Page 77 - Fundamentals of Light Microscopy and Electronic Imaging

60 LENSES AND GEOMETRICAL OPTICS

• Place a sheet of paper (a paper screen) at the 30 cm location to confirm that
the intermediate image of the letter a is both real and magnified. Notice that
the real intermediate image is both inverted and upside-down. Confirm
that it is necessary to position the lens between 1 and 2 focal lengths away
from the object to obtain a real intermediate image of the object.
• Mount the ocular at a position that gives a magnified image of the letter when
looking down the axis of the microscope through the ocular. The eye might
have to be positioned several inches behind the ocular to see the specimen
clearly. Note the image–ocular lens distance.
• Place a paper screen in the plane of the virtual image to confirm that it is
indeed virtual—that is, no image is produced on the screen. You have now
created a compound light microscope!

Answer the following questions about your microscope:

1. Is the object located precisely at the focal length of the objective lens?
2. Is the real intermediate image located precisely at the focal length of the
ocular?
3. Explain why the eye-brain perceives a magnified virtual image, while the
retina receives a real image from the ocular lens.

Lens Aberrations
2
Prepare a specimen consisting of a 5 cm piece of aluminum foil with a dozen
small pinholes contained within a 5 mm diameter circle, and mount it on the opti-
cal bench microscope. The ideal specimen has some pinholes on or close to the
axis and other pinholes near the periphery of the field of view. What lens aberra-
tions do you observe while examining the pinhole images? Here are some tips:

• Chromatic aberration: Move the objective lens back and forth through focus
and notice how the fringes around the points of light change color depending
on the lens position.
• Spherical aberration: Pinholes at the periphery of the visual field look
blurry. The blurriness can be reduced by creating a 5 mm hole in an opaque
mask and placing it in the back focal plane of the objective lens.
• Coma and astigmatism: At best focus, peripheral pinholes look like streaks
with comet tails that radiate from the center of the field (coma). As the objec-
tive is moved back and forth through the plane of best focus, the streaks
become drawn out into elliptical shapes that change their orientation by 90°
on each side of focus (astigmatism).
• Distortion and curvature of field: At best focus, the focal plane is curved like
the shape of a bowl, so only one zone of a certain radius can be focused at
one time (curvature of field). To view pincushion or barrel distortion, replace
the pinhole specimen with a fine mesh copper grid used for electron
microscopy and examine the pattern of the square mesh of the grid on the
viewing screen (distortion).