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CHAPTER 10
GEOMETRICAL OPTICS
Contents:
1.
WHAT IS OPTICS?
2.
TECHNECAL TERM DEFINITIONS.
3.
PATH OF RAYS AND FORMATION OF IMAGE.
4.
POSITON AND NATURE OF IMGE.
5.
THIN LENS EQUATION AND MAGNIFICATION.
6.
MAGNIFYING GLASS AND ITS MAGNIFYING POWER
7.
COMPOUND MICROSCOPE.
8.
TELESCOPE. AND KIND
9.
EYE ANOTOMY
10. EQUATIONS
11. DIMENTIONS
12. SHORT DEFINATIONS
13. SUMMARY
14. SHORT QUESTIONS AND ANSWERS
What is OPTICS: 
Optics is the study of the behavior and properties of light including its interactions with matter and its detection by
instruments. Optics usually describes the behavior of visible, ultraviolet, and infrared light.
GEOMETRIC OPTICS: In this branch of optics some simple laws are established on the basis of experiments. In this branch light
is supposed to travel in straight line path and reflection and refraction of light from materials is studied.
PHYSICAL OPTICS: In this branch the production and propagation and emission of light is studied. In this branch, the nature of
light and related events like interference, diffraction and polarization are studied.
1: TECHNECHAL TERM RELATIVE DEFINITIONS: 
i) Ray of light:
Light travels in a straight line from a light source. Thus the path of light can be represented by straight line. The
direction can be shown by head arrow. The straight line along which the light travels is called ray of light.
 ii) Beam of light:
A light source gives infinite rays of light energy simultaneously. A group of rays which travel along a certain direction is
called beam of light. The beam of light can be of the three types: 1) convergent beam: A beam whose all rays go to meet at a
point. 2) Divergent beam: If the rays of light starts from appoint and spread away from it. 3)
Parallel beam: If all the rays in a beam are parallel to each.
 iii) Mirror:
A mirror is an object with at least one polished and other reflective surface. The most
familiar mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to
produce magnified or diminished images or focus light or simply distort the reflected image.
The image in a plane mirror is:
1) The same size as the object,
2) The same distance behind the mirror as the object is in front,
3) Laterally inverted,
4) Virtual (it can not be formed on a screen).
iv) Lens:
A lens is an optical device with perfect axis, which transmits and refracts light, converging or
diverging the beam. A spherical transparent medium made of glass or plastic that refracts light
waves. Such medium is called “Lens”. The lens can bring light waves together or spread them
apart. Hence, there are two types of lenses one of them is called “convex lens”, while the other is
“concave lens”.
v) Thin lens:
It is a lens with a thickness (distance along the optical axis between the two surfaces of the lens)
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that is negligible compared to the focal length of the lens. Lenses whose thickness is not
vi) Convex lens:
The lens is thicker at the middle and thinner at the edges. Such lens is called “Convex
lens”. A convex lens brings light waves together. This type of lens is also called Double-convex lens.
A lens has one side plane while the other convex is called Plano-convex lens.
A lens has one side concave while the other convex is called Concavo- convex lens.
viI) Concave lens:
The lens is thinner in the middle and thicker at the edges. Such lens is
called “concave lens”. A concave lens spreads light waves apart. This type of lens is
also called Double-concave lens.
A lens has one side plane while the other convex is called Planoconcave lens. A lens has one side concave while the other convex is called
Convexo- concave lens.
viiI) Pole:
A mid point of a lens surface is called “pole” or “optical center.”
Ix) Principal axis:
An imaginary line that passes from a pole of a lens is called principal axis.
x) Focus:
A single point at which rays of light or other radiation converge or from which they
appear to diverge, as after refraction or reflection in an optical system is called the focus of a
lens denoted by “ F ”.
xi) Focal length:
The distance from the focal point of a lens or mirror to the surface of the mirror or the
centre of the lens is called “focal length”, denoted by “ f ”.
xii) Radius of curvature:
f
The focal length of lens is half its “radius of curvature, R = .
2
xiii) Object distance:
A distance from object placed to pole of a lens, such distance is called “object
distance” denoted by “p”.
xiv) Image distance:
A distance from image formed to pole of a lens, such distance is called “image
distance” denoted by “q”.
xv) Aperture:
In optics, an aperture is a hole or an opening through which light travels. More
specifically, the aperture of an optical system is the opening that determines the cone angle of
a bundle of rays that come to a focus in the image plane.
xvi) Image:
If the rays of light starting from an object after reflection or refraction meet at appoint or appears to come from a point, then
this second point miscalled called image point of the first point. Thus the image of each point of the object is formed. The images
are of two types: 1)REAL IMAGE :If the rays of light starting from a point after reflection or refraction actually meet at appoint
then this point is called the real image point . Such image can be taken on the screen. 2)VIRTUAL IMAGE: If the rays of light
starting from a point after reflection or refraction appear to come from second point then this point is called the virtual image of the
first point. Such image can not be formed on the screen.
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xvii) PATH OF RAYS THROUGH CONVEX LENS:
1) Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens
and passes through the focus on the opposite side of the lens.
2) Any incident ray passing through the focus on
the way to the lens will refract through the lens
and travel parallel to the principal axis.
3)An incident ray which passes through the center
of the lens will in effect continue in the same
direction that it had when it entered the lens.
xviii) FORMATION OF IMAGE THROUGH CONVEX LENS:
The image formed by a converging lens can be made using only three principal rays.
Suppose an object AB is placed at a distance “p” from a pole of convex lens. According to the
ray rules, the rays from the object point A, are converge at a point A’; such point is called
image point. Hence, we get complete image A’B’ of an object, at a distance “q” from a pole of
converging lens.
xix) POSITION AND NATURE OF IMAGE THROUGH CONVEX LENS:
1) Suppose an object is at infinity or (p=), its image will be located at focus(q=f). The nature of image
will be real, inverted and very small in size.
2) Suppose an object is away from 2F or (p2f), its image will be located away from F or (2f<q<f). The
nature of image will be real, inverted and small in size.
3) Suppose an object is on to 2F or (p= 2f), its image will be located at 2F or
(q=2f).The nature of
image will be real, inverted and equal in size. Hence, p=q=2f.
4) Suppose an object is between 2F and F or (f<p<2f), its image will be located away from 2For
(q2f).The nature of image will be real, inverted and large in size.
5) Suppose an object is on to F (p=f), its image will be real,
inverted and very large in size located at infinity or (q=).
6)
Suppose an object is between focus and pole of lens
(p f), its image will be located to the same side of object. The
nature of image will be virtual, erect and magnified. Hence a
convex lens is called “diverging lens”, or simple microscope or
magnifying glass.
xx): PATH OF RAYS THROUGH CONCAVE LENS:
1)
If an incident ray is passing from a focus, after refraction
it diverges and appears to come parallel to principal axis.
2)
If an incident is parallel to principal axis, after refraction it
diverges and appears to come from focus.
3) If an incident ray is passing from a pole of lens; it passes away with out refraction.
xxi):FORMATION OF IMAGE THROUGH CONCAVE LEARNS:
The image formed by a diverging lens can be made using only three principal rays:
Suppose an object AB is placed in front of concave lens at a distance “p” from its pole. According to
ray rules the rays from a point A diverge after refraction and appears to come from a point A’,
such point is called “image point”. Hence, we get image A’B’ at a distance “q” from a pole.
xxiii) POSITION AND NATURE OF IMAGE THROUGH CONCAVE LENS:
Suppose an object is placed anywhere on to principal axis of concave lens. Its image always
located between focus and pole of lens, to the same side of object placed. The nature o f image
will be always virtual, erect and diminished.
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DESCRIPTIVE
PART
 2: THIN LENS EQUATION: 
The combined relation between object
Distance (p), image distance (q) and focal length (f), is called “thin lens Equation”.
i) Convex lens equation:
Let us consider an object OÓ high is placed between 2F and F at a distance “p” apart, from pole
of a convex lens. Its real and inverted image Iľ high is to be located at a distance “q”, away from 2F.
Consider OPO’ and ŀPI’ right-angled triangles .Both triangles are similar.
Therefore,
Tan P =
OO'
OP
And
Tan P =
II'
IP
By comparing, we get,
OO' II'
=
OP IP
IP
II'
=
OP OO'
q II'
- - - - - - - eq.(1)
=
p OO'
Consider PFE and IFI’ right angled triangles. Both triangles are similar
FP
Tan F =
PE
FI
Tan F =
And
II'
FP
FI
=
By comparing, we get,
PE
II'
II'
FI
=
- - - - - - - eq.(a)
PE
FP
From the fig., OO’ = PE, FP = f and Fŀ = q – f
II'
q-f
=
Therefore
- - - - - - - eq.(2)
OO'
f
q
(q-f)
By comparing equation #s 1 and 2, we get,
=
p
f
qf=P(q–f)
qf=pq– pf
Divide both sides by p q f, we get,
qf
p qf
=
pq
pq f
-
pf
p q f
1
1 1 1
1
1
= = + This equation is called “ convex lens equation”.
p
f q f
p
q
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 ii)Concave lens equation: 
Let us consider an object OÓ high is placed between 2F and F at a distance “p” apart, from pole of a
convex lens. Its real and inverted image Iľ high is to be located at a distance “q”, away from 2F.
Consider OPO’ and ŀPI’ right-angled triangles .Both triangles are similar.
OO'
Tan P =
Therefore,
OP
And
Tan P =
By comparing, we get,
II'
IP
OO' II'
=
OP IP
IP
II'
=
OP OO'
q II'
- - - - - - - eq.(1)
=
p OO'
Consider PFE and IFI’ right angled triangles. Both triangles are similar Therefore,
FP
Tan F =
PE
FI
Tan F =
And
II'
FP
FI
=
By comparing, we get,
PE
II'
II'
FI
=
- - - - - - - eq.(a)
PE
FP
From the fig., OO’ = PE, FP = f and FI = ( f – q )
 f - q  - - - - - - - - eq.(2)
II'
Therefore,
=
OO'
f
f - q
q
By comparing equation #s 1 and 2, we get,
=
p
f
qf=p(f–q)
qf=pf–pq
qf
Pf
pq
Divide by p q f, both sides we get,
=
pqf pqf
pqf
qf
p qf
=
pf
p q f
-
pq
pq f
1 1
1
=
p
q
f
1
1
1
This equation is called “concave lens equation”.
- =p
p
f
NOTE: 1) The object distance is positive, for real objects, and negative for virtual objects.
2)
The image distance is positive for real images and negative for virtual images.
3)
The focal length is positive for convex lens and negative for concave lens.
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3 :MAGNIFICATION: 
Magnification is the process of enlarging something only in appearance, not in physical
size. This enlargement is quantified by a calculated number also called magnification. When this number is
less than one it refers to a reduction in size, sometimes called magnification.
Mathematically it is, “The ratio between sizes of image to the size of object”.
size of image ( Hi )
Magnification =
size of object ( H o )
H
q
Mag: = i =
Ho p
 4: MAGNIFYING GLASS: 
The simple converging lens is called “Magnifying glass”. It is used to see small objects enlarge.
How large an object appears, depends on the size of the image, it makes on the retina of an eye. This depends on the
angle subtended by the object at the eye. As an object bring up close to our eyes so that it subtended a greater angle. However,
our eyes can accommodate only up to a standard distance of 250 mm called “least distance of distinct vision” denoted by “d”. The
converging lens when acts as the magnifying glass then virtual image, which must be at least 25 cm from an eye, if the eye is to
focus on it.
Magnifying power:
“The ratio of the angle subtended by a object when using the lens (), to the angle subtended using the eye () with an
object at least distance of distinct vision”.
β
Magnifying power =
α
Magnifying power of magnifying glass:
A convex lens is called “magnifying glass”, when object is placed in the range of focal length.
1
1 1
We know that ,
= + , only for convex lens.
f
p q
1
1 1
for virtual image, in convex lens as diverging lens.
= f
p q
1
1 1
for magnifying glass.
= f
p d
multiply both sides by “d”.
d
d d
= f
p d
d
d
= -1
f
p
d
 d
 f + 1 = p
d

M.P.=  + 1
f

5:COMBINATION OF LENSES: 
When two or more lenses are combined this is way to design an optical instrument and to increase the
magnifying power. A light passes through several lenses; the image formed by one lens becomes the object for the next lens.
Total magnification will be the product of the separate magnification of each lens.
Suppose an abject PQ is placed at a distance p1 apart from first lens. It’s real inverted image P’Q’ is located at q1 ,
determined by lens equation,
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1
1
1
=
+
f1
p1 q1
The second lens is placed at L distance apart from the first. Thus L= ( q1 + p2 ).The final image P’’Q’’
located through second lens will be at a distance q2 apart, which can be determined from,
1
1
1
=
+
f2
p2 q 2
Where p2 = ( L- q1) and f1 , f2 are the focal lengths of first and second lens respectively.
Now suppose two lenses are placed in close contact the separation between them is negligible or less than their focal lengths.
A point object O is placed at p from the first lens of focal length f1 .Its real point image l located across the second lens at q1 ,
which can be determined by
1
1
1
=
+
- - - - - - -  eq.(1)
f1
p
q1
This real point image becomes virtual object for the second lens; there is
zero separation between the lenses. Hence q1 = - p2.The final point image
ľ is located at a distance q from the second lens determined by,
1
1
1
=
+
f2
p2 q 2
1
1
1
=+ ------------- eq. (2)
f2
q1 q
Add equation #s I and 2, We get,
 1 1
1 1
1
1
+ =
+ + + 
f1 f 2
p
q1
 q1 q 
1
1
1 1
+
=
+
f1 f 2
p
q
1
1 1
+
=
If,
f1 f 2 f
1
1 1
=
+
Then,
f
p
q
This shows that two lenses act as the single lens have “f” focal length.
6:POWER OF LENS: 
The reciprocal of focal length of a lens is called “power of lens” expressed in meter.
1
Power of lens =
f
It’s unit is “diopter” expressed in meter. Shorter the foal length, greater the power of lens.
7: POWER ACCOMMODATION: 
The ability of an eye to change the focal length of its lens so as to from a clear image of an object on its retina is
called its “power accommodation”.
8:DEFECTS OF LENS: 
Chromatic aberration:
The dispersion of light into its constituent colors gives rise a blurred image, which appears
colored. This defect in the image is called “Chromatic aberration”. A lens acts like two prisms
placed end to end. This gives rise to dispersion of light. Chromatic aberration can be removed by
using a compound lens consisting of a convex lens of one type of glass and concave lens of
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dispersion. The convex lens produces both deviation and dispersion. The concave lens minimized the
deviation and neutralizes completely the dispersion. . Such a combination of lenses is called “Achromatic
lens” (color corrected lens).
Spherical aberration:
The rays, which pass through a lens near its edges, are brought to focus closer to
the lens as compared to the central rays. The image produced will not be well
defined and sharp. This defect in a lens is called “Spherical aberration”. This
defect can be removed by using only the central portion of the lens. A lens
system that is corrected spherical aberration, chromatic aberration and other
an “Astigmatic lens”, whish is used in costly cameras and other optical
distortion is called
instruments.
9:COMPOUND MICROSCOPE: 
Introduction:
optical instrument used to increase the apparent size of an object It has multiple magnification. The compound
microscope was invented by Galileo in 1610 . The lenses with shortest focal length gives the greatest magnification and small
objects are seen enlarged.
Construction:
It consisted of an objective lens and an eyepiece. The objective of microscope is of smaller focal length f o and
eyepiece has a greater focal length fe than the objective (fo< fe ). Both lenses are kept at the two ends of tube.
Working:
A very small in size object is placed between the focus and twice the
distance of the focal length of objective, say very close to the focal length p1  fo .
This lens forms an inverted, magnified real image in front of eyepiece. The
eyepiece is so adjusted that it forms a virtual magnified image of already inverted
image, say L  q2. The eyepiece acts as the magnifying glass.
Magnification of compound microscope:
A compound microscope consists two converging lenses, so that it has multiple
magnifying power. Hence,
M.P. = (M.P)o ( M. P. )e
q  q 
M.P.=  1   2 
 p1  o  p 2  e
 L 
d 
M.P. =    1+
fe 
 fo  
, because eye piece acts as the magnifying
glass.
As larger the focal length of objective than, the smaller focal length of
eyepiece, the magnification will becomes very shorter.
10 : TELESCOPE: 
Introduction:
A telescope is a device used to magnify distant objects. An
object, which is far away, viewed by an optical instrument, known as
“Telescope”. The function of a telescope is to increase the visual angle
which a distant object appears to subtend at the eye and therefore
produces the same effect as if the object were either larger or close to the
eye. A Telescope is used to magnify objects that are very far away.
Construction:
A Telescope is used to magnify objects that are very far away. It contains two converging lens located at opposite ends of
a long tube. The lens closest to the object is called the “objective” and a lens close to an eye is called “eyepiece”, which acts as a
magnifier.
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There are number of kinds of telescope, some of them are: Astronomical telescope, reflecting telescope,
radio telescope, Galilean telescope, terrestrial telescope.
Astronomical Telescope: 
An astronomical telescope used to see
distant objects like stars, planets, moon etc., which is effectively at infinity. In refracting astronomical
telescopes two convex lenses are used. In this telescope only those rays from the object, which are
bounded by the edges of the objective lens enter the instrument. The lens acts as a stop to the light from the
object with a given objective, the best position of the eye is one where it collects as much light as possible
from that passing through the objective.
Working:
A converging lens of larger focal length, which is used as the objective lens fo .It will form the image of a distant object at
infinity as close to the eye as we may desire. The image II’ is real. Another converging lens of smaller focal length f e ( f o >f e )
used as the eye piece near the image formed by objective. The image I and focal planes of both lenses are in conjunction and the
light coming out from the eyepiece is parallel. The final virtual, erect and very large in size image Iľ is at infinity by viewing eye.
Thus, the distance between two lenses is (fo+fe) for an object at infinity.
Magnification of Telescope:
The magnifying power of a telescope is defined as the ratio between the angle subtended at the eye (  ) by the
final image and the angle subtended at eye (  ) by the object itself .
β
Magnifying power =
α
α=
β=
Size of image OI
focal length of objective q1
Size of image OI
focal length of eyepiece p 2
Because, f o  q1
Because, fe  p2
OI
fe
Magnifying power =
OI
fo
focal length of objective f o
Magnifying power =
focal length of eye piece f e
For Greater magnification, the objective have larger focal length and the eyepiece a shorter one.
 11:GALILEAN TELESCOPE: 
The scientist Galileo invented this optical instrument. The concave lens serves as the ocular lens, or the
eyepiece of small focal length of small focal length, while the convex lens serves as the objective of large focal length and
eyepiece. The lens are situated on either side of a tube such that the focal point of the ocular lens is the same as the focal point
for the objective lens. The rays of light from a very distinct object fall parallel making a small
visual angle  with the axis of the objective and an inverted image is formed in its focal length
plane. But before he image is formed the eyepiece diverges the rays. The position of he concave
lens is so adjusted that the image is within the focal length and therefore the virtual and enlarged
image of the object is formed have larger visual angle  .The final image is upright with respect
to object.
β
Thus, Magnifying power =
α
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12: TERRESTRIAL TELESCOPE: 
The construction and working of this telescope is same as the
astronomical telescope. But the only difference is that erecting lens of
smaller focal length is introduced between the objective
and eyepiece. The objective forms a real inverted image of distinct object,
at its focus. This image is situated at distance twice the focal length of
erecting lens. Thus this lens forms inverted and real image at the same
distance to the other side of erecting lens. Thus final image is upright with
respect to object. This image is formed with the focal length of eyepiece,
which forms its enlarged and virtual image.
13:ANATOMY OF THE EYE: 
The human eye resembles cameras in its structure. The eye is an
enclosed volume into which light passes through a lens.
A diaphragm, called the “Iris” (the colored part of an eye), adjusts
automatically to control the amount of light entering the eye.
The hole in the iris through which light passes (the pupil) is black because no light is reflected from it and very little light is
reflected back out from the interior of the eye.
The Retina, which plays the role of the film in camera, is on the curved rear surface. It consists of a complex array of
nerves and receptors known as rods and cons, which act to change light energy into electrical signals that travel along the nerves.
The reconstruction of the image from all these tiny receptors is done mainly the brain. At the center of the retina is a small area
called the Fovea, about bo.25 mm in diameter, where the cones are very closely packed and the sharpest image and best color
discrimination are found.
Unlike a camera, the eye contains no shutter. The equivalent operation is carried out by the nervous system, which analyses
the signals to form image at the rate of about 30 per second.
The lens of the eye does little of the bending of light rays. Most of the refraction is done at the front of the cornea (index of
refraction = 1.376), which acts as a protective covering. The lens acts as a fine
adjustment for focusing at different distances. This is accomplished by the
capillary muscles, which change the curvature of the lens so that the focal length
is changed. To focus on distant objects, the muscles are relaxed and the lens is
thin. When light from a distant object passes through the lens system of the eye it
is refracted and brought to focus on he retina. There a real but inverted image of
the objects formed. While all retinal images are inverted, they are interpreted as
being erect. To focus on nearby objects the muscles contact, causing the center
of the lens to be thicker, thus shortening the focal length. This focusing
adjustment is called accommodation.
The closest distance at which the eye can focus clearly is called the near point
of the eye .For young and adult it is 25cm .A person’s far point is the farthest
distance at which n object can be seen clearly. For some purposes I is useful to speak of a normal eye and far point of infinity.
The important optical features of an eye are:
1.  The Eye –lens, which focuses light entering the eye.
2.  The Ciliary muscles, which are attached to the eye lens surface and alter the focal length.
3.  The Retina, the light sensitive area of calls at the back of the eye.
4.  The Yellow spot (fovea centralism), the most light sensitive spot on the retina
5.  The Iris, the spot colored circle round the eye lens.
6.  The Pupil, the circular opening or diaphragm in the iris through which light passes.
7.  The Cornea, the thick transparent protective covering in front of the eye lens. It refracts the light most
8.  The Aqueous humor and Vitreous humor, which are liquids respectively in front of and behind the eye lens in which the
eye lens is suspended.
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14:DEFECTS OF EYE: 
The defects of vision are due to the inability of the eye lens to produce
sharp images at the retina. The two main defects of vision are:
I) Shortsightedness (Myopia)
II) Long-sightedness ( Hypermetropia)
Short sighted (Myopia):
Some people cannot see long
distance objects clearly without
spectacles. This defect of vision is called
“myopia or short sighted ness It is due to
the
eyeball being too large. Light rays from
distant objects are focused in front of the
retina and a blurred image is produced.
Short sight can be corrected by placing suitable concave lens in front of the eye. Light rays from
the distant objects are now diverging by this lens before entering the eye. Light rays appear to come from a distant point forming a
sharp image.
Long sighted (Hyper metropia):
The disability of eye to form distinct images of nearby objects on its retina is known as Hyper metropia
Equations.
1 1 1
1
L  d
1 1 1
 d
  3. Power =
1.  
2.
4. M.P. =
1+  5. M.P of M.G = 1+ 
f
f
f1 f 2
f p q
fo  fe 
 f
q  q 
f
6 Magnifying power of compound microscope =  1   2  7. magnifying power of telescope = o
fe
 p1  o  p2  e
8.Length = ( q + p ) 9. Length of telescope = f e + fo 10. angular magnifying power =
β
d
11. linear magnifying power =
α
p
Dimensions.
QUANTITY
FORMULA
Object distance = image distance = focal length = p = q = f
Magnifying power = magnification
Length of telescope
1
Power of lens
f
DIMENSION
[L]
dimensionless
[L]
[L-1]
UNIT
meter
no unit
meter
meter-1 = Diopter
Short questions
Q. # 1:
Why should the magnifying lens placed close to the eye?
Answer:
Because, the virtual, erect and enlarged image located at least distance of distinct vision by using magnifying
glass. So that, to see clear object, the magnifying lens is placed close to an eye.
Q. # 2: Why should the objective of compound microscope have small focal length?
Answer:
Because, an object should placed be very closer to the focus of objective to get real, large and inverted image
formed close to the pole of eyepiece. Hence, the objectives of compound microscopes have smaller focal length
Q. # 3: Under what condition a double convex lens acts as a diverging lens?
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Answer:
The condition under which double convex lens acts as a diverging lens is when an object
is placed with in the focal length say object distance less than focal length, then double convex
lens is called diverging lens.
Q. # 4: Why is convex lens of small focal length preferred for a magnifying glass?
Answer:
Because, a lens having smaller focal length, greater magnifying power. We know
d 

according equation that, Magnification = 1 +
. Hence, convex lens of small focal length
f 

preferred for a magnifying glass
Q. # 5:
Distinguish between telescope and compound microscope.
Answer: Telescope:
1. It is used to see far away objects.
2. It is used to see objects; whose visual angle is very small
3. The final image at least distance of distinct vision or at infinity
4. The image formed by objective is small and real.
5. The focal length of objective is larger than eyepiece
6. The final image should be smaller than object but visual angle increases
7. The eyepiece cannot act as the magnifying glass.
8. The length of instrument is fo + fe.
9. The magnifying power is fo / fe .
compound microscope
1. It is used to see close objects.
2. It is used to see small.
3. The final mage will be at least distance of distinct vision.
4. The image formed by objective large and real.
5. The focal length of objective is smaller than eyepiece
6. The final image should be virtual erect and magnified.
7. Eyepiece act as the magnifying glass
8. The length of instruments q1 + p2
9. The magnifying power is (M.P.)o (M.P.)e
Q. # 6:
How can a real image be distinguished from a virtual image?
Answer:
Real image:
Virtual image:
1. It can be projected on to screen.
1. It cannot project on to screen.
2. It is always inverted.
2. It is always erect.
3. This type of image is formed by the actual interaction of reflected or refracted light rays 3. This type of image is
formed by not actual interaction of reflected or refracted rays.
Q. # 7: Explain why the lenses used in expensive optical devices are composed of more than one part?
Answer:
The lenses used in the expensive devices are composed of more than one part only to remove lens defects.
The results should be correct.
Q. # 8: Why the magnifying lens should placed close to the eye?
Answer:
Because, the virtual, erect and magnified image should be located at least distance of distinct vision. So that, the
magnifying lens should placed close to the eye
Q. # 9: When light enters glass from air, its speed becomes less. Is it due to change in frequency or
wavelength?
Answer:
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The speed of light in a medium depends on its wavelength. The decrease of speed is due to
decrease of wave length.
Q. # 10: An object is placed with in the focal length of convex lens. What will be the nature
of image?
Answer:
When an object is placed with in the focal length of convex lens then the nature of image
will be virtual erect and magnified.
Q. #11: What is the difference between Galilean telescope and astronomical telescope?
Answer:
Galilean telescope:
Astronomical telescope:
1. In this telescope concave lens is used as the eyepiece. 1. In this type of telescope convex lens is used as eyepiece.
2. in this telescope erect final image formed
2. In this telescope inverted final image formed
Q. # 12:
Define visual angle?
Answer:
The visual angle is the angle subtended between an object and an eye or image and magnifying glass.
Q. # 13: Explain linear magnification?
Answer:
The ratio between the sizes of image to the size of object is called linear magnification.
height of image
Magnification =
height of object
Q. # 14:
What are the defects of lenses?
Answer:
There are two defects of lenses i) spherical ii) and chromatic aberration.
Q. # 15: Where will be the image formed, if an object is placed between F and 2F of convex lens?
Answer:
When an object is placed between F and 2F then the image will be formed away from 2F. The nature of
image will be real, inverted and large in size.
Q. # 16: Why should the eyepiece of telescope have small focal length?
Answer:
The eyepiece of telescope is used of smaller focal length. Because, in the telescope, object is placed for away
from the focus of objective. Its real and small image formed close to the focus of eyepiece. The final image should be
enlarged and virtual has larger visual angle. To get higher magnification have eyepiece of telescope have smaller
focal length eyepiece of telescope have small focal length.
Q. #17:
What is least distance of distinct vision?
Answer:
The minimum distance to see an object clearly is 25cm called “least distance of distinct vision”.
Q. #18:
Why we combine two or more than two lenses?
Answer:
Two or more lenses are combined to design an optical instrument. By this combination the power of lens also
increases. By this process, we avoid lens defects.
Q. # 19:
What is power of convex lens, whose focal length is 20cm?
Answer: We know that,
1
power =
f
1
Power=
20cm
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100
.Thus Power = 5 Diopter
20 meter
Q. #20: What Is the Power Of Accommodation of the Eye?
Answer:
Accommodation in the eye is the process by which the eye increases optical power to
maintain a clear image on the retina.
The focus of the eye is controlled by a number of factors including the iris, cornea, and
muscle tissue that alters the shape of the lens so that the eye can focus on both near and far
objects.
Sometimes these muscles don't work properly because the eye is slightly altered in shape, and as people age,
the lens becomes harder and cannot be properly focused leading to poor vision.
If the point of focus is short of the retina, it's known as "nearsightedness" which means that the eye cannot focus on
distant objects. "Farsightedness" is where people have problems focusing on nearby objects
Q. # 21: can you see stars by a compound microscope.
Answer:
No, it is not possible that e can see the stars by compound microscope. Because, eyepiece is of larger focal
length than the objective. So this is not possible.
Q. # 22: what do you understand by the power of lens? Write its unit obtains formula for the power of two
lenses placed in contact.
Answer:
The reciprocal of focal length is called power of lens expressed in Diopter.
1
Power of lens =
f
The unit of power of lens is Diopter expressed in m-1.
When two or more lenses are used in close combination, that is, with no space in between them, the equation to
1
1
1
+
+ +- - - - calculate the effective power of the combination is: Powercombination =
f1
f2
f3
Q. # 23: Why is a candle flame yellow?
Answer:
The received color of an object is determined by the spectrum of its radiant flux factored by the luminous
efficacy of the human eye .If the human eye responded equally to all wavelengths in the visible range; the candle
flame would appear red. But since the eye sensitivity peaks in the green and diminishes toward the red wavelengths,
the eye perceives the color as yellow.
Q. # 24: why a pin hole placed in front of a lens leads to a good image even when the image is not quit in focus.
Answer:
A pin hole placed in front of a lens reduces its aperture. The image is formed by the central rays. Thus we
obtain a good image even of the lens is not in focus.
Q. # 25: what happens when light hits the object?
Answer:
When a light wave hits an object, what happens to it depends on the energy of the light wave, the natural
frequency at which electrons vibrate in the material and the strength with which the atoms in the material hold on to
their electrons. Based on these three factors, four different things can happen when light hits an object:
The waves can be reflected or scattered off the object.
The waves can be absorbed by the object.
The waves can be refracted through the object.
The waves can pass through the object with no effect.
And more than one of these possibilities can happen at once.
Q. # 26: Why sky seem to be blue?
Power =
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Answer:
Because, atmosphere is containing molecules of many different sizes, including
nitrogen, oxygen, water vapor and various pollutants so that the Earth's atmosphere is rough.
This assortment scatters the higher energy light waves, the ones we see as blue light. This is why
the sky looks blue.
Q. # 27: Why focal length of convex lens is more for red colour light?
Answer:
Because, when light possesses from convex lens it bends to wads the normal. This
coloured light has larger wave length smaller frequency. So that focal length of convex lens is more.
Q. #28: Two thin lenses are in contact. Now they are separated and placed coaxially. What will be the power of
lens combination?
Answer:
When two thin lenses are in contact the power of lens combination increases. Now when they are separated
and placed coaxially the power of lens combination decreases.
Q. # 29: Why does sunlight drain colour?
Answer:
The sunlight (electromagnetic radiation) chemically breaks down dyes. Washing detergents and bleach do the
same thing. The exact chemical formula of the dyes will make a difference in how long the color lasts but, nothing
lasts forever. Sunlight carries ultraviolet (UV) light along with visual spectrum light. The UV light causes a chemical
reaction with color molecules and they fade over time.
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Q. # 21: What is the use of spectrometer?
Answer:
Q. # 22: What are the defects of vision? How can these be corrected?
Answer:
We know
Q. # 32: How is the magnifying power of a a) telescope b) compound microscope, affected by increasing the focal
length of their objective
Answer:
Q. # 32: Define in short the construction working and magnifying power of a compound microscope?
Answer:
Q. # 32: Show how the position nature and size of the image are formed by a) convex lens b) concave lens.
Answer:
Q. # 32: show that a real image of a man formed by a converging lens only inverts him but that he and his image still
have the same right hand.
Answer:
Q. # 32:if a telescope is focused on a moon and then directed towards a tree 50m away , what adjustment would be
necessary to focus the tree sharply/
Answer:
Q. # 32: what wavelength of light would you recommended for use with a microscope if the maximum detail was to
be seen.
Answer:
The wavelength of light doesn't limit magnification. It limits resolution. You can have magnifications that go far
beyond the limits of resolution. In that situation, things would look larger, but you would not see any more detail.
Two factors limit resolution in a light microscope. The wavelength of light and the Numerical Aperture of the lens
system. The shortest visible wavelength is ~400 nm. The highest Numerical Aperture for a lens system to date is 1.4.
This means the best resolving power on a microscope is ~200 nm.
Q. # 32: what are the different arrangements used for decreasing the length of terrestrial telescope.
Answer:
Q. # 32: What is an image?
Answer: In optics, the likeness or counterpart of an object produced when rays of light coming from that object are
reflected from a mirror or are refracted by a lens
Q. # 32: What is a virtual image?
Answer: On the other hand, a virtual image occurs when the prolongations of the light rays converge to form an
image, but the light rays themselves do not reach the point of convergence.
Q. # 32: What is a real image?
Answer: A real image occurs when the rays of light from the object actually converge to form an image and can be
seen on a screen placed at the point of convergence
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