 1: Relationship among the three models  ^

**Photon Model**\\
The photon model focuses on the particle nature of light, and its fundamental energy unit is called the **photon**.\\
|  {{ :yanding:成像基础知识:光学:光度学:wavelet.gif? |}}  |
^  Figure 2: Photon  ^
(Image source: https://www.robotlab.com/hs-fs/hubfs/Wavelet.gif)\\

Photons have no rest mass and carry energy and momentum. Their energy formula is $$E=h\nu=\frac{hc}{\lambda}$$
where: h is Planck's constant, ν is frequency, c is the speed of light, and λ is wavelength. The energy is directly proportional to the frequency and inversely proportional to the wavelength (see Figure 3).\\
|  {{ :yanding:成像基础知识:光学:光度学:光3.png?750 |}}  |
^  Figure 3: Electromagnetic spectrum  ^
(Image source: https://en.wikipedia.org/wiki/Spectrum#/media/File:EM_Spectrum_Properties_edit.svg)

In a vacuum, the speed of light is constant at $3\times10^8 m/s$. This property defines the energy exchange process between light and matter (such as the photoelectric effect, see Figure 4). When the energy carried by incident photons strikes electrons in a material, it transfers its energy to the electrons. When the energy is sufficient to overcome the atomic binding, the electrons escape to form photoelectrons, which is the core process of the photoelectric effect.\\
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^  Figure 4: Photoelectric effect  ^
(Image source: https://en.wikipedia.org/wiki/Photoelectric_effect#/media/File:Photoelectric_effect_in_a_solid_-_diagram.svg)\\

In low-light environments (i.e., when fewer photons reach the sensor per unit time), photon shot noise (see Figure 5) has a relatively significant impact on the image.\\
|  {{ :yanding:成像基础知识:光学:光度学:photon-noise.jpg?600 |}}  |
^  Figure 5: Photon shot noise  ^
(Image source: https://commons.wikimedia.org/wiki/File:Photon-noise.jpg)\\

**Wave Model**\\
The wave model focuses on the wave nature of light, treating light as an electromagnetic wave propagating in the form of transverse waves. Light requires no medium for propagation and manifests as periodic alternating changes in the electric and magnetic field vectors.
{{ :yanding:成像基础知识:光学:光度学:光波1.png |}}
Its core wave characteristics are manifested in phenomena such as **interference** (e.g., double-slit interference fringes), **diffraction** (light propagating around obstacles), and **polarization** (the directional nature of light's vibration). These not only reveal the physical essence of wave phase superposition and diffraction but also clarify the relationship between frequency and wavelength: $$c=\lambda*\nu$$
where: c is the speed of light, $\lambda$ is the wavelength, and $\nu$ is the frequency. The frequency $\nu$ and wavelength $\lambda$ are inversely related, reflecting the constraint between light in the time domain (frequency) and the spatial domain (wavelength).\\

|  {{ :yanding:成像基础知识:光学:光度学:doubleslit.gif? |}}   |  {{ :yanding:成像基础知识:光学:光度学:wave_diffraction_4lambda_slit.png?360 |}}  |  {{ :yanding:成像基础知识:光学:光度学:圆偏振.gif |}}  | 
^  Double-slit interference   ^  Single-slit diffraction  ^   Circular polarization  ^
(Image source: https://upload.wikimedia.org/wikipedia/commons/a/a9/Doubleslit.gif)\\
(Image source: https://en.wikipedia.org/wiki/Diffraction#/media/File:Wave_Diffraction_4Lambda_Slit.png)\\
(Image source: https://upload.wikimedia.org/wikipedia/commons/d/d1/Circular.Polarization.Circularly.Polarized.Light_Left.Hand.Animation.305x190.255Colors.gif)\\

**Ray Model**\\
The ray model ignores the wave-particle duality of light (such as interference, diffraction, and quantum effects) and abstracts the complex propagation of electromagnetic energy into "rays" traveling along geometric line segments. Its theoretical cornerstone is Fermat's principle—that light propagates along a path of extremal optical path length (maximum, minimum, or constant). Based on this principle, the ray model unifies the propagation behavior of light in isotropic media. In a homogeneous medium, [[yanding:成像基础知识:光学:几何光学:光的直线传播定律|light propagates in straight lines]], which explains solar eclipses, pinhole imaging, and shadow formation. When light evolves at the interface between media, its path strictly follows the [[yanding:成像基础知识:光学:几何光学:光的反射定律|law of reflection]] (i.e., the angle of incidence equals the angle of reflection) and the [[yanding:成像基础知识:光学:几何光学:光的折射定律|law of refraction]] (i.e., Snell's law $n_1 \sin \theta_1 = n_2 \sin \theta_2$).\\
**Light propagates in straight lines:**\\
{{ :yanding:成像基础知识:光学:光度学:直线传播7.png |}}

**Law of reflection:**\\
{{ :yanding:成像基础知识:光学:光度学:反射2.png?800 |}}

**Law of refraction:**\\
{{ :yanding:成像基础知识:光学:光度学:折射.png |}}