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REFLECTION OF LIGHT
Light obeys the law of refection that states that: "The
angle of incidence is equal to the angle of reflection."
Angles are read from the normal.
Angle of Reflection = Angle of Incidence
Angles are measured with respect to the normal line
Light reflection from a smooth surface is called regular
or specular reflection. Light reflection from a rough or
irregular surface is called diffuse reflection.
FLAT MIRRORS
A flat mirror reflects light
rays in the same order as
they approach it. Flat
mirrors are made from
pieces of plate glass that
have been coated on the
back with a reflecting
material like silver or
aluminum.
The image is the same size as the object and the same
distance behind the mirror as the object is in front of
the mirror.
Notice that the images formed
by a flat mirror are, in truth,
reflections of real objects.
The images themselves are
not real because no light
passes through them.
These images which appear to the eye to be formed by
rays of light but which in truth do not exist are called
virtual images.
On the other hand real images are formed when rays of
light actually intersect at a single point.
Left-Right Reversal
CURVED MIRRORS
A curved mirror is a mirror
that may be thought of as a
portion of a reflecting sphere.
If the inside of the spherical
surface is the reflecting
surface, the mirror is said to
be concave or converging.
If the outside portion is the
reflecting surface, the mirror
is convex or diverging.
A curved mirror has a geometric center or vertex A. The
center of curvature ( C )or radius R. The focal length f of
the mirror is half the radius:
1
f  R
2
IMAGES FORMED BY CURVED SPHERICAL MIRRORS
The best method of understanding the formation of
images by mirrors is through geometrical optics or ray
tracing.
IMAGES FORMED BY CURVED SPHERICAL MIRRORS
The principal rays are:
Ray 1. A ray parallel to the mirror axis passes through
the focal point of a concave mirror or seems to come
from the focal point of a convex mirror.
Ray 2. A ray that passes through the focal point of a
concave mirror or proceeds toward the focal point of a
convex mirror is reflected parallel to the mirror axis.
Ray 3. A ray that proceeds along a radius of the mirror is
reflected back along its original path.
Ray 1. A ray parallel to the mirror axis passes through
the focal point of a concave mirror or seems to come
from the focal point of a convex mirror.
Ray 2. A ray that passes through the focal point of a
concave mirror or proceeds toward the focal point of a
convex mirror is reflected parallel to the mirror axis.
Ray 3. A ray that proceeds along a radius of the mirror is
reflected back along its original path.
9.2 a. Find the images formed by the following mirrors
using the Ray Tracing method.
b. Write the characteristics of each image:
real or virtual,
larger, smaller or same size as object and
inverted or upright.
VIRTUAL
SAME SIZE
UPRIGHT
REAL
SMALLER
INVERTED
REAL
SAME SIZE
INVERTED
REAL
LARGER
INVERTED
NO IMAGE IS FORMED
RAYS ARE PARALLEL
VIRTUAL
LARGER
UPRIGHT
VIRTUAL
SMALLER
UPRIGHT
REFRACTION
The bending of a ray of light as it passes from one
medium to another is called refraction.
The speed of light c in a material is generally less than
the free-space velocity of 3 x108 m/s. In water light
travels about three-fourths of its velocity in air. Light
travels about two-thirds as fast in glass.
The ratio of the velocity c of light in a vacuum to the
velocity v of light in a particular medium is called the
index of refraction n for that material.
c
n
v
c = 3 x108 m/s
SNELL’S LAW
The ratio of the sine of the incident angle to the sine of
the refracted angle is constant.
n1 sinθ1 = n2 sinθ2
n1 = index of refraction of the incident medium
n2 = index of refraction of the second medium
THIN LENSES
Lenses are an essential part of telescopes, eyeglasses,
cameras, microscopes and other optical instruments. A
lens is usually made of glass, or transparent plastic.
The two main types of lenses are convex and concave
lenses.
The focal length (f) of a lens depends on its shape and
its index of refraction.
A converging (convex) lens is thick in the center and
thin at the edges.
A diverging (concave) lens is thin in the center and thick
at the edges.
IMAGE FORMATION BY LENSES
The three principal rays are:
Ray 1. A ray parallel to the axis passes through the
second focal point F2 of a converging lens or appears to
come from the first focal point F1 of a diverging lens.
Ray 2. A ray which passes through the first focal point F1
of a converging lens or proceeds toward the second
focal point F2 of a diverging lens is refracted parallel to
the lens axis.
Ray 3. A ray through the geometrical center of a lens will
not be deviated.
A real image is always formed on the side of the lens
opposite to the object. A virtual image will appear to be
on the same side of the lens as the object.
2.
a. Find the images formed by the following lenses using
the Ray Tracing method.
b. Write the characteristics of each image:
- real or virtual,
- larger, smaller or same size as object and
- upright or erect.
REAL
SMALLER
INVERTED
REAL
SAME SIZE
INVERTED
REAL
LARGER
INVERTED
No image is formed
Rays are parallel
VIRTUAL
LARGER
UPRIGHT
VIRTUAL
SMALLER
UPRIGHT
VIRTUAL
SMALLER
UPRIGHT
THE MIRROR AND LENS EQUATION
The mirror/lens equation can be used to locate the
image:
1 1 1
 
d o di
f
Where do is the object’s distance, di is the image
distance and f is the focal length.
hi
di
M

ho
do
The ratio M is called the magnification, ho is the object’s
size and hi is the image size.
R
radius of
curvature
+ converging
- diverging
f
focal
length
+ converging
- diverging
do
object
distance
+ real object
+ real object
di
image
distance
+ real
images
- virtual
images
ho
object size
+ if upright
- if inverted
hi
image size
+ if upright
- if inverted