Four optical principles of microscope
1. Refraction and Refractive Index
Light travels in a straight line between two points in a homogeneous isotropic medium. When passing through transparent objects with different densities, refraction occurs, which is caused by the different propagation speeds of light in different media. When the light that is not perpendicular to the surface of the transparent object enters the transparent object (such as glass) from the air, the light changes direction at its interface and forms a refraction angle with the normal.
2. Lens performance
Lenses are the most basic optical components that make up the microscope optical system. Components such as objective lenses, eyepieces, and condensers are composed of single or multiple lenses. According to their shapes, they can be divided into two categories: convex lenses (positive lenses) and concave lenses (negative lenses). When a beam of light parallel to the optical axis passes through a convex lens and intersects at a point, this point is called the "focus point", and the plane passing through the intersection point and perpendicular to the optical axis is called the "focal plane". There are two focal points, the focal point in the object space is called "object focal point", and the focal plane there is called "object focal plane"; conversely, the focal point in the image space is called "image focal point". The focal plane at is called the "image square focal plane". After light passes through a concave lens, it forms an erected virtual image, while a convex lens forms an erected real image. Real images can appear on the screen, but virtual images cannot.
3. Key Factors Affecting Imaging—Aberrations
Due to objective conditions, any optical system cannot generate a theoretically ideal image, and the existence of various aberrations affects the imaging quality. The various aberrations are briefly introduced below.
1). Chromatic aberration Chromatic aberration is a serious defect of lens imaging, which occurs when polychromatic light is used as the light source, and monochromatic light does not produce chromatic aberration. White light is composed of seven kinds of red, orange, yellow, green, cyan, blue, and purple. The wavelengths of various lights are different, so the refractive index when passing through the lens is also different. In this way, a point on the object side may form a color spot on the image side. The main function of the optical system is to achromatize.
Chromatic aberration generally includes position chromatic aberration and magnification chromatic aberration. Positional chromatic aberration makes the image appear blurred or blurred at any position. The magnification chromatic aberration makes the image have colored fringes.
2). Spherical Aberration Spherical aberration is the monochromatic aberration of the on-axis point due to the spherical surface of the lens. The result of spherical aberration is that after a point is imaged, it is no longer a bright spot, but a bright spot with a bright edge in the middle that gradually blurs, which affects the imaging quality.
The correction of spherical aberration is usually eliminated by lens combination. Since the spherical aberration of convex and concave lenses is opposite, convex and concave lenses of different materials can be glued together to eliminate them. For old type microscopes, the spherical aberration of the objective lens is not completely corrected, and it should be matched with the corresponding compensating eyepiece to achieve the correcting effect. Generally, the spherical aberration of new microscopes is completely eliminated by the objective lens.
3). Coma Coma is a monochromatic aberration of an off-axis point. When an off-axis object point is imaged by a large-aperture beam, the emitted beams pass through the lens and do not intersect at one point, then the image of a light point will be in the shape of a comma, which is shaped like a comet, so it is called "coma aberration".
4). Astigmatism is also an off-axis point monochromatic aberration that affects sharpness. When the field of view is large, the object point on the edge is far away from the optical axis, and the beam tilts greatly, causing astigmatism after passing through the lens. Astigmatism makes the original object point become two separated and perpendicular short lines after imaging, and after synthesis on the ideal image plane, an elliptical spot is formed. Astigmatism is eliminated through complex lens combinations.
5). Field curvature Field curvature is also known as "field curvature". When the lens has field curvature, the intersection point of the entire beam does not coincide with the ideal image point. Although a clear image point can be obtained at each specific point, the entire image plane is a curved surface. In this way, the entire image surface cannot be seen clearly during the mirror inspection, which makes it difficult to observe and take pictures. Therefore, the objectives of research microscopes are generally plan objectives, which have been corrected for field curvature.
6). Distortion All kinds of aberrations mentioned above, except field curvature, affect the sharpness of the image. Distortion is another type of aberration in which the concentricity of the beam is not compromised. Therefore, the sharpness of the image is not affected, but the image is compared with the original object, causing distortion in shape.
