CHAPTER 25: THE REFLECTION
OF LIGHT: MIRRORS
WAVEFRONTS AND RAYS
In describing the propagation of light as a wave we
need to understand:
 wavefronts: a surface passing through points of a
wave that have the same phase and amplitude.
 rays: a ray describes the direction of wave
propagation.
 A ray is a vector perpendicular to the wavefront.

TYPES OF REFLECTION
Specular reflection- when parallel light rays
strike a smooth, plane surface, the reflected rays
are parallel to each other. (mirrors do this)
 Diffuse reflection – a rough surface reflects the
light rays in all directions

THE REFLECTION OF LIGHT
Most objects reflect a certain portion
of the light falling on them.
 For a ray of light incident on a mirror
 The angle of incidence, Ɵi , is the
angle that the incident ray makes with
respect to the normal
 The normal is a line drawn perpendicular
to the surface at the point of incidence.
 The angle of reflection, Ɵr , is the angle
that the reflected ray makes with the
normal.

LAW OF REFLECTION

The incident ray, the reflected ray, and the
normal to the surface all lie in the same plane,
and the angle of reflection Ɵr equals the angle of
incidence Ɵi
 Ɵr
= Ɵi
CHECK YOUR UNDERSTANDING
Consider the diagram below.
 Which angle (A, B, C, or D) is the angle of
incidence? ______
 Which angle is the angle of reflection? ______

CHECK YOUR UNDERSTANDING

A ray of light is incident towards a plane mirror
at an angle of 30-degrees with the mirror surface.
What will be the angle of reflection?
PLANE MIRRORS

1.
2.
3.

When you look into a plane
(flat) mirror, the image you
see has 3 properties:
The image is upright
The image is the same size
you are
The image is located as far
behind the mirror as you
are in front of it.
The image is also reversed
left to right
PLANE MIRRORS

1.
2.
3.
When you look into a plane (flat) mirror, the image
you see has 3 properties:
The image is upright
The image is the same size you are
The image is located as far behind the mirror as
you are in front of it.
PLANE MIRRORS
A mirror is an object that
reflects light. A plane mirror
is simply a flat mirror.
 Plane mirrors are ground to
be flat – the flatter the more
expensive. (Typically good
ones have no hills or valleys
larger than 500nm).
 Consider an object placed at
point P in front of a plane
mirror. An image will be
formed at point P´ behind the
mirror.

PLANE MIRRORS
do = distance from object to
mirror
 di = distance from image to
mirror
 ho = height of object
 hi = height of image

ho
For a plane mirror:
 do = di and ho = hi
hi

do
di
IMAGES
An image is formed at the point where the rays of
light leaving the object either actually intersect
or where they appear to originate.
 If the light rays actually do intersect, then the
image is a real image.
 If the light only appears to be coming from a
point, but is not physically there, then the image
is a virtual image.

IMAGES

The image is called virtual because it does
not really exist behind the mirror
CHECK YOUR UNDERSTANDING
To save expenses, you would like to
buy the shortest mirror that will
allow you to see your entire body.
Should the mirror be
 (a) half your height
 (b) two-thirds your height, or
 (c) equal to your height?
 Does the answer depend on how far
away from the mirror you stand?

ASSIGNMENT
Check Understanding: p 778 #1-3
 Focus: p 792 #2, 4, 7
 Problems: p. 793 #3-6
