Optical Instruments
lecture 3
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Dr. Abdallah Ramadan
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3- simple magnifier , cont..
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• Ex1. In case of a simple microscope, the object is placed at
(a)Focus f of the convex lens
(c) Beyond 2f
Ex2.
(b) A position between f and 2f
(d) Between the lens and f
The ratio of the magnifying power of a convex lens used as a
simple microscope when it is used to receive an image at the least
distance of distinct vision to the magnifying power when it is used to
receive an image at infinity is 6:5 If the focal length of the lens
is 4.8 cm,
a) find the least distance of distinct vision
b) 02/11/2023
What is the distance at which
we should keep the lens from3the
Dr. Abdallah Ramadan
object?
Defects in lens: (i) Chromatic aberration
• Magnifying power of a simple microscope is inversely proportional
to the focal length of the lens. What then stops us from using a
convex lens of smaller and smaller focal length and achieving
greater and greater magnifying power?
Defects in lens
• (i) Chromatic aberration : Image of a white object is coloured
and blurred because n –refractive index (hence f ) of lens is
different for different colours. This defect is called chromatic
aberration.
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(ii) Spherical aberration
(ii) Spherical aberration : Inability of a lens to form the point
image of a point object on the axis is called Spherical aberration.
• In this defect all the rays passing through a lens are not focussed at
a single point and the image of a point object on the axis is blurred.
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4) Compound Microscope
A compound microscope consists of two converging lenses called
objective and eye-piece lens.
▪ Gives greater magnification than a single lens
▪ The objective lens has a short focal length, ƒo< 1 cm
▪ The 02/11/2023
ocular lens (eye-piece) has
a focal length, ƒe, of a few cm
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Compound Microscope, Cont.
▪ The lenses are separated by a distance L ,
L=q0+pe
L >> ƒo , ƒe is much greater than either focal length
▪ The approach to analysis is the same as for any two lenses in a row:
The image formed by the first lens becomes the object for the
second lens
▪ The image seen by the eye, I2, is virtual, inverted and very much
enlarged
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Eye-piece lens
Magnification of compound
microscope
compound microscope has
3 magnifications (objective lenses):
❑ Scanning
❑ low
❑ High
Objective lens
Eye-piece lens
Total Magnification
4X
10X
40X
Scanning
10X
10X
100X
Low
40X
10X
400X
high
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Ex1. A compound microscope has a magnifying power 30.
The focal length of its eye-piece is 5cm. Assuming the final
image to be at the least distance of distinct vision. The
magnification produced by the objective will be
(a) +5
(b) – 5
(c) +6
(d) – 6
Ex2. In a compound microscope, the focal lengths of two lenses are
1.5 cm and 6.25 cm an object is placed at 2 cm form objective and
the final image is formed at 25 cm from eye lens. The distance
between the two lenses is
(a) 6.00 cm (b) 7.75 cm
(c) 9.25 cm (d) 11.00 cm
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Ex3. The focal lengths of the objective and the eye-piece of a
compound microscope are 2.0 cm and 3.0 cm respectively. The
distance between the objective and the eye-piece is 15.0 cm. The
final image formed by the eye-piece is at infinity. The two lenses are
thin. The distances in cm of the object and the image produced by
the objective measured from the objective lens are respectively.
(a) 2.4 and 12.0
( b) 2.4 and 15.0
(c) 2.3 and 12.0
(d) 2.3 and3.0
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Resolution
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5) Electron Microscopy
❑Electron microscopes use electrons instead of
photons (visible light) to image cells and
structures.
❑Electrons are accelerated across large voltages
❑A high velocity electron has a wavelength of about
0.003 nm
❑the electron beam is focused, and images are
formed using magnetic lenses
❑reflection and transmission imaging are both
possible
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SEM vs TEM
Scanning Electron Microscopy (SEM)
• an electron beam scans the surface, and the reflected
(backscattered) electrons are collected
• sample must be electrically conductive
• material surface is observed
• 200,000x magnification possible
Transmission Electron Microscopy (TEM)
• an electron beam passes through the material
• thin samples
• details of internal microstructure observed
• 1,000,000x magnification
possible
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Electron optics
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Ex. In an electron microscope if the potential is increased from 20 kV
to 80 kV, the resolving power of the microscope will change from R to
…..
(a) R/4
(b) 4R
(c) 2R
(d) R/2
Ex. Find out voltage applied to an electron microscope to
produce electron of wavelength 0.6˚A
Ex. An electron microscope uses electrons accelerated by a voltage of
50 kV. Determine the de Broglie wavelength associated with the
electrons. If the numerical aperture =0.9, how does the resolving
power
of an electron microscope
compare with that of an optical
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microscope which uses yellow light?
6) Telescopes
• By telescope distant objects are seen.
• Two fundamental types of telescopes
– Refracting telescope uses a combination of lenses
– Reflecting telescope uses a curved mirror and a lens
In both types, two optical elements in
the
image
of Dr. Abdallah
the Ramadanfirst
element
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the object of the second element
a row:
becomes
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Refracting Telescope
❑ The objective forms
a real, inverted image I1
❑ The image is near the focal
point of the eyepiece Fe
❑ The two lenses are separated by
the distance ƒo + ƒe about the
length of the tube
❑ The eyepiece forms an enlarged,
inverted image I1 of the distant
object
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Refracting Telescope
6-1) Astronomical telescope
• Used to see heavenly bodies.
and d objective  deye-lens
• Intermediate image is real, inverted and small.
▪ Final image is virtual, inverted and small.
▪
f objective  feye-lens
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6-2) Terrestrial telescope
❑Used to see far off object on the earth.
❑It consists of three converging lens : objective, eye lens
and erecting lens.
❑It’s final image is virtual erect and smaller.
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