4.5.3 Experimental principles
The polarization of light confirms that light waves are transverse waves.The research on the polarization properties of light waves not only deepens people’s understanding of the law of light propagation and the law of interaction between light and matter,but also has played important roles in the fields of optical metrology,photoelastic technology,and thin film technology.
Light waves are electromagnetic waves,which are transverse waves.Its electric field component E and magnetic field component B are perpendicular to each other and perpendicular to the direction of light propagation.Because it is the electric field component E that causes the photosensitive and physiological effects,the electric field component E is generally called the light vector,the vibration of the electric field component E is called the light vibration,and the plane formed by the light vector and the propagation direction of the light is called the vibration surface.In the plane perpendicular to the propagation direction,the light vector may have various vibration states,which are called the polarization states of light.As shown in Table 4.5.1(a),if the direction of the light vector is arbitrary,and the time average of the magnitude(or amplitude)of the light vector in all directions is equal,such light is called natural light.After natural light is reflected,refracted or absorbed by some material,if the direction of the light vector is always the same,but its amplitude changes with the phase,and the end track of the light vector is a straight line,then this kind of light is called linear polarized light or plane polarized light,as shown in Table 4.5.1(b).If the light vector can take any direction,but the amplitude of the vibration in a certain direction is more dominant than the amplitude of the vibration in the perpendicular direction,the light is called partially polarized light,as shown in Table 4.5.1(c).If the same monochromatic polarized light passes through the birefringent material,the two polarized lights produced are coherent light.The vibration directions of the two polarized lights are perpendicular to each other,so like the combination of two perpendicular vibrations of the same period,the end of the light vector will be described as an elliptical trajectory in a plane perpendicular to the direction of propagation,and this light is called an elliptically polarized light,as shown in Table 4.5.1(d).In special cases when the amplitudes of the two polarized lights are equal,they become circularly polarized lights,as shown in Table 4.5.1(e).
In optical experiments,some devices are often used to remove part of the vibration in natural light to obtain the polarization state of light.The device or instrument that can turn natural light into polarized light is called a polarizer,and the device or instrument used to check whether a beam is polarized light is called an analyzer.In fact,the polarizer can also be used as an analyzer.
Table 4.5.1 The polarization states of light
1)Metho d of generating linearly polarized light
(1)Generated by reflection and refraction.As shown in Figure 4.5.2,when natural light is reflected and refracted at the interface between two media(air and isotropic glass),both the reflected light and the refracted light will become partially polarized light.When natural light is incident at the deflection angle iB(also called Brewster’s angle),the reflected light is fully polarized light and the vibration plane is perpendicular to the incident plane.And the transmitted light is partially polarized light.
Figure 4.5.2 The generation of linearly polarized light by polarizer
Figure 4.5.3 The generation of linearly polarized light by reflection and refraction of light
(2)Generated by the birefringence of the crystal.When natural light is incident on some anisotropic crystals(such as calcite and Nicol prism),the ordinary light(o light)and extraordinary light(e light)produced by the birefringence of the crystal are linearly polarized light.
(3)Generated by the polarizer.As shown in Figure 4.5.3,Polarizers are made using the dichroism of certain organic compound crystals(such as cinchona sulfate crystals).When natural light is irradiated on the polarizer,the light vibration perpendicular to the direction of the polarizer’s vibration is almost completely absorbed.It only allows light vibration in a certain direction to pass through.This direction is called the polarization direction,which is represented by“↑↓”.The transmitted light basically becomes linearly polarized light.
As mentioned earlier,polarizers can also be used as analyzers.Let the linearly polarized light generated by passing through the polarizer A incidents pass through the polarizer B,as shown in Figure 4.5.4.When the polarization direction of B is the same as the polarization direction of A,the polarized light can fully pass through the polarizer B.If the polarizer B is rotated through an angle of 90°,that is,when the polarization direction of B is perpendicular to the polarization direction of A,the polarized light cannot be emitted through B,as shown in Figure 4.5.5.If we take the direction of light propagation as the axis and rotate the polarizer B in the same direction,you will find that the polarized light passing through B undergoes a periodic change process of light→dark→bright in turn.
Figure 4.5.4 Two polarizers in the same polarization direction
Figure 4.5.5 Two polarizers in perpendicularly polarization directions
2)Wave-plate and generation of elliptically and circularly polarized light(https://www.daowen.com)
As shown in Figure 4.5.6,when a beam of parallel linearly polarized light is perpendicularly incident on a uniaxial wave-plate C(the optical axis is parallel to the crystal plane),the light is decomposed in the wafer into o light and e light whose vibrating surfaces are perpendicular to each other.Although the two beams of light propagate in the same direction in the wave-plate C,the propagation speed is different.Therefore,after passing through the wave-plate C,a fixed optical path difference δ occurs between the two beams of light,namely:
The generated phase difference is
Figure 4.5.6 The generation of elliptically and circularly polarized light
Where λ is the wavelength of the monochromic light;d is the thickness of the wave-plate C;no,ne are the refractive index of the o light and e light,respectively.
It can be seen from Equation(4.5.2)that the combined vibration of o light and e light has different polarization states depending on the phase difference.
①WhenΔΦ=kπ(k=0,1,2,…),the polarization state is plane polarization(also called linear polarized light).
②WhenΔΦ=(k+1/2)π(k=0,1,2,…),the polarization state is a standard elliptically polarized state with the long and short axis of the elliptical trajectory overlapping with Ex and Ey axis.
③WhenΔΦ is none of the value in①or②,the polarization state is an arbitrary elliptically polarized state.
A wave-plate that can produce a phase differenceΔΦ=(k+1)π for a monochromatic light at a certain wavelength(mainly choose the thickness d of the wave-plate),that is,when the optical path difference is an odd multiple of λ/2,it is called the half wavelength plate of the monochromatic light;When the phase difference is equivalent toΔΦ=(k+1/2)π,that is an odd multiple of the optical path difference of λ/4,and it is called the quarter wavelength plate of the monochromatic light.
When linearly polarized light with a certain amplitude propagates through the λ/4 plate perpendicularly,and the vibration direction is at an angle α with the optical axis of the wave plate,the transmitted light is generally a standard elliptically polarized light.And when
①α=π/4,the transmitted light is circularly polarized light.
②α=0 or α=π/2,the transmitted light is linearly polarized light(No matter what kind of wave plate is used,its linear polarization property will not change).
3)Light intensity changes after linearly polarized light passes through the analyzer
The intensity of linearly polarized light with intensity I0 after passing through the analyzer will be
Equation(4.5.3)was called Malus’law,which means that changing the angle between the direction of light vibration and the optical axis of the wave-plate can change the intensity of light transmitting through the analyzer,and α is the angle between the linearly polarized light vibrating surface and the transmission axis of the analyzer.When the angle between the polarizer and the analyzer makes the intensity of the transmitted light at the maximum value,they are called parallel(at this time α=0°);when the angle makes the transmitted light to zero,they are called orthogonal(at this time α=90°).
