Microscope is an optical instrument consisting of lens for making enlarged or magnified images of minute objects. Robert Hook (1635 – 1703) made and used a first compound microscope. The basic difference between simple and compound microscope is that simple microscope uses only one lens whereas, compound microscope uses more than one lenses to further magnify the object.
The ability of microscope to distinguish two adjacent object as separate and distinct images rather than single blurred image is called resolving power of microscope. Theoretically, size of image can be increased by adding lenses. Magnification of object is only useful if the enlarged image is clearly visible. So, effective magnification depends on resolving power of microscope.
A microscope’s resolving power is dependent on wavelength of beam for illumination and optical quality of lenses. Shorter wavelength gives better resolution. Resolution or shortest distance (d) can be calculated by using the formula, d = λ/2nsinϴ where, d = resolution, λ = wavelength of light, n = refractive index and ϴ = angle subtended.
Note: Oil is used with oil immersion lenses because it has higher numerical aperture than air and also higher refractive index and this greatly improves resolution. Read more