Magnifiers
Perhaps the simplest optical instrument is the lens magnifier. Without optical aid, we cannot “see” things close up. The eye will simply not focus closer than about 0.25 m (unless you are nearsighted!). But an object places just inside the focal point of a converging lens will produce a large virtual image that can be viewed more easily. Let’s look at our Convex Lens again.
Her we can see that the ratio of the heights of the subject and image, the magnification M is
M = hi/hs = i/s
Usually we are able to get good magnification and place the image near 0.25 m if the object is close to the focal point on the object side of the lens. Using i ~ 0.25 m and s ~ f, we get
M ~ 0.25/f
as long as f is measured in meters. (For f in cm, the constant in the numerator is 0.25 x 100 = 25).
Microscopes
The standard optical microscope consists of two lenses (each can be a compound lens). By placing the object to be observed very close to the focal point of the first or objective lens, a larger real (but inverted) image will be produced. This real image is then observed with a second lens, the eyepiece, which acts as a magnifier to make the image even larger. If you wear eyeglasses or reading glasses, removing them will allow you to view the microscope properly.
The net magnification of the entire system is the product of the magnifications of the objective and eyepiece. For these we just use the magnifications given for a simple lens and a magnifier:
M = MoMe = (io/so)(0.25/fe)
Because the image is much larger than the object, it usually requires that the object be brightly lit, or it will be too dark to see well.
Another practical limit on an optical microscope comes from the fact that the wavelength of visible light is so “large”. The fineness of detail that can be observed, measured in radians, is given by Rayleigh’s criterion:
θR = 1.22λ/d
where d is the diameter of the opening through which the light passes (such as the objective), and l is the wavelength of light used. This limitation comes from the wave properties of light. Light passing through a narrow opening undergoes diffraction, which spreads the beam out. Diffraction is basically just the interference pattern of a light wave with other portions within the same opening instead of a different opening.
Better resolution can be obtained using UV light, but that only helps a little.
Electrons have much smaller wavelengths than visible light, and so can be used to see smaller details. This is the basis of the electron microscope, which uses magnetic fields to focus the electrons. However, this is getting off the subject of light & color….
Refracting Telescopes
Telescopes come in many different designs. Those that use a lens as the objective to gather and focus the light are refracting telescopes, while those that do this with mirrors are reflecting telescopes…
Read more below-
