Light and Reflection
Level 1 Physics

Facts
about
Light
It is a form of Electromagnetic Energy
 It is a part of the Electromagnetic Spectrum and the only part we can
really see
More facts…
Speed of light, c, in a vacuum is a constant value
8 m
c  3x10
s
Light is an electromagnetic wave where electric and magnetic
fields fluctuate together.

Light can be:
Reflected
Refracted
Absorbed
Reflection of Light
Angle of
Incidence
θi
Incident Ray
Angle of
Reflection
θr
Reflected Ray
Both angles are
measured from
a line drawn
perpendicular
to the surface:
Mirror
Law of Reflection: The angle of incidence (θi)
EQUALS the angle of reflection (θr)
NORMAL
SPECULAR VS. DIFFUSE
Specular: Important in
determining properties
of mirrors. Rays are
always parallel to one
another.
Diffuse: Law of reflection
applies to each ray, but
irregular surface reflects
light in various directions.
Size of irregularity is ≥ the
wavelength of light
Plane Mirror
A duck is located 15 cm in front of a vertical mirror. WHERE IS THE
OF THE DUCK LOCATED?
MIRROR
On the surface?
Behind the mirror?
Object Distance, Do = 15 cm
On the same side?
Plane Mirror
A duck is located 15 cm in front of a vertical mirror. WHERE IS THE
OF THE DUCK LOCATED?
MIRROR
Virtual Image
Object
height
ho
Behind the mirror?
Object Distance, Do = 15 cm
Image Distance, Di = 15 cm
Image
height
hi
Virtual Image
To the eye, it appears
that the ray originates
from behind the mirror
Rays of light seem to
Come from the image,
But they DO NOT originate
From behind the mirror
Since none of the rays
actually come from the image
it is called a VIRTUAL IMAGE
Virtual Images continued…
Virtual Images are basically images which cannot be visually projected on a
screen.
If this box gave off
light, we could project
an image of this box
on to a screen
provided the screen
was on the SAME
SIDE as the box.
You would not be able to project the image of the
vase or your face in a mirror on a screen, therefore it
is a virtual image.
CONCLUSION: VIRTUAL IMAGES are ALWAYS on the
OPPOSITE side of the mirror relative to the object.
Real Image
If the image can be seen on a screen, it is considered a REAL IMAGE
Real images are always on SAME SIDE
of the mirror as the object (for MIRRORS)
The characteristics may
be different from the
object:
SIZE – reduced, same,
enlarged
POSITION - same side or
opposite side
ORIENTATION – right side
up or
inverted
Spherical Mirrors
Also called DIVERGING mirror
Also called CONVERGING mirror
Concave Mirror - Converging
A converging mirror will focus parallel light rays to a single (fixed)
point in front of the surface of the mirror. This point is said to be
the FOCAL POINT. To find this point, you must use light from
INFINITY
Converging Mirror
Thought to be a slice of a sphere and having a line drawn through
The exact center of the mirror – Principle Axis.
Since it is technically a sphere,
the mirror has a
CENTER OF CURVATURE, C
The focal length, f is
half the distance
of C
C
f 
2
Ray Diagrams
C
Principal Ray Pineault:
f
Principal
Axis
Draw a ray from the top of the object parallel
to the principal axis and then through f after
reflection.
Ray Diagrams
C
f
Principal
Axis
Vice Principal Ray Lanczycki: Draw a ray from the top of the object through
the focal point, then parallel to the principal
axis.
Ray Diagrams
C
f
Principal
Axis
Vice Principal Ray Alves: Draw a ray from the top of the object, through
C, then back along itself.
Ray Diagrams
C
f
Principal
Axis
Converging Mirror – Image
Characteristics
C
f
Principal
Axis
Is the image on the SAME or OPPOSITE side of mirror as object?
SAME SIDE, therefore considered a REAL IMAGE
Is the image ENLARGED or REDUCED?
Is the image INVERTED or UPRIGHT?
Diverging Mirror
Reflected rays will NEVER cross on
object side of mirror, therefore, continue
reflected rays behind the mirror to
locate the image.
Image is:
REAL or VIRTUAL
SMALLER or BIGGER
INVERTED OR UPRIGHT
Diverging Mirror
A diverging mirror (convex) will
ALWAYS form a VIRTUAL IMAGE
Because of its shape, a diverging
mirror gives a wider field of view
than other mirrors.
The Mirror/Lens Equation
Is there any OTHER way to predict image characteristics besides the ray
diagram? YES!
One way is to use the MIRROR/LENS equation to CALCULATE the
position of the image.
1 1 1


f do di
Mirror/Lens
Equation
Assume that a certain concave spherical mirror has a
focal length of 10.0 cm. Locate the image for an
object distance of 25 cm and describe the image’s
characteristics.
1
1 1
1
1 1

  

f do di
10 25 d i
di 
16.67 cm
What does this tell us? First we know the image is BETWEEN “C” & “f”. Since the
image distance is POSITIVE the image is a REAL IMAGE.
Real image = positive image distance
Virtual image = negative image distance
What about the size and orientation?
Magnification Equation
To calculate the orientation and size of the image we
use the MAGNIFICATION EQUATION.
di h i
M  
do h o
16.67
M 
25
M  0.67x
Here is how this works:
•If we get a POSITIVE magnification, the image is
UPRIGHT.
•If we get a NEGATIVE magnification, the image is
INVERTED
•If the magnification value is GREATER than 1, the
image is ENLARGED.
•If the magnification value is LESS than 1, the image
is REDUCED.
•If the magnification value is EQUAL to 1, the image
is the SAME SIZE as the object.
Using our previous data we see that our image was INVERTED, and REDUCED.
Example
Assume that a certain concave spherical mirror has a focal
length of 10.0 cm. Locate the image for an object distance of
5 cm and describe the image’s characteristics.
1
1 1
1 1 1

   
f do di
10 5 d i
d i  -10 cm
di
M  
5
Characteristics?
2x
•VIRTUAL (opposite side)
•Enlarged
•Upright