Cl10 light reflaction and refraction

LIGHT-REFLACTION AND
REFRACTION
SUB TOPIC- REFLECTION
Prepared by- Mrs.Priya Jha

Light
• Our primary source of light is the sun.
• Light travels in straight lines at a speed of
186,000miles per second.
• *Light waves travel faster than sound waves!
• Light energy from the sun travels through
space , reaches earth, and some of it turns to
heat energy and warms the earth’s air.
• Light from the sun also travels to the cells of
green plants (producers) and is stored as
energy.
• When light reaches an object, it is absorbed,
reflected, or passes through it.

What is light really?
Electromagnetic radiation waves
• Light waves are three
dimensional.
• Light waves vibrate in all
planes around a center
line.
• The waves have high
points called “crests.”
• Waves also have low
points called “troughs.”
• *The distance from one
crest to the next crest is
called a “wavelength.”
• *The number of waves
passing a given point in
one second is called the
“frequency.”
wavelength

Essential Knowledge, Skills,
and Processes
• Diagram and label a representation of a
light wave (wavelength, peak, trough)

We will investigate the following:
• Reflection of light
• Spherical mirrors
• Images formation by spherical mirrors
• Representation of images formed by
spherical mirrors using ray diagrams
• Mirror formula and magnification

Reflection of light
• Reflection –
Bouncing back of
light waves

. The laws of reflection are as
follows
• The incident ray, the reflected ray and the
normal to the reflection surface at the point of
the incidence lie in the same plane.
• The angle which the incident ray makes with
the normal is equal to the angle which the
reflected ray makes to the same normal.
• The reflected ray and the incident ray are on
the opposite sides of the normal.

- Reflection
• Reflection from a
mirror:
Incident ray
Normal
Reflected ray
Angle of
incidence
Angle of
reflection
Mirror
Angle of
incidence
Angle of reflection

• The Law of ReflectionThe Law of Reflection
Angle of incidence = Angle of reflectionAngle of incidence = Angle of reflection
The
same !!!
The same !!!
Mirror

LIGHT & ITS USES: Mirrors
• Plane Mirrors – Perfectly flat
– Virtual – Image is “Not Real” because it
cannot be projected
– Erect – Image is right side up
© 2000 Microsoft Clip Gallery

LIGHT & ITS USES: Mirrors
• Reflection & Mirrors (Cont.)
–Convex Mirror
• Curves outward
• Enlarges images.
–Use: Rear view mirrors, store
security…
CAUTION! Objects are closer than they appear!

Light & Its Uses: Mirrors
• Reflection Vocabulary
–Optical Axis – Base line through the center
of a mirror or lens
–Focal Point – Point where reflected or
refracted rays meet & image is formed
–Focal Length – Distance between center of
mirror/lens and focal point

Spherical mirrors
• A curved mirror is a mirror with a curved
reflective surface, which may be either convex
(bulging outward) or concave (bulging inward).
Most curved mirrors have surfaces that are
shaped like part of a sphere, but other shapes
are sometimes used in optical devices. The
most common non-spherical type are
parabolic reflectors, found in optical devices
such as reflecting telescopes that need to image
distant objects, since spherical mirror systems
suffer from spherical aberration.

Curved mirror
• A curved mirror can be thought of as
consisting of a very large number of small
plane mirrors oriented at slightly different
angles. The laws of reflection always
apply, regardless of the shape or
smoothness of the surface

Spherical mirror
• A spherical mirror consists of a portion of
a spherical surface.
• A converging mirror has a concave
reflecting surface.
• A diverging mirror has a convex
reflecting surface.

Rules for drawing ray diagrams for
converging and diverging mirrors:
• (Parenthetical remarks refer specifically to diverging
mirrors. Rules 1 and 2 apply to parabolic mirrors only.)
• An incident ray that is parallel to the principal axis is
reflected such that it passes through the principal focus
(or appears to have originated at the principal focus).
• An incident ray passing through (or heading toward) the
principal focus is reflected such that it travels parallel to
the principal axis.
• An incident ray passing through (or heading toward) the
centre of curvature reflects back along the same path.

Rules of Reflection for Curved Mirrors
• 1. Any light ray traveling parallel to the principal axis
is reflected by the curved mirror through the
principal focus.
It either actually passes (for a concave mirror) or
appears to pass (for a convex mirror) through the
principal focus.
• 2. Any light ray that passes (for a concave mirror) or
principal focus is reflected by the curved mirror
parallel to the principal axis.
• 3. Any light ray that passes (for a concave mirror) or
center of curvature retraces its initial path after
reflection by the curved mirror.

• 1. Any light ray
traveling parallel to
the principal axis is
reflected by the
curved mirror
through the principal
focus.
It either actually
passes (for a
concave mirror) or
appears to pass (for
a convex mirror)
through the principal
focus.
FocusC
PRINCIPAL AXIS
i
r

• Any light ray that
passes (for a
concave mirror) or
appears to pass (for
a convex mirror)
through the
principal focus is
reflected by the
curved mirror
parallel to the
principal axis.
F

• Any light ray that
passes (for a
concave mirror) or
appears to pass
(for a convex
mirror) through
the center of
curvature retraces
its initial path after
reflection by the
curved mirror.
C F

Centre of Curvature : The centre of curvature of a curved mirror is
defined as the center of the hollow glass sphere of which the curved
mirror was (previously) a part.
Radius of curvature :The radius of curvature of a curved mirror is
defined as the radius of the hollow glass sphere of which the
spherical mirror was (previously) a part. Note that any line drawn from
the center of curvature C to the mirror surface meets it at right
angle and equals the radius of curvature.
Principal Axis :The principal axis of a curved mirror is defined as the
imaginary line passing through its pole P and center of curvature C.
Focus :The principal focus is defined as the point on the principal axis
where the light rays traveling parallel to the principal axis after
reflection actually meet (for a concave mirror) or appear to meet (for a
convex mirror).
Parts of a spherical mirror

The principal focus is in front of the concave
mirror and is behind the convex mirror.
The focal length (denoted by FP in the figure) is
the distancebetween the pole P and the principal
focus F of a curved mirror.
Focal length is half the radius of curvature.
Focal Length = Radius of Curvature/2
Pole : The pole is defined as the geometric center
of the curved mirror.

Convex mirrors
• A convex mirror,
fish eye mirror or
diverging mirror, is a
curved mirror in which
the reflective surface
bulges toward the
light source. Convex
mirrors reflect light
outwards, therefore
they are not used to
focus light

Convex mirror
• A convex mirror, fish eye mirror or diverging
mirror, is a curved mirror in which the reflective
surface bulges toward the light source. Convex
mirrors reflect light outwards, therefore they are
not used to focus light. Such mirrors always form
a virtual image, since the focus (F) and the
centre of curvature (2F) are both imaginary
points "inside" the mirror, which cannot be
reached. As a result, images formed by these
mirrors cannot be projected on a screen, since
the image is inside the mirror

Effect on image of object's position relative
to mirror focal point (convex)
Object's
position (S),
focal point (F)
Image Diagram
Virtual
Upright
Reduced
(diminished/sm
aller)

Mirror Ray Tracing
• Mirror ray tracing is that rays parallel to
the optic axis and through the focal point
are used. A third useful ray is that through
the center of curvature since it is normal to
the mirror and retraces its path backward.

Concave mirrors
• A concave mirror, or
converging mirror, has
a reflecting surface that
bulges inward (away from
the incident light).
Concave mirrors reflect
light inward to one focal
point.They are used to
focus light. Unlike convex
mirrors, concave mirrors
show different image
types depending on the
distance between the
object and the mirror.

Effect on image of object's position
relative to mirror focal point (concave)
Object's
position (S),
focal point (F)
Image Diagram
S < F
(Object
between focal
point and
mirror
Virtual
Upright
Magnified
(larger)

Object's
position (S),
focal point (F)
Image Diagram
S = F
(Object at
focal point)
Reflected rays are parallel
and never meet, so no
image is formed.
In the limit where S
approaches F, the image
distance approaches infinity,
and the image can be either
real or virtual and either
upright or inverted
depending on whether S
approaches F from above or
below.

Object's
position (S),
focal point (F)
Image Diagram
F < S < 2F
(Object
between focus
and centre of
curvature)
Real
Inverted
(vertically)
Magnified
(larger)

Object's
position (S),
focal point (F)
Image Diagram
S = 2F
(Object at
centre of
curvature
Real
Inverted
(vertically)
Same size
Image formed
at centre of
curvature

Object's
position (S),
focal point (F)
Image Diagram
S > 2F
(Object
beyond centre
of curvature)
Real
Inverted
(vertically)
Reduced
(diminished/sm
aller)

Image Characteristics
• The image characteristics found in a
converging mirror depend on the location
of the object. The table below summarizes
the characteristics of images found in a
converging mirror based on the location of
the object:

Object
location
Magnifi
-cation
Attitude Type Position
near
infinity
< -1 Inverted real at F
beyond
C
< -1 Inverted real between
F & C
at C -1 Inverted real at C
between
F and C
> -1 Inverted real beyond
C
between
F and V
> +1 erect virtual behind
mirror
at F undefin
ed

Sign convention for Reflection
by Spherical Mirrors
New Cartesian Sign
Convention

New Cartesian Sign
Convention
XX’
Y
Y’
(+x, +y)
(-x, +y)
(-x, -y)
(+x, -y)

i. The object is always placed to the left of the
mirror. This implies that the light from the
objects falls on the from the left-hand side.
ii. All distance parallel to the principal axis are
measured from the pole of the mirror.
iii. All the distance measured to the right of the
origin (along +x-axis ) are taken as positive
while those measured to the left of the origin
(along -x-axis) are taken as negative.
iv. Distance measured perpendicular to and
above the principal axis (along +y-axis) are
taken as positive.
v. Distance measured perpendicular to and
below the principal axis (along -y-axis) are
taken as negative.

Mirror equation and
magnification
• Gaussian form
• The Gaussian mirror
equation relates the
object distance =do
and
• image distance =di
to
• the focal length= f:

• The magnification of a mirror is defined
as the height of the image divided by
the height of the object:
By convention, if the resulting
magnification is positive, the image is
upright.
If the magnification is negative, the
image is inverted (upside down).

Mirror Equations
• 1/f = i/do + 1/di
• Magnification (M) = di/do
• f – Focal length of the mirror
• di Distance of the image from the Pole
• Mirror Equations
• *f is +ve in the case of concave mirror.
• *f is ve
• in the case of a convex mirror.
• * di is +ve if the image is a real image and located on the
object's side of the mirror.
• * di is ve if the image is a virtual image and located
• behind the mirror

power of mirrors and lenses (in
dioptres):
where f (metre) is the focal length.
(Some texts use D for the power)

curved mirror and lens equation:
,since
(The equations apply for mirrors and lenses.
The derivation of the equations using similar triangles is
optional.)

Sign conventions for the use of the
lens equations:
• The focal length (f) is positive for converging mirrors
and lenses, and negative for diverging ones.
• The object distance (do) is positive. (The distance is
negative for a virtual object.)
• The image distance (di) is positive for all real images
and negative for virtual images.
• Heights (Ho and Hi) are positive if measured upward
from the principal axis and negative if measured
downward.
• Magnification (m) is positive if the image is erect and
negative when inverted.

Uses of Convex mirror
• Convex mirror lets motorists see around a corner.
• The passenger-side mirror on a car is typically a convex mirror. In
some countries, these are labeled with the safety warning "Objects
in mirror are closer than they appear", to warn the driver of the
convex mirror's distorting effects on distance perception. Convex
mirrors are preferred in vehicles because they give an upright,
though diminished, image. Also they provide a wider field of view as
they are curved outwards.
• Convex mirrors are used in some automated teller machines as a
simple and handy security feature, allowing the users to see what is
happening behind them. Similar devices are sold to be attached to
ordinary computer monitors.
• Some camera phones use convex mirrors to allow the user to
correctly aim the camera while taking a self-portrait.

• Light travels VERY FAST –
around 300,000 kilometres per
second.
At this speed it can
go around the world 8
times in one second.

Cl10 light reflaction and refraction

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