The foundation of the Leica biological microscope
In order to develop instruments with higher resolving power, the scientific research team of Leica Microscope must look for a shorter wavelength 6t illumination substance and a "lens" that can focus and control it. An electron microscope based on the principle of electron optics is such an instrument. The so-called electron optics refers to a discipline that studies and utilizes the deflection, focusing and imaging laws of electron flow. It is based on the following three findings;
(one). J． J． Thomson (1872) proved the existence of electrons;
(two). L． deBroglie's (1923) corollary to the particle-wave duality of matter'
(3). H.Busch (1926) discovered the lensing effect of axisymmetrically distributed electric and magnetic fields on charged particles.
First, let's discuss the illumination substance in the Leica biological microscope-electron flow. According to the above items (1) and (2), we can regard the moving electron flow as an electron wave, which advances toward the direction of electron movement at a constant speed and sinusoidally changes with time. In 1927, D9v confirmed the volatility of electrons more definitely than the electron diffraction phenomenon discovered by On and Germer, and then measured and verified the relational formula. In order to calculate the electron wavelength, we assume that the mass is M and the charge is (one') The electrons have zero velocity. When it passes through a region where the potential changes from o to Yo, the velocity becomes ?. Therefore, the momentum and kinetic energy x of electrons are respectively: Finally, the expression of electron wavelength can be obtained: It should be pointed out that for electrons moving at high speed, their mass will increase with the increase of speed. For example, when the accelerating voltage yo=lookV, the electronic mass characteristic changes by 5%. For this reason the relativistic correction of the electron mass must be considered. The revised formula is: in the formula, the unit of the electron wavelength A is M, and the unit of the relativistic correction voltage vL is held). The following example shows the relationship between electron wavelength and accelerating voltage
Another necessary part of the Leica biological microscope is the lens that can focus the electron beam - the electron lens. In order to qualitatively illustrate its working principle, a simple example can be used, that is, a long hollow cylinder made of a helical coil, also known as a long solenoid. When a current is passed through such a coil, an approximately uniform magnetic field is generated near its central axis. According to the hand rule, this magnetic field is along the pumping (Z) direction. When the high-speed moving electrons (-') enter this field area, they will be affected by the krentan force (evil) of the magnetic field. It is proportional to the cross product value of the electron velocity and the magnetic field strength, that is, ten thousand = one Mx ten thousand. The initial velocity of electrons entering the magnetic field region; It can be divided into two parts to discuss state=seat l. The velocity parallel to the direction of the magnetic field is 5z, and its force with the magnetic field is zero, so the velocity of electrons along the axial direction will not change. The magnetic field force on the velocity component 5L perpendicular to the direction of the magnetic field is not only perpendicular to the direction of the velocity component, but also perpendicular to the direction of the magnetic field, so it is a uniform centripetal force. The final effect is that the electrons move in a uniform circular motion around the central axis while advancing along the toughness, and their spatial trajectory is a helical line.
Leica microscope can prove that electrons with different initial velocities emitted from the same object point (product) will converge on the same image point (Pf) after a certain distance. This is the prototype of the magnetic lens. It should be emphasized that the contact lens has the function of rotating and converging (imaging) the electrons moving at high speed. Electron trajectories in a uniform magnetic field.
Electron lenses in Leica biological microscopes can be electrostatic or (electro)magnetic. It is an electrostatic lens composed of multiple electrodes, which has high requirements for shielding and vacuum systems. At present, (electro)magnetic lenses are mostly used. Only the design and structure of the lens can be different according to different requirements at different positions.
