The document discusses refraction rules for converging and diverging lenses. It states that for a converging lens, any ray parallel to the principal axis will pass through the focal point on the opposite side, and any ray through the focal point will exit parallel to the principal axis. For a diverging lens, any ray parallel to the principal axis will pass through the focal point, and any ray toward the focal point will exit parallel to the principal axis. Additionally, any ray passing through the center of either lens will continue in the same direction.

1. By
Slide_maker4u
(Abhishek Sharma)

2. Introduction
• First lets consider a double convex lens. Suppose
that several rays of light approach the lens; and
suppose that these rays of light are traveling
parallel to the principal axis. Upon reaching the
front face of the lens, each ray of light will
refract towards the normal to the surface. At
this boundary, the light ray is passing from air
into a more dense medium (usually plastic or
glass). Since the light ray is passing from a
medium in which it travels fast (less optically
dense) into a medium in which it travels
relatively slow (more optically dense), it will
bend towards the normal line. This is the FST
principle of refraction. This is shown for two
incident rays on the diagram below. Once the
light ray refracts across the boundary and enters
the lens, it travels in a straight line until it
reaches the back face of the lens. At this
boundary, each ray of light will refract away
from the normal to the surface. Since the light
ray is passing from a medium in which it travels
slow (more optically dense) to a medium in
which it travels fast (less optically dense), it will
bend away from the normal line; this is the SFA
principle of refraction.

3.  Any incident ray traveling parallel to the principal axis
of a converging lens will refract through the lens and
travel through the focal point on the opposite side of
the lens.

 Now suppose that the rays of light are traveling
through the focal point on the way to the lens. These
rays of light will refract when they enter the lens and
refract when they leave the lens. As the light rays enter
into the more dense lens material, they refract towards
the normal; and as they exit into the less dense air,
they refract away from the normal. These specific rays
will exit the lens traveling parallel to the principal axis.

4. Refraction Rules for a Converging Lens
• Any incident ray traveling
parallel to the principal
axis of a converging lens
will refract through the
lens and travel through
the focal point on the
opposite side of the lens.
• Any incident ray traveling
through the focal point on
the way to the lens will
refract through the lens
and travel parallel to the
principal axis.

5.  These two "rules" will greatly simplify the task
of determining the image location for objects
placed infront of converging lenses. This topic
will be discussed in the next part of Lesson 5.
For now, internalize the meaning of the rules
and be prepared to use them. As the rules are
applied in the construction of ray diagrams,
do not forget the fact that Snells' Law of
refraction of light holds for each of these
rays. It just so happens that geometrically,
when Snell's Law is applied for rays that strike
the lens in the manner described above, they
will refract in close approximation with these
two rules. The tendency of incident light rays
to follow these rules is increased for lenses
that are thin. For such thin lenses, the path
of the light through the lens itself contributes
very little to the overall change in the
direction of the light rays. We will use this so-
called thin-lens approximation in this unit.
Furthermore, to simplify the construction of
ray diagrams, we will avoid refracting each
light ray twice - upon entering and emerging
from the lens. Instead, we will continue the
incident ray to the vertical axis of the lens
and refract the light at that point. For thin
lenses, this simplification will produce the
same result as if we were refracting the light
twice.
The Thin Lens Approximation

7.  Refraction Rule for a Diverging Lens
 Any incident ray traveling parallel to the principal axis of a diverging
lens will refract through the lens and travel in line with the focal point
(i.e., in a direction such that its extension will pass through the focal
point).
 Now suppose that the rays of light are traveling towards the focal
point on the way to the lens. Because of the negative focal length for
double concave lenses, the light rays will head towards the focal point
on the opposite side of the lens. These rays will actually reach the
lens before they reach the focal point. These rays of light will refract
when they enter the lens and refract when they leave the lens. As the
light rays enter into the more dense lens material, they refract
towards the normal; and as they exit into the less dense air, they
refract away from the normal. These specific rays will exit the lens
traveling parallel to the principal axis.

8.  Any incident ray traveling parallel
to the principal axis of a diverging
lens will refract through the lens
and travel in line with the focal
point (i.e., in a direction such that
its extension will pass through the
focal point).
 Any incident ray traveling towards
the focal point on the way to the
lens will refract through the lens
and travel parallel to the principal
axis.
Refraction Rules for a Diverging Lens

9.  The above discussion focuses on the manner
in which converging and diverging lenses
refract incident rays that are traveling parallel
to the principal axis or are traveling through
(or towards) the focal point. But these are not
the only two possible incident rays.There are a
multitude of incident rays that strike the lens
and refract in a variety of ways.Yet, there are
three specific rays that behave in a very
predictable manner.The third ray that we will
investigate is the ray that passes through the
precise center of the lens - through the point
where the principal axis and the vertical axis
intersect.This ray will refract as it enters and
refract as it exits the lens, but the net effect of
this dual refraction is that the path of the light
ray is not changed. For a thin lens, the
refracted ray is traveling in the same direction
as the incident ray and is approximately in line
with it.The behavior of this third incident ray
is depicted in the diagram below..

10.  Any incident ray traveling parallel to the
principal axis of a converging lens will refract
through the lens and travel through the focal
point on the opposite side of the lens.
 Any incident ray traveling through the focal
point on the way to the lens will refract
through the lens and travel parallel to the
principal axis.
 An incident ray that passes through the center
of the lens will in effect continue in the same
direction that it had when it entered the lens.

11.  Any incident ray traveling parallel to the
principal axis of a diverging lens will refract
through the lens and travel in line with the focal
point (i.e., in a direction such that its extension
will pass through the focal point).
 Any incident ray traveling towards the focal
point on the way to the lens will refract through
the lens and travel parallel to the principal axis.
 An incident ray that passes through the center of
the lens will in effect continue in the same
direction that it had when it entered the lens.

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the topic “REFRACTION THROUGH LENSES". Hope you like it.
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By- Slide_maker4u (Abhishek Sharma)
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