Plane Mirror Ray Diagrams
Law of Reflection
• The angle of incidence (ray hitting the mirror)
…is equal to the angle of reflection (ray bouncing
off the mirror)
• Measure angles with respect to the surface normal
(unless otherwise stated)
• The surface normal makes a 90o angle with the
surface causing the reflection
θi
θr
mirror
Law of
Reflection
θi = θr
θi
θr
mirror
Finding the “image”
The object is the source of the light that is hitting the mirror
The image is what you see in the mirror
mirror
But where is the image?
What does this light ray do when it hits the mirror?
object
θi
θr
Law of Reflection!…θi = θr
Image is formed where
reflected rays appear to
cross…
behind the mirror!
for plane mirrors…
do = di
object
image
do
di
for plane mirrors…
image is virtual
object
image
do
di
Virtual image is formed
because light rays do not
actually cross in space.
(They only appear to cross.)
Images Formed by Multiple
Mirrors
Regular old mirror images
Formed by reflection from a
single mirror
Where does this image come
from?
Light originally traveling
to the left of the object
now traveling to the
right…corner image
appears reversed left-to
right
Blue – incident ray
Green – 1st reflection
Purple – 2nd reflection
Reflections
bouncing off both
mirrors!
Summary – plane mirrors
• Law of Reflection
– Angle of incidence is equal to angle of reflection, θi = θr
– Law of reflection works for all mirrors when angles are
defined w.r.t. surface normal
• Image is formed where reflected light rays cross
– If behind the mirror then the image is a virtual image
• For plane mirrors
– do = di
– Image magnification: none…object and image are the
same size
– Image is virtual
Ray Diagrams
• We are going to develop steps that will take
us through ray diagrams with plane mirrors
Ray Diagrams
• Let’s draw a plane mirror and place an
object.
mirror
.
object
Ray Diagrams
• Let’s use Do = Di and locate the image
.
image
.
object
mirror
Ray Diagrams
• Let’s put a person at position A and let’s see
if they can see the image from that position
.
A
.
Ray Diagrams
• Statement 1: If you want to see something,
you have to look at it.
.
A
.
Ray Diagrams
• We know at position A the person can see it
because the sight line crosses the mirror.
.
A
.
Ray Diagrams
• Statement 2: For you to see something,
light has to go from the object to your eye
.
A
.
Ray Diagrams
• If you drop a normal and measure the
angles, they will be equal, angle i = angle r
.
r
A
i
.
Ray Diagrams
• What if we put person B at this location.
Can they see the image?
.
.
B
Ray Diagrams
• The answer is NO because the sight line
does not cross the mirror
.
.
B
Height and Mirrors
How much mirror do you need to see self?
Height and Mirrors
Draw your image (Do = Di)
Height and Mirrors
Draw your image (Do = Di)
Height and Mirrors
Look at your head
Height and Mirrors
Look at your feet
Height and Mirrors
This is how much of a mirror you need
Height and Mirrors
½ your height!
Height and Mirrors
How do you see yourself?
Height and Mirrors
Light goes from object to eyes
Height and Mirrors
Light goes from object to eyes
r
i
Types of Reflection
• Specular
Smooth Surfaces so all rays bounce off
at same angle (this creates glare)
Types of Reflection
• Diffuse
Rough surface which reflects light in all
different directions (because of the law of
reflection)
Homework
Wkst 1