REFRACTION THROUGH LENSES
REFRACTION
THROUGH LENSES
Concepts Of Refraction Through
Lenses.
Refraction
through lenses involves the same change in direction of light rays. A lens is
any transparent material with two faces, of which at least one is curved. It
causes a beam of light passing through it to either converge or diverge.
Types Of Lenses.
There
are two kinds of lenses- I. The converging or convex lens II. The diverging or concave lens
Converging
(convex) lens makes a parallel beam of light to converge to a point. Such
lenses are thicker at the centre than at the edges.
Diverging
(concave) lens makes a parallel beam of light to appear to diverge from a
point. Such lenses are thinner at the centre than at the edges.
Classification Of Lenses:
The
various classification of Concave lenses
and convex lenses are biconcave and biconvex lens which a mostly used in school
laboratory experiments. Plano-concave and plano-convex lenses are used in
optical instruments. The concavo-convex ( or converging meniscus) and
convexo-concave ( or diverging meniscus) are used as contact lenses to correct
defective vision because they fixed the curvature of the eyeball.
Most
lenses are made from glass. A few are made with quartz or plastic. The eye has
a crystalline biconvex lens.
TERMS USED IN REFRACTION THROUGH LENSES:
The Principal focus (F)
of a converging or convex lens is the point from which all rays parallel and
close to the principal axis appear to converge after refraction through the
lens.
The
Principal focus of a diverging or concave lens is the point from which all rays
parallel and close to the principal axis appear to diverge after refraction through the lens. A
lens has two principal foci since light may pass through a lens in either
direction.
Note:
The principal focus of a converging lens is real while The principal focus of a
diverging lens is virtual.
The optical center (C) of a lens
is a point through which rays of light pass without being deviated by the lens.
Principal axis:
The line passing through the optical centre of the lens and joining the centre
of curvature of its surface
The focal length (f):
is the distance between the optical centre and the principal focus of the lens.
The power (P) of a lens:
is equal to the reciprocal of the focal length and is measured in dioptres when f is in metres. P =
Image
Formation by Concave and Convex Lenses:
Convex
Lenses
When an object is placed at infinity,
the real image is formed at the focus. The size of the image is much smaller as
compared to that of the object.
When
an object is placed behind the center of curvature, the real image is formed
between the center of curvature and focus. The size of the image is same as
compared to that of the object.
When
an object is at the center of curvature, the real image is formed at the other
center of curvature. The size of the image is same as compared to that of the
object.
When
an object is placed in between the center of curvature and focus, the real
image is the formed behind the center of curvature. The size of the image is
smaller as compared to that of the object.
When
an object is placed at the focus, a real image is formed at infinity. The size
of the image is much larger as compared to that of the object.
When
an object is placed in between focus and pole, a virtual image is formed. The size
of the image is larger as compared to that of the object.
Concave Lenses
When
an object is placed at infinity, a virtual image is formed at the focus. The
size of the image is much smaller as compared to that of the object.
When
an object is placed at a finite distance from the lens, a virtual image is
formed between pole and focus of the convex lens. The size of the image is
smaller as compared to that of the object.
Difference between real and
virtual image.
|
Real image |
Virtual image |
|
The
images are inverted |
The
images are erect |
|
Converging
lens are used to produce the image |
Diverging
lens are used to produce the image |
|
Concave
mirror is used to produce the image |
A
plane mirror or convex mirror is used to produce the image |
Linear magnification and Power of lens
The
object distance, u, the image distance, v, and the focal length, f, of a lens
are related by the equation
Sign convention
When
the mirror formula is used in solving practical problems, it is necessary to
add a positive(+) or a negative (-) sign to each of the distances according to
a sign rule or convention.
|
Real is Positive |
New Cartesian |
|
f is +ve for a converging lens |
f is +ve for a converging lens |
|
f is -ve for a diverging lens |
f Is -ve for a diverging lens |
|
Distances
of real objects and real images are positive. Distances of virtual objects
and images are negative. |
All
distances to the left of the lens are negative and all those to the right are
positive |
Linear magnification
Linear
magnification (M) produced by a mirror
given by M =
Example
A
small image is viewed through a converging lens held close to the eye. If the
focal length of the lens is 10 cm and a virtual image of height 2 cm is formed
30 cm away from the lens, obtain by calculation or scale diagram: I. the
distance of the object from the lens. II. The size of the object.
Solution
f
= 10 cm, hi = 2 cm, image distance, v,= -30 cm, do = ?
i. -
Assignment
1.
Find the position, magnification and
nature of the image formed by a lens of power +5 Dioptres when the object is 20
m away from it.
2.
What is the power in dioptres of a.
converging lens of focal lengths 2 m, 0.5 m, 50 mm. b. diverging lens of focal lengths 0.4 m, 0.1
m, 80 mm.
Solution
1. P =5; P =
2.a.
i. f = 2 m ; P =
b.
i. f = 0.4 m ; P =
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