Microscopes

Question 1

Objective lens

Answer 1

• short focal length/group of lenses
• biggest part of the magnification of the microscope
• some lenses remove optical defects of the frontal lens (distortion, chromatic aberration)
• forms true real magnified image

Question 2

Eyepiece lens

Answer 2

Ocular

• two spherical lenses
• forms large virtual image
• final image virtual, magnified and reversed

Question 3

Optical condenser

Answer 3

• collimate the light beam of the light source and to focus it onto the object
• contributes to quality of image

Question 4

Magnification

Answer 4

ratio of the linear dimensions of the image and of the object

• magnification of the optical microscope W_m is given by the product of the linear magnification of the objective W_obj and the angular magnification of the eyepiece W_oc

W_m = W_obj x W_oc

Question 5

Resolution

Answer 5

• ability to distinguish neighbouring objects
• smaller distance, able to distinguish, larger resolution

Question 6

Distance d, in the optical microscope

Answer 6

• is defined by the size and the position of the frontal lens of the objective
• by the refractive index n, of the medium between the frontal lens and the object
• and by the wavelength of the light illuminating the object

d = 0.61 x λ/n.sinα
α - half maximal angle between the incident beam and frontal lens of the objective = aperture angle

Question 7

Numerical aperture

Answer 7

A = n sin α

Question 8

Refractive index

Answer 8

dry object, vacuum, air, = 1

• if space between frontal lens of the objective and the investigated object is filled with a substance with n>1, d decreased and resolution increases
• substance used = immersion liquids, cedar oil n=1.52
• necessary when microscope has high magnification

Question 9

Dark field observation

Answer 9

• used when investigated object is small, transparent, and unstained
• when it strongly disperses the incident light
• light transmitted by object is minimised and only light dispersed by object falls on objective
• object seen bright

Question 10

Phase contrast observation

Answer 10

• used when object is unstained and contains structural elements with different n’s
• inserting suitable rings in condenser and objective, difference between n is transformed into an amplitude difference thus increasing contrast

Question 11

Observation with polarised light

Answer 11

• suitable for investigation of a transparent unstained object containing optically active structures
• object placed between polariser and analyser of the polarising microscope only the optically active structures are visible