The Four Optical Principles of Microscopes
1, Refraction and refractive index
Light propagates in a straight line between two points in a homogeneous isotropic medium. When passing through transparent objects of different densities, refraction occurs due to the different propagation speeds of light in different media. When light rays that are not perpendicular to the surface of a transparent object (such as glass) are emitted by air, the direction of the light rays changes at its interface and forms a refraction angle with the normal.
2, Performance of lenses
Lenses are the most basic optical components that make up the optical system of a microscope. The objective, eyepiece, and condenser components are all composed of a single or multiple lens. According to their different 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 intersects at a point through a convex lens, this point is called the focal plane, and the plane passing through the intersection 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 the "object focal point", and the focal plane at that point is called the "object focal plane"; On the contrary, the focal point in the image space is called the "image focal point", and the focal plane at that point is called the "image focal plane". After passing through a concave lens, light forms an upright virtual image, while a convex lens forms an upright real image. Real images can be displayed on the screen, while virtual images cannot.
3, The key factor affecting imaging - aberration
Due to objective conditions, no optical system can generate theoretically ideal images, and the presence of various aberrations affects the imaging quality. Below is a brief introduction to various aberrations.
1. Color difference is a serious flaw in lens imaging, which occurs when multiple colors of light are used as light sources, and monochromatic light does not produce color difference. White light is composed of seven types: red, orange, yellow, green, blue, blue, and purple. The wavelengths of each type of light are different, so the refractive index when passing through a 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 optical systems is to eliminate chromatic aberration.
Color difference generally includes positional color difference and magnification color difference. The positional color difference causes the image to have spots or halos when observed at any position, making the image blurry. And the magnification chromatic aberration causes the image to have colored edges.
2. Spherical aberration refers to the monochromatic aberration of points on the axis, caused by the spherical surface of the lens. The result of spherical aberration is that after imaging a point, it is no longer a bright spot, but a bright spot with gradually blurred middle edges, which affects the imaging quality.
The correction of spherical aberration is often achieved by using lens combinations. As the spherical aberration of convex and concave lenses is opposite, different materials of convex and concave lenses can be selected and glued together to eliminate it. The spherical aberration of the objective lens in the old model microscope was not completely corrected, and it should be matched with the corresponding compensating eyepiece to achieve the correction effect. The spherical aberration of general new microscopes is completely eliminated by the objective lens.
3. Huixia Huixia belongs to the monochromatic aberration of off-axis points. When an off-axis object is imaged with a large aperture beam, the emitted beam passes through the lens and no longer intersects at a point. The image of a light point will form a dot like shape, resembling a comet, hence the name "coma".
4. Astigmatism is also an off axis monochromatic aberration that affects clarity. When the field of view is large, the object points on the edge are far from the optical axis, and the beam tilts too much, causing astigmatism after passing through the lens. Astigmatism causes the original object point to become two separate and perpendicular short lines after imaging, which are combined on the ideal image plane to form an elliptical spot. Astigmatism is eliminated through complex lens combinations.
5. Field curvature, also known as "image field curvature". When there is field curvature in the lens, the intersection point of the entire beam does not coincide with the ideal image point. Although clear images can be obtained at each specific point, the entire image plane is a curved surface. This makes it difficult to see the entire image surface clearly during microscopic examination, making observation and photography difficult. Therefore, the objective lenses used for studying microscopes are generally flat field objectives, which have already corrected the field curvature.
6. The various aberrations mentioned earlier, except for field distortion, all affect the clarity of the image. Distortion is another type of aberration where the concentricity of the beam is not compromised. Therefore, it does not affect the clarity of the image, but causes distortion in shape when compared to the original object.
