Question

what is compound microscope and its expression

Answer 1

MAGNIFICATION AND RESOLVING DISTANCE It is relatively easy to think of the microscope in terms of magnification; the importance of which is without dispute. However, the importance of magnification is meaningless without a clear crisp image and resolution is a measure of clarity. Resolution is defined by resolving distance. Resolving distance is the smallest distance between two points that allows the observer to see those points as distinctly separate. The most limiting factor in obtaining good resolution is the wavelength of light used for illumination. Bear in mind that the bacteriologist observes objects whose own dimensions are of the same order of magnitude as the wavelength of light. Blue light (360 nm to 420 nm) permits greater resolution than red light (650 nm to 800 nm). Sometimes it is possible to magnify an image beyond the smallest resolving distance of the lens system; this is termed "empty magnification" because although the image is magnified, it is not distinct but blurry and can not be seen as well as if it were magnified to a lesser amount within the resolving distance of the objective. Another factor that determines resolving distance is the refractive index of the medium through which the light rays pass. The refractive index of glass is 1.52 compared to air (N = 1.00). Light rays passing through a glass slide, using the high-dry lens, will pass through air and be bent before reaching the objective lens. Less bending will occur with water than air and even less with oil. The maximal angular aperture of the lens (the angle of greatest divergence of light rays that the objective lens can collect) will not be realized with air. With an oil immersion lens, the air space between the glass slide and the lens is replaced with oil that has a refractive index very close to that of glass. These factors are combined in the Numerical Aperture of the lens. The numerical aperture of a lens is defined as: Numerical Aperture = N (sin α) where "N" is the refractive index of the material between the object and the objective lens and "α" is one half of the angular aperture of the objective lens. If a high-dry lens has an angular aperture of 111o 48'(sin α = 0.828), its numerical aperture working in air (N = 1.00) will be: N.A. = 1.00 x 0.828 = 0.83 If that same lens could be used in oil (most likely it couldn't), it would have a numerical aperture of: N.A. = 1.52 x 0.828 = 1.26 assuming a refractive index of 1.52 for the oil. These calculations are important because of their relationship to the resolving distance. The formula for Resolving Distance is: R.D. = wavelength/(2 N.A.) The reason the numerical aperture is multiplied by two is that two numerical apertures are involved: that of the objective lens and that of the condenser. When the condenser is in perfect focus it has the numerical aperture of the objective. The denominator of the Resolving Distance equation is really the numerical aperture of the objective plus the numerical aperture of the condenser. If one assumes that "average blue light" is being used (wavelength = 400 nm), using an oil lens of numerical aperture of 1.25 will allow you to resolve distinctly objects as small as the resolving distance: R.D. = 400 nm/(2 x 1.25) = 160 nm or 0.16 μm This means that under the most ideal conditions, this lens is capable of distinguishing two objects as separate if they are 0.16 um or greater apart. If the two objects are less than 0.16 μm apart, say 0.10 μm, then they will be blurred together at the point where they are 0.10 μm apart

Answer 2

an optical instruments for forming magnified images of small objects consisting of an object lens with a very short focal length and an eyepiece with a longer focal length both lense mounted in the same tube origin of compound microscope