Components of Optical Microscope
This is a lens located on the side of the sample and the sample image cannot be observed by magnifying the image without this lens. Leeuwenhoek’s microscope had used the only objective lens. Although the early objective lens was made with the only one lens, the latest objective lens is the combination lens eliminating the aberration that is the cause of the distortion. Also there is a lens called oil immersion lens designed to eliminate the influence of the difference in the refractive index between air and lens by filling the oil having the same refractive index as the glass, between the lens and the sample.
Cause of aberrations is on the shape of the lens itself and also on what the light is refracted at the lens.
Chromatic aberration: Please remember a prism. Newton discovered that the sunlight (white light) splits into the colors of the rainbow if it passes through the prism. When white light passes through the glass, the light has the different refraction angles depending on the wavelengths (color) so that the light having the different color goes through different route and as the result, the colors blur after the light goes through the lens.
Spherical aberration: Surface of the lens was spherical until recently. If the spherical lens is used, the parallel light enters into the outside of the lens and the parallel light enters into the center of the lens are not focused at the same one point. If the lens is designed to focus the parallel lights on the same one point, the curved surface of the lens is no longer spherical. A non-spherical lens (aspherical lens) has been able to be designed recently and it has been applications/ed to the DVD pickups, CD and glasses.
Objective lens is designed to make the aberration smaller by the combination of several lenses.
In the electron microscope, the electromagnet to focus the electrons becomes the objective lens. The lens is not used for a scanning probe microscope.
This is the lens closer to the eye. Since the binocular is used for an optical microscope it is so called binocular or eyepiece. The magnification of the microscope is determined by multiplying the magnification of the objective lens by the magnification of the eyepiece.
Recently, the optical microscope with a CCD camera is becoming popular. With regard to a laser scanning microscope, the image is observed through a detector such as a CCD camera. In this case, the lens in front of the detector acts as the eyepiece.
The diaphragm is placed in front of the eyepiece for a confocal microscopy to eliminate the ambient light that is the cause of the blurred image. The only one point can be seen in this configuration so the scanning function is required to observe the image.
The illumination plays an important role for the optical microscope.
The most common illumination is the bright-field illumination. This is the illumination to fully enter the illumination light into the diameter of objective lens to reduce the extra diffuse reflection light. The image contrast is created by the light absorption rate of the sample for the transmitted illumination and the reflectance of the light for the reflected illumination.
The dark-field illumination is the illumination to introduce the illumination light into just barely around the objective lens and enter the only refracted or diffuse reflected light by the sample into the objective lens.
In the fluorescence microscope, the sample is dyed with a special dye to generate fluorescence. The fluorescence is emitted by the illumination of certain wavelength of light (excitation light). The wavelength of the fluorescence light is different from that of the excitation light so the only fluorescence light can be observed in the dark field by separating the fluorescence light from the excitation light with the filter.
There are other illumination methods such as a polarized and phase contrast illumination.
If there is no sample, nothing is observed. But the sample is not always simply placed in front of the objective lens.
If the oil immersion lens described above is used, the sample should be created as a preparation to demonstrate the effect of the emulsion oil. Furthermore, if the fluorescent observation is practiced, the sample must be dyed with a fluorescent dye as far as the fluorescent material is not already contained. To increase the contrast, the sample is also dyed with the color.
In the transmitted electron microscope, the sample should be precisely sliced to make the very thin film to uniformly pass the electrons through it. There is a dedicated machine to make the thin film sample. In the scanning electron microscope, the non-conductive sample must be coated with a metal to avoid the charge up on the sample surface by the irradiation of the electron beam.