Light travels through the path between two points
in space
that takes the shortest time!!
x
B
X
x
A
X
Starting from A, make one touch at the wall and finish at B.
Which path takes the shortest time?
x A
x B
θ
θ
Reflection and Mirrors
qi
qi = qr
qr
Law of reflection
Specular Reflection
Diffuse Reflection
Who has the better chance to save this
drowning guy?
Pam Anderson
David Hasselhoff
Jeremy Jackson
Pamela Anderson
People run faster on sand than swim in water!
Refraction
Bending of light at the interface between two different media
A
air
water
B
Light travels slower in water than in air.
Ex 29-1 A light ray emitted from a point source is reflected from
two flat mirrors as shown in the figure. What is the angle A?
30°
A
60°
60°
30°
30°
45°
45° 45°
45°
45°
(180 - 30 - 45)°
45°
Q1 A light ray emitted from a point source is reflected from
two flat mirrors as shown in the figure. What is the angle A?
1.
2.
3.
4.
45
30
60
75
A
30°
Optical Components
Mirrors
(Reflection)
Lenses
(Refraction)
Every object is considered as a
collection of point light sources.
An image is formed at the
converging point of the rays passed
through optical components.
Optical
Components
Ray Diagram
Some Definitions
parallel rays: a collection of rays propagating in parallel
light coming from a point source infinitely far away (e.g. sunlight)
focal point: point where the parallel rays (along the optical axis) converge
after passing through a specific optical component.
f
focal length (f): distance from the optical component to the focal point.
object length (p): distance from the optical component to the object.
image length (q): distance from the optical component to the image.
optical axis: symmetry axis of an optical component.
Focal point should be on the optical axis.
Rules for Images
• Trace principle rays from one end of an object or a point source.
(you trace at least two rays to form a converging point)
• Follow the rules of a specific optical component when the rays
pass through the component.
• Continue to trace the rays until they converge at a point. It not
make imaginary extension of each ray to the other direction to
find a converging point.
We never know that the path of light rays coming into our eyes has
been modified by optical components. We perceive the light as coming from the
virtual image.
VIRTUAL
IMAGE
p
q
Image forms at the point where the light rays converge.
When real light rays converge  Real Image
When imaginary extension of L.R. converge  Virtual Image
Only real image can be viewed on screen placed at the spot.
VIRTUAL
IMAGE
p
q
For a plane mirror: p = q
p = q
No mirror at this point
 Cannot see the image!
p
q
To be able to see the whole body
we do not need mirror below red line!
Q2 I am standing 2 m away from a vertical plain mirror. When
I see the image of myself, how far is it from me?
1.
2.
3.
4.
2m
4m
1m
Infinitely far