Absorption, reflection, refraction, scattering and diffraction of light by titanium dioxide particles

Definition of absorption, reflection, refraction, scattering and diffraction of light

1. Absorption of light: Since light is a kind of energy flow, when light propagates through a material, it will cause valence electron transition of the material or cause the atom to vibrate, thereby causing a part of the light energy to become thermal energy, resulting in attenuation of light energy. This phenomenon is called absorption of light. The absorbed light intensity is proportional to the thickness of the absorber.

2. Reflection of light: When light passes from one medium to the interface of another medium, part of the light returns to the original medium, and the direction of light propagation changes. This phenomenon is called reflection of light. The law of reflection of light: the reflected light is on the same plane as the incident ray and the normal; the reflected ray and the incident ray are separated on both sides of the normal; the angle of reflection is equal to the angle of incidence can be summarized as: "three lines are coplanar, two lines are separated, two The angle is equal."

3. Refraction of light: When light is obliquely reflected from one medium to another, the direction of its propagation changes. This phenomenon is called the refraction of light. After the light is refracted, its frequency does not change, but the wavelength and wave speed change. When the light is refracted, the ray is refracted, the incident ray is in the same plane, and the refracted ray and the incident ray are respectively located on both sides of the normal. The angle of refraction changes with the change of the incident angle, but the two are not equal.

4. Scattering of light: When a beam of light passes through a medium, part of the light deviates from the main direction of propagation. This phenomenon is called light scattering. The essence of the phenomenon is the result of the interaction of the light electromagnetic field with the medium molecules. Light scattering is a re-radiation process in which an atom or molecular system changes its direction and phase of propagation, or even changes its frequency, after it has obtained energy from the incident light. When a light wave strikes a medium, under the action of the electric field of the light wave, the molecule or atom obtains energy to induce polarization, and forcibly vibrates at a certain frequency to form a vibration electric dipole (the dipole refers to a symbol that is close to each other) The opposite pair of charges). These vibrating dipoles become secondary sources that emit electromagnetic waves in all directions. In a pure homogeneous medium, the interference of these secondary waves causes light to propagate only in the direction of refraction and cancels out in other directions, so no scattered light appears. However, when a uniform medium is mixed with particles subjected to Brownian motion, or when the system causes local density fluctuation or concentration fluctuation due to thermal motion, the coherence of the secondary wave is destroyed, and the emitted light appears in other directions. .

5. Diffraction of light: The phenomenon that light leaves the straight path and wraps around the shadow of an obstacle is called diffraction of light.

Diffraction is also called diffraction, and the phenomenon that light rays continue to be emitted in space by scattering after being irradiated to the edge of the object.

The essence of light diffraction: the diffraction of light has nothing to do with the rigid rebound of particles. Here we use the volatility of light rather than the particle nature of light. The reason is easy to understand: since light is wave-propagating, the route it takes is naturally a curve like a sine function. It is only at a large scale that we can't tell but think that light is spreading along a straight line. The curve of light is the diffraction of light, which gives us the illusion that light deviates from the direction of motion.

Absorption, reflection, refraction, scattering and diffraction of light by titanium dioxide particles

The particle size of titanium dioxide (aggregate of titanium dioxide particles) is generally less than 1 micron, less than or close to the wavelength of visible light (380-780 nm), so that both light diffraction and light scattering can occur, and its light scattering behavior Meet Rayleigh's law of scattering.

Light has wave-particle duality, and the diffraction phenomenon of light proves the volatility of light. In turn, the diffraction phenomenon of light can also be explained by the volatility of light. Similarly, the scattering and deflection of light demonstrates the photoelectric effect (particle property of light), and the scattering of light can also be explained by the particle nature of light.

Light is an electromagnetic wave that has volatility. Although it is different from mechanical waves, its propagation path should be similar to a sinusoidal function curve. When the scale of the obstacle is relatively large, the incident light wave and the particle rebound rigidly. When the obstacle scale is small to a certain extent, the light will be diffracted into the shadow of the obstacle, or the light will continue to scatter through the edge of the object. Launch in space. Obviously, the smaller the obstacle scale, the more obvious the diffraction phenomenon. If the ray propagation path is similar to a sinusoid, then the half-wavelength can be seen as a critical size point, and the diffraction of particles smaller than half a wavelength will be very noticeable. Therefore, considering the hiding power of titanium dioxide, the particle size of the titanium dioxide particles should not be less than half a wavelength, and the larger the better. Light waves are a form of energy transmission. Light is particle-like and can be seen as a collection of countless photons. The photon has zero static mass, no electricity, but carries energy, which is a medium that transmits electromagnetic interactions. Under the action of the electric field of the light wave, the titanium dioxide molecule or atom obtains energy to induce polarization and form a vibration dipole. The titanium dioxide dipole is forced to vibrate and becomes a secondary wave source, emitting electromagnetic waves in various directions, resulting in scattering of light.

The scattering effect of titanium dioxide particles on light is in accordance with Rayleigh scattering law. According to the Rayleigh scattering formula (see below), we can deduce the following conclusions:

Rayleigh scattering formula

1) The total energy of the scattered light is inversely proportional to the fourth power of the wavelength of the incident light. The shorter the wavelength of the incident light, the more significant the scattering. Therefore, in visible light, blue and purple light scattering is strong.

2) The greater the difference in refractive index between the dispersed phase and the dispersion medium, the more significant the scattering effect. The sol is heterogeneous and has a large difference in refractive index, so the scattering effect is obvious; while the true solution is homogeneous and the scattering effect is weak.

3) The intensity of scattered light is proportional to the number of particles per unit volume. That is, the smaller the particle volume, the more significant the scattering.

4) The intensity of the scattered light is proportional to the intensity of the incident light.

5) The intensity of the scattered light is related to the scattering angle, and the intensity of the scattered light of a certain wavelength is proportional to (1+cos θ).

The coating added with titanium dioxide is obviously a heterogeneous solution, and scattering will definitely occur. Moreover, the smaller the titanium dioxide particles, the more obvious the scattering effect.

The factors affecting the hiding power of titanium dioxide are light absorption, reflection, refraction, scattering and diffraction. Considering the above optical properties of titanium dioxide, increasing the absorption, reflection, refraction and scattering of light by titanium dioxide particles is beneficial to enhance the hiding power of titanium dioxide. Diffraction will weaken the hiding power of titanium dioxide. Therefore, we must try to avoid diffraction phenomenon and try to exert the absorption, reflection, refraction and scattering of light by titanium dioxide.

The intensity of light absorption depends on the thickness of the coating, and is not directly related to the size of the titanium dioxide particles; the titanium dioxide particles have the same degree of strong reflection on all wavelengths of visible light, so they appear white under the illumination of visible light; the refraction of light Depending on the refractive index of titanium dioxide, rutile titanium white has the largest refractive index and can achieve good refractive effect. The smaller the titanium dioxide particles, the stronger the scattering effect, but when the particle size is less than half wavelength, the diffraction becomes significant. . Therefore, the main factor determining the rutile titanium white hiding power falls on the balance between the scattering and the diffraction intensity, which just falls at the half-wavelength position. Therefore, when the titanium dioxide particle size is near the half wavelength, the titanium dioxide has the largest hiding power.