Chapter 10 Optical Instruments-2.ppt slides

FSC Physics
Chapter 10: Optical Instruments

Least Distance Of
Distinct Vision
Magnifying Power
And Resolving
Power Of Optical
Instruments
Simple Microscope
Compound
Microscope
Astronomical
Telescope
Spectrometer Speed Of Light
Introduction Of
Fibre Optics
Fibre Optic
Principles
Types Of Optical
Fibres
Signal
Transmission And
Conversion To
Sound
Losses Of Power
ECAT Past
Questions

MCQ: An object is placed between to parallel
mirrors. The number of images formed is:
A. 2
B. 4
C. 8
D. infinite

MCQ: An observer moves towards a plane mirror
with a speed of 2 m/s. The speed of the image
with respect to the observer is:
A. 1 m/s
B. 2 m/s
C. 4 m/s
D. 8 m/s

MCQ: A bulb is placed between plane mirrors
inclined at an angle of 60o. The number of
images formed is:
A. 5
B. 6
C. 4
D. 3

Least Distance of Distinct Vision
1. Near Point:
• “Minimum distance from the eye at which object can be seen
clearly”.
• d = Near Point = 25 cm = 10 inch.
• d = Far Point = ∞.
• Near point increases with age.
• If an object is held closer to the near point of
eye than image formed will be blurred and fuzzy.

2. Mirror:
• “A highly polished surface from which most of the light is
reflected”.
i. Types of Mirror:
3. Lens:
• “A piece of transparent medium bounded by two surface at
least one of which is curved”.
• Every lens is a part of sphere.

i. Types of Lens:
ii. Types of Convex and Concave lens:

4. Image:
• “An image which is located in the plane of convergence for
the light rays that originate from a given object”.
i. Types of Image:
a. Real Image:
• “Image made by convergence of light rays”.
• Real image is always inverted.
• Real image can be projected to the screen.
b. Virtual Image:
• “An image formed when the outgoing rays from a point on an object
always diverge”.
• Virtual image is erect.
• Virtual image cannot be projected
on screen.

ii. Number of Images:
• Number of images formed by two mirrors inclined at an angle
′θ′.
• K(number of mirrors) =
360
θ
• Number of image = n = K ( if K is odd )
n= K – 1 ( if K is even )

5. Some important terms:
i. Centre of Curvature:
• “Center of sphere from which spherical surface of lens is
obtained”.
• Every lens has two centers of curvature.
ii. Radius of Curvature:
• “Center of sphere from which spherical surface of lens is
obtained”.
• Every lens has two radii of curvature that may not be equal.
iii. Principle Axis:
• Line joining the two centers of curvature.
iv. Optical Center:
• A point inside the body of lens through which light rays pass
undeviated.

v. Principle Focus:
a. Convex Lens:
• A point of convergence of refracted light rays.
• It is a real point.
b. Concave Lens:
• A point of divergence of refracted light rays.
• It is imaginary point.
vi. Focal Length:
• A distance between principle focus and optical center.
• Convex lens + ve.
• Concave lens – ve.
vii. Aperture:
• The size of diameter of lens.

viii. Ray of Light:
• A straight line path along which the transfer of light energy take
place.
• It is represented by a straight line with arrow marked on it.
a. Types of rays:
ix. Beam of Light:
• A relatively large bundle of rays of light.

MCQ: A man of height 1.6 m wishes to see his full image in
a plane mirror placed at a distance of 2 m.
The minimum height of the mirror should be:
A. 0.5 m
B. 0.8 m
C. 1.6 m
D. 2.4 m

Magnifying Power and Resolving Power of
Optical Instruments
1. Magnifying Power:
i. Linear Magnification:
• “The ratio of size of image to size of object”.
• M =
hi
ho
=
q
p
ii. Angular Magnification:
• “The ratio of the angle subtended by the image as seen through
the optical device to that subtended by the by the object”.
• M =
tanα
tanβ

• When the same object is viewed at a shorter distance, the
image on the retina of the eye is greater, so the object appears
large.
• For very small angle ‘linear magnification’ is equal to ‘angular
magnification’.
2. Resolving Power:
• “Ability to reveal the mirror details of the object under
examination”.
i. Angle of Resolution:
• “The minimum angle that allows two point sources to appear
distinctly separated”.
• It is expressed as ∝min.

∝min = 1.22
λ
D
.
• Resolving power of lens of aperture D, under light source of
wavelength λ is,
R =
D
1.22λ
=
1
∝min
Resolving Power
Human Eye
𝛂 = 𝟏 𝐦𝐢𝐧
R ∝
𝟏
𝛂
Microscope
d =
𝛌
𝟐𝛍𝐬𝐢𝐧𝛉
R ∝
𝟏
𝐝
Telescope
𝛂 =
𝛌
𝟏. 𝟐𝟐𝐃
R ∝
𝟏
𝛂
Grating
R = N × m
R =
𝛌
∆𝛌

MCQ: When object is placed between F and 2F, the
image are formed:
A. At 2 F
B. Between F and 2F
C. Beyond 2F
D. At F

MCQ: What is the minimum and maximum distance
between the convex lens and object for the
magnification of a real image to be greater than 1?
A. f and infinity
B. 2 f and infinity
C. f and 2 f
D. 0 and f

MCQ: The focal length of a simple magnifier is
12.5 cm. Its magnifying power if the final image is
formed at 25 cm in front of the eye is:
A. 2
B. 3
C. -3
D. 1

Simple Microscope
1. Simple Microscope:
• “An instrument used to see objects that are too small for the
naked eye”.
i. Angular Magnification:
• M =
β
α
=
I
O
ii. Magnification for Real and Virtual Image:
• For real image:
• M =
q
p
=
d
p

• For virtual image:
• M = 1 +
d
f
iii. Position of Object and Image:
Sr. no. Position of Object Position of Image
1. At infinity At focus
2. At focus At infinity
3. Beyond 2F Between F and 2F
4. At 2F At 2F
5. Between F and 2F Beyond 2F
6. Between F and optical centre On the same side as the object

MCQ: In compound microscope, the objective lens
produces:
A. Real, magnified and erect
B. Virtual, magnified and erect
C. Real, magnified and inverted
D. Virtual, magnified and inverted

MCQ: The magnifying power of a compound microscope is
33. It uses an eyepiece of focal length 2.5 cm. The
magnifying power of objective is:
A. 33 / 2.5
B. 33 / 11
C. 2.5 / 33
D. 2.5 / 11

MCQ: A compound microscope has a magnification of 24.
The focal length of the eye piece is 5 cm. If the final
image is formed at the least distance of distinct
vision, the magnification produced by the objective
is:
A. 3
B. 4
C. 5
D. 12

Compound Microscope
1. Statement:
• “Microscope which have high magnification”.
2. Construction:
i. Object lens:
• Short focal length
• Real image.
ii. Eye – piece:
• Longer focal length
• Virtual image.

3. Magnification:
• M = M1M2
• M =
q
p
1 +
d
f𝑒
• M =
L
fo
1 +
d
f𝑒
L = qo + pe
• The values of M are marked as × 5, ×10, ×40 etc.
4. Image:
• Virtual.
• Inverted.
• Magnified.
5. Resolving power:
• Width of the objective.
• Wavelength.

MCQ: If a convex lens of focal length f is cut into
identical halves along the lens diameter, the
focal length of each half is:
A. 2 f
B. f / 2
C. f
D. Zero

MCQ: The focal length of a simple magnifier is
12.5 cm. Its magnifying power? If the final
image is formed 25 cm in front of the eye is:
A. 2 f
B. f / 2
C. f
D. Zero

MCQ: Two convex lenses (f1 , f2) are joined side by
side their focal length will reduce to half if:
A. f1 < f2
B. f1 = f2
C. f1 > f2
D. f1 = f2 = ∞

MCQ: Magnification of a compound microscope is 10. If
magnification of eyepiece is 5 cm and focal length of
objective is 8 cm. Then the length of microscope is:
A. 4 cm
B. 16 cm
C. 400 cm
D. 32 cm

MCQ: If a lens is immersed in water its focal length
will be:
A. Increase
B. Decrease
C. Remain same
D. None of these

Astronomical Telescope
1. Statement:
• “An optical device used for viewing distant objects”.
• In telescope visual angle is bigger than angle made by naked
eyes.
• It is used for astronomical observation.

2. Construction:
i. Objective lens:
• Long focal length ‘fo’.
• Formed real, inverted and diminished image.
ii. Eye piece:
• Short focal length ‘fe’.
• Magnifying the real image.
3. Image:
• Virtual
• Enlarge
• Inverted

4. Magnification Power:
M =
fo
fe
• Normally the distance between objective and eye-piece is
equals to the length of the telescope.
L = fo + fe
• Good telescope has an objective of long focal length and
large aperture.

Spectrometer
1. Statement:
• “An optical device used to study spectra from different
sources of light”.
2. Uses of Spectrometer:
• Deviation of light.
• Refractive index of material.
• Wavelength of light.

3. Components of a Spectrometer:
i. Collimator:
• It contain metallic tube with convex lens at one end and an
adjustable tube at the other end.
• “It makes the light beams parallel”.
ii. Turn Table:
• A prims (for rotating vertical axis).
• A circular scale (graduated in half degree).
iii. Telescope:
• Telescope is attached with vernier scale
and rotatable with vertical axis.

Speed Of Light
1. Measurement of Speed of Light:
• 1st measured by Galileo
• Measured accurately by Michelson
2. Michelson’s Formula:
• c = 16fd
• f= frequency of rotation of octagonal mirror
• d= distance between plane mirror and octagonal mirror
• c = 3.00 x 108 ms-1

3. Speed of Light:
i. In Space:
• 3 x 108 ms-1
ii. In other Materials:
• Always less than 3 x 108 ms-1

Introduction Of Fibre Optics
1. Optical Fibre:
• A device that propagates signals using light
• “A flexible, transparent fiber made by drawing glass (silica) or
plastic to a diameter slightly thicker than that of a human
hair”.

2. Photophone:
• Transmit voice message via beam of light
• Invented by Graham Bell
• Light is used as a transmission carrier wave
3. Introduction:
• Alexander Graham Bell invented ‘photo phone’.
• Bell transmit voice message via a beam of light.
4. Advantages:
• Light use as transmission carrier wave due to;
• Wider bandwidth capability.
• Immunity from electromagnetic interference.

5. Importance of Fibre Optics:
• 1000’s telephone calls, several TV channels through 1 or 2
hair - thin threads.
• Word processing, image transmitting and receiving
equipment's to operate efficiently.
• Much thinner and light weight cables.
• Optical fibre = 6.0 mm.
• Copper wire = 7.62 cm

Fibre Optic Principles
1. Principle of Transmitting Signals in Optical Fibres:
i. Total Internal Reflection:
• “A phenomenon which occurs when a propagating wave strikes
a medium boundary at an angle larger than a particular critical
angle with respect to the normal to the surface”.
• n =
C
V
• n1sinθ1 = n2sinθ2
• sinθc =
n2
n1
• For glass θc= 41.8o.

ii. Continuous Refraction:
• Light is refracted continuously.
• Density decrease from centre to boundary.

Least Distance Of
Distinct Vision
Magnifying Power
And Resolving
Power Of Optical
Instruments
Simple Microscope
Compound
Microscope
Astronomical
Telescope
Spectrometer Speed Of Light
Introduction Of
Fibre Optics
Fibre Optic
Principles
Types Of Optical
Fibers
Signal
Transmission And
Conversion To
Sound
Losses Of Power
ECAT Past
Questions

Types Of Optical Fibres
1. Single Mode Step Index Fibre:
• Core 5 μm diameter.
• Cladding (glass or plastic)
• Monochromatic light source
• 14 TV channels
• 14000 phone calls.

2. Multimode Step Index Fibre:
• Mode of transmission is total internal reflection
• Useful for short distances
• Core 50 μm diameter.
• White light.
• Short range.
• Refractive Index;
• Core = 1.52
• Cladding = 1.48

3. Multimode Graded Index Fibre:
• Core diameter is 50 -100𝜇m
• No noticeable boundary
• Mode of transmission is continuous refraction
• suitable for long distance travel

Signal Transmission And Conversion To Sound
1. Components of Fiber Optics Communication System:
i. Light Source:
• Semiconductor laser and LED (invisible infra-red signals)
ii. Transmitter:
• Convert electrical signals to light
iii. Optical Fibre:
• For guiding of signals
iv. Receiver:
• Capture light signals at other end of the fibre
• Reconvert to electrical signals

2. Digital Modulation of Light Waves:
• LED flashed on and off on extremely fast rate
• On  1
• Off  0
i. Coding:
• Represented by a particular pattern of 1s and 0s
ii. Decoding:
• 1s and 0s pattern is decoded in analogue form i.e picture, voice,
video
iii. Unit of Digital Modulation:
• bits (a 1 and a 0)
• megabits

v. Flaws of Signal Transmission:
• Signals become dim due to power loss
vi. Measures to Reduce Power Loss:
a. Repeater:
• Devices used to regenerate signals
• Placed every 30-100km
b. Photodiodes:
• Convert light signals to electric signals
c. Amplifiers:
• Amplification of electric signals

Losses Of Power
1. Reasons:
• Absorption of light due to impurities
• Scattering of light due to group of atoms at joints
• Narrow band of radiation refracted in different directions
2. Consequences:
• Distorted information transfer

3. Measures to reduce Power Loss:
• Use graded index fibre instead of step index fibre

MCQ: From going one medium to another which
does not change:
A. Frequency
B. Speed
C. Wavelength
D. All
ECAT
2012

MCQ: A convex lens of 20 cm, focal length is used to
form an erect image is twice as large as the object.
The position of the object from the lens is:
A. 10 cm
B. 30 cm
C. 20 cm
D. 15 cm
ECAT
2012

MCQ: In astronomical telescope the final image is
formed at:
A. Near point
B. Far point
C. Infinity
D. None of these
ECAT
2011

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