Do now…
Describe what you see in this image.
Extend: Explain what you see in this image.
The Law of Reflection
Learning outcomes
By the end of this lesson you should be able to:
o state the law of reflection
o describe the features of a virtual image
o draw labelled ray diagrams and use them to
explain imaging in mirrors
Plane mirrors & virtual images
o Plane mirrors reflect light regularly,
so they produce a clear image
o The image that we see in a plane
mirror is called a virtual image
o Virtual images are the same size
and the same distance behind the
mirror as the object in front of it
What’s the difference?
TASK – In your groups, discuss what is different about the
virtual image compared to the object in front of the mirror.
Lateral inversion
o The type of reversal for
which right appears as
left and left appears as
right is known as lateral
inversion
o All virtual images are
laterally inverted
What is a normal line?
o A normal line is a line
that is perpendicular to
a surface (it makes a 90°
angle with the surface)
o Protractors must always
be used when drawing
normal lines
normal
90°
plane mirror
o Incident ray – the ray coming into the
surface of the mirror (arrow pointing
towards the mirror)
o Reflected ray – the ray coming out of
the surface of the mirror (arrow pointing
away from the mirror)
o Angle of incidence – the angle
between the incident ray and the
normal
o Angle of reflection - the angle between
the normal and the reflected ray
Plane mirror
Ray diagrams
The law of reflection
𝜽𝒊 = 𝜽 𝒓
The angle of
incidence is equal to
the angle of reflection
How do we use reflections?
We use our understanding of
mirrors and reflection in
periscopes.
The rest of the PPT will not be in your exam
Image formation in plane mirrors
The next slides
(16 – 20) show the steps
for drawing a ray
diagram to display the
formation of a virtual
image on the other side
of a plane mirror
Step 1
a. Copy the pencil and the
plane mirror in your book
b. Identify the top and the
bottom of the pencil and
label these A and B
Step 2
a. Draw an incident ray starting at
point A
b. Draw a normal line where the
incident ray meets the mirror
c. Use a protractor to draw the
reflected ray (remember that i = r)
Step 3
Repeat step 2 (a. – c.)
with a second incident
ray at a different angle
Step 4
a. Extend both reflected rays
behind the mirror until they
meet
b. Label the point of
intersection Ai
Step 5
a. Repeat steps 2 – 4 for
point B.
b. Draw the virtual image
of the pencil using
points Ai and Bi as a
guide
Assessment for learning
TASK – Copy the diagrams below in your books. Make sure
you leave enough space around the diagrams to complete
them. Draw the virtual images of the circle and the heart.
mirror
mirror