Optical principles and performance of microscopes
The traditional optical microscope consists of an optical system and a mechanical structure to support them. The optical system includes an objective lens, an eyepiece and a condenser lens, all of which are complicated magnifying lenses made of various kinds of optical glass. Objective lens will magnify the specimen image, its magnification M thing by the following formula: M thing = Δ ∕ f 'thing, where f 'thing is the focal length of the objective lens, Δ can be understood as the distance between the objective lens and the eyepiece. The eyepiece will be the image of the objective lens again magnified into an imaginary image in front of the person 250mm for human observation, which is the majority of people feel **** observation position, the eyepiece of the magnification of the M eye = 250/f'eye, f'eye is the eyepiece of the focal length. The total magnification of the microscope is the product of the objective and the eyepiece, i.e., M = M object * M eyepiece = Δ * 250∕f'eye * f;object. It can be seen, reduce the focal length of the objective lens and eyepiece will make the total magnification, which is the microscope can see ** and other microorganisms of the key, but also the difference between its and ordinary magnifying glass.
So, is it conceivable to reduce the f'objective f'eyepiece without limit in order to increase the magnification so that we can see more subtle objects? The answer is no! This is because the nature of the light used to image is a kind of electromagnetic wave, and thus in the propagation process inevitably produce diffraction and interference phenomena, just as the daily seen ripples on the surface of the water when encountering obstacles can be rounded, the two columns of water waves can meet each other to strengthen or weaken the same. When the light waves from a point-shaped light-emitting object point into the objective lens, the objective lens of the rim impedes the propagation of light, diffraction and interference, after the objective lens can no longer be gathered in a point, but the formation of a certain size of the spot, there is also a series of intensity of the periphery of the weak and gradually diminishing halo, we call the centre of the bright spot for the Avery spot, two light-emitting point close to a certain distance when the two spots will overlap until it can not be confirmed for the two spots. Riley proposed a criterion, that when the two spot centre distance is equal to the radius of the Airy spot, the two spots can be distinguished, calculated that the distance between the two light-emitting points e = 0.61 into the ∕n.sinA = 0.61 into the ∕N.A, where the into the wavelength of light waves, the human eye can be received by the wavelength of light waves of about 0.4-0.7 um, n for the light-emitting point of the medium refractive index, where the light-emitting point is located in the refractive index of the light-emitting point. Refractive index of the medium in which the light-emitting point, such as in the air, n ≈ 1, in the water, n ≈ 1.33, and A for the light-emitting point of the objective lens bezel angle of the half, N.A known as the numerical aperture of the objective lens. From the above formula, the objective lens can distinguish the distance between the two points by the wavelength of light and the numerical aperture of the limitations of the human eye, due to the human eye visual * sharp wavelength of about 0.5 um, and the A angle is not more than 90 degrees, sinA is always less than 1, for the available light-transmitting medium * the refractive index of about 1.5, so the e-value is always greater than 0.2 um, this is the optical microscope can distinguish the * smallest limit of the distance. Through the microscope magnification imaging, if you want to be able to have some N.A value of the objective lens resolution of the object point spacing e enlarged to enough to be distinguished by the human eye, it is necessary to M.e ≥ 0.15mm, where 0.15mm for the experimental human eye can distinguish between the two micro-objects placed in front of the eye at 250mm in the distance between the * small, so M ≥ (0.15 ∕ 0.61 into the) N.A ≈ 500N.A, in order to make observation In order to make the observation is not too laborious, M doubled will be enough, that is, 500N.A ≤ M ≤ 1000N.A, is a reasonable selection of the total magnification of the microscope range, and then the total magnification is meaningless, because the numerical aperture of the objective lens has been limited to the * small resolvable distance to increase the magnification has been impossible to distinguish the details of the smaller objects.
