Geometrical optics, microscopy, electron microscopy Flashcards
Define the index of refraction!
The index of refraction (n) gives the speed of light (c) in a given material according to the following equation:
c = c0/n , where c0 is the speed of light in vacuum.
Write Snell’s law of refraction!
A light beam is refracted when it travels from a material with a refractive index of n1 into a material with a refractive index of n2. Refraction is described by the following equation:
sin alpha/ sin beta = c1/c2 = n2/n1 , where alpha and beta are the angles of incidence and refraction, respectively, c1 and c2 are the speeds of light in the two materials.
What is the shortest resolvable distance in a light microscope?
approximately 200 nm
How can the resolving power of a microscope be increased?
-by decreasing the wavelength of light
-by increasing the index of refraction of the material between the objective and the object
-by increasing the half angle of the objective
What is numerical aperture?
It is the product of the index of refraction of the material between the object and the objective (n), and the sine of the half angle of the objective (sin alpha): n sin alpha
Give the formula for the resolving power (f) of a conventional light microscope!
f=1/d=2n sin alpha/ lambda, where: n = refractive index of the medium between the coverslip and the objective, alpha = half angle of the objective, lambda = wavelength of light, d = the minimum distance between two points at which they are resolvable.
What is the function of the dichroic mirror in a fluorescence microscope?
It reflects the excitation light, and is transparent for the emitted photons, therefore it separates the excitation and emission light paths.
What is the function of the excitation filter in a fluorescence microscope?
It is transparent only in the wavelength range in which the fluorescent dye can be excited, therefore it allows only those photons to reach the sample which can excite the fluorescent molecule.
What is the function of the emission filter in a fluorescence microscope?
It is transparent only in the wavelength range in which the fluorescent dye emits photons, therefore only the photons emitted by the fluorescent dye will reach the detector.
List at least three of the imaging aberrations in optical systems!
-chromatic aberration -spherical aberration -astigmatism
-coma
-curvature of the field of the image
-barrel-shaped and cushion-shaped distortion of the image
Give the equation for the relationship between the image distance (i), object distance (o) and the focal distance (f)!
1/I + 1/o = 1/f
Give the definition and SI unit of diopter!
D (diopter)=1/f, is the refractive power of the lens, where f is the focal length of a given lens.
SI unit: 1/m.
What were those two discoveries that made construction of an electron microscope possible?
-an electron can be regarded as a wave, and its wavelength is only a fraction of the wavelength of visible light
-an electron beam can be focused with a magnetic field
List at least three signals that can be detected during an electron microscopic measurement!
-back-scattered electrons
-secondary electrons
-characteristic X-rays -Auger electrons -absorbed electrons -cathode luminescence -transmitted electrons
What are the two types of electron microscopes?
transmission electron microscope (TEM) scanning electron microscope (SEM)
