How may the microscope's resolution be increased?
One of the primary testing tools is the microscope, and resolution is a crucial metric for assessing the tool's performance. Resolution is the capacity to discern between two lines or small points distinctly at close range. The eye itself functions as a microscope. The resolution of the human eye at the distance of vision, which is universally accepted to be 25 cm, is around 1/10mm under normal lighting circumstances. Because straight lines can excite a number of nerve cells, the resolution of the eyes can be increased while watching two straight lines.
Since the human eye's resolution is only 1/10mm, it cannot discern between two extremely small objects that are closer than 1/10mm apart. So the development of the optical microscope for microscopic inspection came first, followed by the development of the electron microscope. The shortest distance between two tiny spots that can be clearly distinguished on a specimen is referred to as the microscope resolution. D=0.61/NA is its calculating formula.
In the equation, D stands for resolution in um, for the light source's wavelength in um, and NA for the objective lens's numerical aperture in um (also called the aperture ratio).
The formula indicates that the numerical aperture of the matching objective lens and the wavelength of the incident light source determine the microscope's resolution. As can be seen, the optical microscope can be improved by:
1. Reduce the wavelength of the light source.
Visible light has a shorter wavelength of 390nm. This wavelength of ultraviolet light can degrade the optical microscope's resolution to 0.2um when utilized as the illumination source. The majority of common materials' glass, however, absorbs a lot of light with a wavelength below 340 nm, making it impossible for UV light to produce a clear and brilliant image even after significant attenuation.Since the UV light microscope cannot be seen with the naked eye and even the observed sample, expensive materials like quartz and fluorite, which can pass through ultraviolet light as low as 185nm and 200nm respectively, must be utilized. This technique to increase the resolution of the microscope is not frequently employed because of its own limits, which are compounded by the high cost and restrictions of the microscope.
2. Increase the numerical aperture NA of the objective lens.
Numerical aperture NA=n*sin(u)
In the equation, n stands for the medium's refractive index that lies between the objective lens and the specimen, and u for the objective lens's half-aperture angle. Because of this, from the standpoint of optical design, correctly adopting a bigger aperture angle or increasing the refractive index has emerged as a popular technique to enhance the resolution of an optical microscope.The medium for low magnification objective lenses, such as those with magnifications below 10X, is typically air, which has a refractive index of 1, making it a dry medium. The medium for water immersion is distilled water, which has a refractive index of 1.33. The medium for oil immersion objective lenses, such as the 100X oil lens from Olympus, has a refractive index that is generally around 1.52. Due to the use of high refractive index medium, the water immersion objective lens and oil immersion objective lens not only have high magnification but also enhance the resolution of the objective lens.
