Lesson 19: Reflection!
!
Reflection is the bouncing back of a wave without absorbing its energy. !
Mirrors are excellent reflectors of light waves. !
!
Three types of mirrors!
Plane MIrrors
Spherical Mirrors
Concave Mirror
!!
!!
!!
!
Part 1: Plane Mirrors!
!
!
!
!
!
!
!
The normal line is:!
!
The angle of incidence is:!
!
The angle of reflection is!
!
!
Law of Reflection:!
!
!
!
!
lig
ht
ra
y
Convex Mirror
!
!
!
!
!
!
!
!
If a bundle of parallel rays strike a smooth surface (plane mirror), they
all obey the law of reflection and emerge as reflected parallel rays. This
is an example of specular reflection.!
!
!
!
When parallel light rays strike
an irregular or rough surface
they will not be reflected as
parallel rays. In each case, the
ray obeys the law of reflection,
but the rays are reflected in a
multitude of directions. The
surface is said to produce
diffuse reflection.!
!
!
!
!
!
Example: What is the angle of reflection for the light ray in the diagram below?!
!
!
!
!
!
!
!
!
!
!
!
Example: What is the angle of reflection for the light ray bouncing off of mirror II in the diagram below?!
!
Part 2: Spherical Mirrors!
!
Spherical mirrors are mirrors that have the shape of a piece cut out of a spherical surface. !
!
Concave mirrors are the inside of the sphere
Convex mirrors are the outside of the sphere
Spherical mirror vocabulary:!
!
!
!
!
!
Centre (C):!
Principal axis (PA):!
Vertex (V):!
Real:!
Virtual:!
A concave mirror is also referred to as a !
A concave mirror is also referred to as a !
because any ray that hits the mirror parallel to the
principal axis will converge on the same point - we
call this point the focus (F)
because any ray that hits the mirror parallel to the
principal axis will diverge from the same point - we
call this point the focus (F)
!!
!!
!!
!
!
Concave Focus:
Convex Focus:
!
!
Characteristics of images!
Orientation
Size
Type
!
!
!
!
Memorize these image attributes for concave and convex mirrors!
!
Concave:!
!
V
F
C
For an object
placed
The image will
be
Beyond C
At C
Between C and F
!!
!!
!
!
Convex!
!
!
!
!
For an object placed ANYWHERE in front of a
convex mirror
The image will be
!
!!
!!
At F
Between F and V
Ray diagrams:!
!
You can sketch the relative location and size of an image in a mirror using a ray diagram.!
!
To draw a ray diagram you use a ruler to sketch three rays from a single point on the object in front of
the mirror:
Ray number
Comes in
Reflects
1
2
!
3
Examples: !
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
Part 2: Spherical Mirror Equations!
Equation 1
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
Equation 2
!!
!
Definition
do
di
ho
hi
f
M
Positive (+)
Negative (-)
!
!
!
!
!
!
!
Example: A 5 cm tall object located 40 cm from a mirror produces an inverted image that is 2 cm tall.
What is the focal length of the mirror?!
!
Predict:!
!
!
!
!
!
Calculate:!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
Example: A 12 cm object is placed 25 cm from a convex mirror with a focal length of 7.5 cm. How tall is
the image?!
!
Predict:!
!
!
!
!
!
Calculate:!
!
!
!
!
!
!
!
!
!
!
Example: A 10 cm tall object is located at the centre of curvature which is 30 cm from the vertex of a
concave mirror. What is the magnification?!
!
Predict:!
!
!
!
!
!
Calculate:!
!
!
!
!
!
!
!
Practice problems !
1.!
!
!
!
!
!
!
!
2.!
!
!
!
!
!
!
!
!
!
!
!
What is the angle of reflection for the ray shown below? (50°)!
40°
Light strikes mirror A at an angle of 20° to the mirror. Find the angle of reflection from
mirror C. (75°)!
B
135°
130°
C
A
3.!
!
!
Draw the image of the following object in the mirror.!
!
mirror
4.!
A hidden surveillance camera is mounted on a wall 2.5 m above the floor. It “sees” by
looking into a mirror on the opposite wall which is 3.5 m away. The mirror starts 1.0 m up
from the floor and is 1.0 m high. How much floor does the camera actually see? (1.17 m)!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
5.
!
6.
!
7.
!
!
!
!
!
!
!
!
camera
mirror
An object which is 5 cm tall is placed 14 cm in front of a concave mirror which has a
radius of 10 cm.
A. What is the image distance? (7.8 cm)
B. Describe the image. (real, inverted, smaller)
C. What is the size of the image? (-2.8 cm)
An object which is 5 cm tall is placed 14 cm in front of a convex mirror which has a radius
of 10 cm.
A. What is the image distance? (-3.68 cm)
B. Describe the image. (virtual, erect, smaller)
C. What is the size of the image? (1.32 cm)
The inverted image of an object is a quarter the size of the object. If the object is 30 cm
from the mirror:
A. What kind of mirror is it? (concave)
B. What is the focal length of the mirror? (6.0 cm)
8. Draw ray diagrams for the following situations. Locate and describe the image.
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
A.
(virtual, erect, smaller)
principal axis
f
B.
•
f
•
C
•
f
•
C
•
f
•
C
•
f
(real, inverted, larger)
principal axis
E.
•
C
(no image forms)
principal axis
D.
•
C
(virtual, erect, larger)
principal axis
C.
•
(real, inverted, smaller)
principal axis
F.
!
!
!
!
!
!
!
9.
!
10.
!
11.
!
12.
!
13.
!
14.
!
15.
!
16.
!
17.
(real, inverted, same size)
principal axis
•
C
•
f
A 6.0 cm tall object is placed 40 cm in front of a concave mirror with a radius of curvature
of 60 cm.
A. What is the image distance? (120 cm)
B. What is the size of image produced? (-18 cm)
C. Describe the image. (inverted, real, larger)
A 6.0 cm tall object is placed 40 cm in front of a convex mirror with a radius of curvature
of 60 cm.
A. What is the image distance? (-17 cm)
B. What is the size of image produced? (2.6 cm)
C. Describe the image. (erect, virtual, smaller)
An object located 40 cm in front of a mirror produces an erect image 80 cm from the
mirror.
A. What is the radius of curvature for the mirror? (160 cm)
B. What type of mirror is it? (concave)
An object located 40 cm in front of a mirror produces an inverted image 120 cm from the
mirror.
A. What is the radius of curvature for the mirror? (60 cm)
B. What type of mirror is it? (concave)
An object located 40 cm in front of a mirror produces an erect image 20 cm from the
mirror.
A. What is the radius of curvature for the mirror? (-80 cm)
B. What type of mirror is it? (convex)
A 20 cm object located 30 cm in front of a mirror generates an erect image that is 10 cm
tall. What is the size of image produced when the object is moved 60 cm further from the
mirror’s surface? (5.0 cm)
An object located in front of a concave mirror with a radius of curvature of 80 cm
produced an inverted image that is three times the size of the object. What is the object
distance? (53 cm)
An object located in front of a concave mirror with a radius of curvature of 180 cm
produced an erect image that is two times the size of the object. What is the object
distance? (45 cm)
An object located in front of a convex mirror with a focal length of 60 cm produced an
erect image that is 1/6 the size of the object. What is the object distance?
(300 cm)