Hello everyone.
Please, which one of the following materials can reflect the visible light more efficiently :
semiconductors, insulators, metals or dielectric materials and why ?
Hello Balsam,
the reflectivity of a material depends on the refractive index, the conductivity, the surface properties and the angle of incidence (k-vector) of light.
For smooth surfaces and vertical incidence, the reflection coefficient R in dielectric media (conductivity zero) is equal to
R = (n-1)2/(n+1)2. This means, for glass with an refractive index of n = 1,5, you get R = 0,04. With increasing n, R increases. For silicon with n = 3,4, you find R = 0,3. If you take into account the conductivity, the reflection increases, due to the effect of the high mobily electrons. In metals, the reflectivity lies near 1. If you deviate from vertical incidence, the reflectivity increases as a function of angle of incidence and polarization. For specular incidence, the reflectivity is 1. Generally, Fresnels equations describe the behaviour exactly. You must differ between s- and p-waves (polarization vertical or within the plane of incidence). For p-waves, the reflectivity has a minimum at the brewster angle teta = arctan (n2/n1). You can test this property using a laser pointer. Look for a dielectric medium (smooth plastic surface), calculate the angle and rotate the laser around his axis. Then you see a point of maximum (s-wave) and minimum (p-wave) brightness.
The effect of total reflection you can observe if you have a boundary with n1 > n2.
A very complex behaviour you have for rough surfaces. Every point of surface has another reflection coefficient because the angle of incidence is distributed randomly. You must integrate over all contributions but of course, the reflectivity is higher than for vertical incidence.
You see, the reflectivity can be well described but the number of influencing factors is high.
With regards
R. Mitdank
Hello Balsam,
the reflectivity of a material depends on the refractive index, the conductivity, the surface properties and the angle of incidence (k-vector) of light.
For smooth surfaces and vertical incidence, the reflection coefficient R in dielectric media (conductivity zero) is equal to
R = (n-1)2/(n+1)2. This means, for glass with an refractive index of n = 1,5, you get R = 0,04. With increasing n, R increases. For silicon with n = 3,4, you find R = 0,3. If you take into account the conductivity, the reflection increases, due to the effect of the high mobily electrons. In metals, the reflectivity lies near 1. If you deviate from vertical incidence, the reflectivity increases as a function of angle of incidence and polarization. For specular incidence, the reflectivity is 1. Generally, Fresnels equations describe the behaviour exactly. You must differ between s- and p-waves (polarization vertical or within the plane of incidence). For p-waves, the reflectivity has a minimum at the brewster angle teta = arctan (n2/n1). You can test this property using a laser pointer. Look for a dielectric medium (smooth plastic surface), calculate the angle and rotate the laser around his axis. Then you see a point of maximum (s-wave) and minimum (p-wave) brightness.
The effect of total reflection you can observe if you have a boundary with n1 > n2.
A very complex behaviour you have for rough surfaces. Every point of surface has another reflection coefficient because the angle of incidence is distributed randomly. You must integrate over all contributions but of course, the reflectivity is higher than for vertical incidence.
You see, the reflectivity can be well described but the number of influencing factors is high.
With regards
R. Mitdank
In Short: Metals
Why?
Reflectance is a measure of the incident electromagnetic radiation that is reflected by a given interface. The reflectance may be calculated by comparing the amount of reflected radiation to the amount of incident radiation.
Any polished surface will be more reflective to the visible light compared to the unpolished surface. Remember, Reflection, Transmission and Absorption. Smooth/polished surfaces have higher Reflective index. Dielectrics are transparent and hence have higher transmission coefficient. Non-smooth unpolished surfaces have higher dispersion.
Metals: Metals are very smooth and have high reflection coefficient.
Dielectrics/Insulator: They have high transmission for the visible light. It also depends on the thickness of dielectric of course. But the reflection coefficient
is small compared to metals.
Semiconductors: Not as smooth as metals, low reflection coefficient and no/low transmission/absorption.
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