Properties of Light and
Reflection
Reflection
 Is the change in direction of a wave when it reaches a
surface and bounces off that surface.
Rays of Light
 Light travels in s straight line as long as it is
moving through a medium.
 Medium is the substance through which light
travels.
 You can use a technique called ray tracing to
make ray diagrams.
 A ray is a straight line with an arrowhead that
shows the direction in which light waves are
traveling.
Fermat’s Principle
 Predicts the path that light will take after
reflecting from a surface or passing through
more than one medium.
 According to the principle, light follows the
path that will take the least time.
 When light reflects from a surface and stays in
the same medium, its speed is constant.
Laws of Reflection
 A ray of light coming toward a surface is called
an incident ray.
 The angle of incidence is measured between
the incident ray and a perpendicular line drawn
from the point of contact of the incident ray at
the surface.
 Perpendicular line is called the normal.
 The reflected ray begins at the point of contact.
 The angle of reflection is measured between
the reflected and the normal.
 The angle of reflection and the angle of
incidence are the same.
 The reflected ray always lies on the plane
that is defined by the incident ray and the
normal.
 These laws of reflection apply to light
and all other forms of waves (sound
waves).
Drawing Ray Diagrams
 Step 1: Draw the incident ray using a ruler
 Step 2: At the contact point, draw a normal at
90with a protractor
 Step 3: measure the angle of incidence (i)
between incident ray and the normal.
 Step 4: Draw the reflected ray
 Step 5: label the reflected ray, the incident ray,
the angle of incidence (i), the angle of
refraction (r) and the normal.
normal
Incident ray
Reflected ray
i
Reflecting surface
r
Images in Plane Mirrors
 If you apply the laws of reflection to rays going
from an object, you can predict where the
image will be and what the image will look like.
 A plane mirror is a mirror with a flat, reflective
surface.
 The brain assumes light travels in a straight
line, so to find out where the eye sees the
image, extend the rays that reach the eye
backwards until the meet at a point beyond the
mirror.
Image in Plane Mirror
Image in Plane Mirror
Virtual Images
 An image formed by rays that appear to be
coming from a certain position, but are not
actually coming from this position; image does
not form a visible projection on a screen.
 If light rays hit a screen and form an image, the
image is real and not virtual.
 If the image is behind a mirror, there is no way
that light rays could get there, and it would be
virtual.
Ray Diagrams and Plane
Mirrors
 An image has four characteristics:
Its location (closer than, farter than, same
distance as the object to the mirrow)
Orientation (upright or inverted)
Size (same size, larger, smaller than the
object)
Type (real image or virtual image).
 You can predict characteristics by
drawing ray diagrams to locate the image
of an object.
 The four characteristics of an image in a
plane mirror are always:
Same size
Same distance
Same orientation
Virtual image
You Try It!
 On page 416
 Follow the instructions in table 10.1 in
order to draw a pencil.
 P. 418 # 1-4, 6-8.
 Section 10.2 Review Handout