Lecture 2 Jan 29 2018 Flashcards
The term microscopy used to describe:
Analytical approach in which a microscope plays a central role in maximizing extraction of useful information from a variety of samples.
Identification (ID) of general unknown is greatly facilitated with a______________________.
Preliminary Microscopical Examination
Applications of the microscope in forensics include:
- Document examination
- Tool mark comparison
- Firearms ID
- Serology
- Drug chemistry
- Trace evidence
Microscope used as a symbol of science but seems less exotic than recent instrumentation, in some cases____________________.
old fashioned
________________and______________are the two main purposes of a microscope too create an enlarged image of an object that is clear and distinct.
Magnification
Resolution
Magnification and resolution conflict with each other:
As magnification increases, resolution can decrease
Greater resolution means greater detail, while greater magnification means ________________.
Larger image
A converging (also known as positive )lens can form images in two ways:
Real image
Virtual image
____________ is formed on the side of the lens opposite the object and is inverted with respect to object, light rays converge at a given plane in space and thus image falls on the screen.
- slide projector
Real image
_____________is when a lens is used as a magnifying glass; rays from this type appear to diverge from a plane on the same side of the lens as the object:
- Use of another lens is necessary to convert virtual into real
- When virtual image is examined visually, the lens of the eyes serves this function so that real image is projected onto the retina
Virtual image
___________________ distance from the lens to the principle focus (for thin lens).
Focal length
If an object placed at infinity, the image will form at:
Principle focus
If object placed at principle focus, image will form at _____________.
Infinity
_________ of a simple lens is defined by a straight line running through the centers of curvatures of the two surfaces.
Axis
The on axis point object at infinity on left side of lens, the lens will form real image in the right-hand space known as:
Principle focus of the lens
_____________(for thick lenses) measured from the image for an object at infinity (focal point for incident rays parallel to lens axis) to nearest principle plane.
Equivalent focal length
Magnification (M) for a real image can be defined by the equation:
M = Q/P
If, P=2F=Q, magnification is:
1
If P>Q, then M<1 :
Means there is a reduction
If P1:
We have conditions for magnification by microscope objective
__________________the departures in performance between actual and hypothetical ideal simple lens.
- Aberrations do not rise from defects introduced from manufacturing process
- Inherent even in perfectly made simple lens
Aberrations
The variation of focal length with distance from the lens axis is called__________________.
Spherical Aberration (fuzzy image)
- Focal length will vary with part of lens being used
- Areas or zones of the lens that are closer to its periphery are stronger that have shorter focal lengths than zones near axis of the lens.
__________________refractive substances disperse light to some extent, hence lenses depart from ideal behavior.
Chromatic Aberrations
- Transparent materials do not refract all wavelengths equally.
________________variation of focal length with wavelength.
Longitudinal chromatic aberration
- Image will have out of focus color fringes at edge of objects.
_______________variation of magnification with wavelength.
Lateral Chromatic Aberration
- Size of an object imaged in blue light may be significantly different from the image formed in red light.
Not to be confused with spherical aberration, which is unlike ___________________, an image cannot be sharply focused for any part of the field no matter how much refocusing is done.
Curvature of Field
- Image defect produced by lens exhibits a moderate of curvature which may not be apparent visually.
- Only possible to get acceptable focus for only one area of the field of view (center, edge etc.)
- When attempt is made to photograph such image, difficulty of recording curved image surface on a flat film plane becomes apparent.
If the magnification is not constant across the field of view, the image will not be an exact representation of the object.
Distortion
______________is when magnification fall off, when moving from the center of the field toward periphery.
Barrel distortion
_______________when magnification increases toward the edge of the field.
Pin-cushion distortion
Magnification takes place in one stage, no more than a magnifying glass.
Simple microscope
Ex: hand lenses, fingerprint magnifiers
Magnification of a simple microscope:
M= least distance of vision + 1
focal length of lens
Magnification happens because the lens allows the image of the object to remain focus when the object is placed closer to the eye increasing the angle and resulting in larger image on the retina.
- Distance of closest vision for normal human eye is about 25 cm.
Simple microscope
Modern ______________have magnification in two stages.
Compound microscope (s)
_______________resembles a projector + magnifying glass.
Compound microscope
Last stage of the magnification in the ______________is essentially the same as in a _____________ ( a virtual image is formed) and it takes place in the eyepiece (ocular).
Compound microscope
Simple microscope
Ocular is used to examine a real image produced by the first image-forming lens system, the objective
- objective produces an enlarged real image of the object.
Compound microscope
Other factors for evaluating a microscope are:
Image quality
Visibility
Resolution