Download File

Refraction through a lens
• we have seen people using spectacles
for reading.
• The watchmakers use a small glass
to see tiny parts.
• Pistol or rifle shooters use glass for
clear view.
• In your biology class you must have worked
with a magnifying glass.
 Have you ever touched the surface of a
magnifying glass with your hand?
 Is the surface of above mentioned glasses
is plane or curved?
 The glasses used in spectacles and that by a watchmaker,
shooter are examples of lenses.
Learning outcome
 What is lens?
 How lenses are made?
 How lenses bend light rays?
 Application of lenses.
LENS
 A transparent material bounded by two or at least one spherically
curved surface is known as lens.
 Lenses are cut from an optical glass sphere.
 There are two types of lenses
Two types
CONVEX
CONCAVE
CONVEX LENS
 Convex lens is thicker in the middle and thinner at the edges.
 There are three types of convex lenses.
CONVEX LENS
BICONVEX
PLANO CONVEX
CONVEXO CONCAVE
 Construction of convex lens using optical sphere
CONVEX LENS
 Convex lens can be
considered as a set of
prisms and a glass slab.
 So, convex lens behaves
as a converging lens.
 Convex Lens as a set of prisms
and a slab.
CONCAVE LENS
 Convex lens is thinner in the middle and thicker at the edges.
 There are three types of concave lenses.
CONCAVE LENS
BICONCAVE
PLANO CONCAVE
CONCAVO CONVEX
 Construction of concave lens using optical sphere
CONCAVE LENS
 Concave lens can also be
considered as a set of
prisms and a glass slab.
 Concave lens behaves
as a diverging lens.
 Concave lens as a set of prisms
and a slab.
ASSIGNMENT 1
1) Define a lens. How will you identify the nature of lens
without touching it.
2) Explain why convex lens shows converging action and
convex lens show diverging action with diagram ?
TERMS RELATED TO LENS
1) Optical centre: The centre of a lens is called optical centre.
• It is denoted by symbol O.
TERMS RELATED TO LENS
2) Centre of curvature: The centres of the spherical surfaces
forming the lens are known as centre of curvature.
• It is denoted by symbol C.
• A lens has two centre of curvatures.
TERMS RELATED TO LENS
3) Radius of curvature: The distance between the optical centre
and the centre of curvature is known as radius of curvature.
• It is denoted by the symbol R.
4) Principal axis: The line joining the centres of curvature C1 and
C2 of two spherical surfaces of the lens is called the principal
axis of the lens.
TERMS RELATED TO LENS
5) First Focus: It is a point situated on the principal axis of the lens
such that a beam of light starting from it in case of a convex
lens, or directed towards it in case of a concave lens, becomes
parallel to principal axis after refraction.
TERMS RELATED TO LENS
6) Second Focus: It is a point situated on the principal axis of the
lens such that a beam of light incident parallel to the principal
axis, after refraction from the lens, pass through it (convex lens)
or appear to be diverging from this point (concave lens).
FACTORS AFFECTING FOCAL LENGTH
Focal length of a lens depends on two factors.
i. Refractive index of material of lens relative to its surrounding
medium
ii. The radii of curvature of the two surfaces of lens.
(This formula is known as lens
maker’s formula) (Not in the
syllabus)
 If the lens is placed in water instead of air, its focal length
increases.
 A thick lens has less focal length than a thin lens.
 If a part of the lens is covered,
•
•
Focal length remains unchanged
Intensity of the image decreases.
ASSIGNMENT 2
1) Light ray posses undeviated through optical centre, explain with
diagram?
2) If the medium is different on both side of the convex lens them
how will the first and second focal length differ? If a part of
lens is covered how will:1. focal length differ
2. Intensity of image differ
3. Image differ
To locate the position of an image in a convex lens we use two
of the following rays of light
1 parallel to the principal axis emerging through focus
2 striking the centre of the lens passes straight through (if lens is thin)
3 through the focus emerging parallel to principal axis.
2F
F
F
2F
Images in Convex lens
Image formed in convex lens when the object is placed at
infinity
2F
F
F
f
u
Image
Real, inverted & highly
diminished
v
Use
As a camera lens when object is
very far, burning lens
2F
Images in Convex lens
Image formed in convex lens when the object is placed
beyond 2f
object
2F
F
F
f
u
Image
Real, inverted & diminished
v
Use
As a camera lens when object
is not very far
2F
Images in Convex lens
Image formed in convex lens when the object is placed at
2f
object
F
2F
F
2F
f
u
v
Image
Use
Real, inverted & same size as
object
In terrestrial telescope for erecting
the image
Images in Convex lens
Image formed in convex lens when the object is placed
between f and 2f
object
2F
F
2F
F
f
u
Image
Real, inverted & magnified
v
Use
Cinema and slide projectors
Images in Convex lens
Image formed in convex lens when the object is placed at
the focus
object
2F
F
F
f
u
Image
Use
At Infinity, inverted and
highly magnified
In collimator of a spectrometer
to obtain parallel beam
2F
Images in Convex lens
Image formed in convex lens when the object is placed
between the Focus and optical centre
object
2F
F
F
f
u
Image
Use
Virtual, magnified & upright
As a reading glass
2F
Concave lens
To locate the position of an image in
a concave lens we use two of the
following rays of light
1. A ray which strikes the lens
travelling parallel to principal
axis is refracted as if it came
from focus
3. A ray heading for
the focus on striking
the lens is refracted
parallel to principal
axis
2F
F
F
2F
2. A ray striking the
centre of the lens
passes straight
through (if lens is thin)
Images in Concave lens
Image formed in concave lens when the object is placed
at infinity
2F
F
F
f
Image
Use
Virtual, erect & diminished
In Galilean telescope
2F
Images in Concave lens
Image formed in concave lens when the object is placed
in front of lens
object
2F
F
F
2F
f
v
u
Image
Use
Virtual, erect & diminished In spectacles for correcting shortsightedness
EXPERIMENTAL DETERMINATION OF
FOCAL LENGTH OF A CONVEX LENS
(I) BY DISTANT OBJECT METHOD:
• This is based on the following principle.
• “ a beam of parallel rays incident from a distant object is
converged in the focal plane of the lens.”
• The lens is kept between the
distant object and a screen. The
position of the lens is changed till
the clear image of the object is
formed on the screen.
• The distance between the screen and the lens is measured.
This distance is known as focal length of the lens.
EXPERIMENTAL DETERMINATION OF
FOCAL LENGTH OF A CONVEX LENS
(II) BY AUXILLARY PLANE MIRROR METHOD :
• Place the lens L on a plane mirror kept on the horizontal
surface of the vertical stand and arrange the pin P
horizontally in the clamp so that its tip is vertically above
the centre O of the lens L.
EXPERIMENTAL DETERMINATION OF
FOCAL LENGTH OF A CONVEX LENS
(II) BY AUXILLARY PLANE MIRROR METHOD :
• Adjust the height of the pin until it has no parallax with its
inverted image as seen from vertically above the pin.
• If the pin and its image shift together, then parallax is
removed.
• Measure the distance x of the pin from the lens and the
distance y of the pin from the mirror.
• Calculate the focal length using fromula
• f = x+y
2
ASSIGNMENT 3
1) Draw a ray diagram showing the formation of image by a convex
lens, if the size of image is same as that of object.
2) Which of the following lenses would you prefer to use while reading
small letters found in a dictionary- (a) Convex lens of focal length 50
cm (b) Concave lens of focal length 50 cm (c) convex lens of focal
length 5 cm (d) concave lens of focal length 5 cm.
3) Parallel light rays inclined at some angle to principal axis fall on
concave lens. Draw a neat ray diagram to show the position of the
image.
4) Under what circumstances we obtain a virtual image with the help of
convex lens.
5) An object is brought from infinity towards the pole of convex and
concave lens. Mention the change in position, size and nature of
image.
Cornea
Suspensor
ligament
Retina
Iris
Pupil
Optic nerve
Lens
Short-sight defect
falls short of
retina
light from
distant object
Corrected
falls on
retina
light from
distant object
with help of a
diverging
(concave) lens
33
Long-sight defect
light from
near object
falls ‘behind’
retina
Corrected
falls on
retina
light from
near object
with help of a
converging (convex)
lens
34
POWER OF LENS
• The power of lens is a measure of deviation produced by it in
the path of rays refracted through it.
• Power of lens = _______1________
focal length(m)
• The unit of power is dioptre.
• The power of convex lens is positive.
• The power of concave lens is negative.
MAGNIFYING GLASS
(SIMPLE MICROSCOPE)
MAGNIFYING GLASS (SIMPLE MICROSCOPE)
• To see an object by the naked eye, it is necessary to place it at a
minimum distance of 25 cm. (which is known as the least distance
of distinct vision).
• The eye is able to see the object if it subtends a minimum angle of
1’ at it.
• To observe a small object which subtends an angle less than 1’ at
the eye when placed at least distance of distinct vision from the eye,
convex lens is used.
• The object is placed between the optical centre and the focus of
the lens. The image is erect, virtual, magnified and on the same side
of the object.
ASSIGNMENT 4
1) Identify the following uses under the category of concave lens and
convex lens.
a.
b.
c.
d.
e.
f.
g.
Lens that enables our eyes to form a real and inverted image on retina.
Lens used in Galillean telescope
Lens used in camera
Lens used in microscopes
Lens used in terrestrial telescopes
Lens used in spectacles to cure short sightedness ( Myopia)
Lens used in spectacles to cure long sightedness (Hypermetropia).
2) State uses of concave and convex lenses. (three each)
3) Explain with ray diagram working of microscope. Write the formula
for its magnification?
4) Name the lens use for correcting short sightedness and the lens used
for correcting long sightedness?