A Brief History of Light
1000 AD
It was proposed that light consisted of tiny particles
Huygens-1678
Explained many properties of light by proposing light
was wave-like
Newton-1752
Used this particle model to explain reflection and
refraction
Young-1801
Strong support for wave theory by showing
interference
Maxwell-1865
Electromagnetic waves travel at the speed of light
Well- not-so-brief History of Light
20th Century
Planck
EM radiation is quantized
Implies particles
Explained light spectrum emitted by hot objects
Einstein
Particle nature of light
Explained the photoelectric effect
What does light do?
Travel in a straight line
Except when it passes from one substance
to another
Geometric Optics
We are using the Ray Approximation
Angles: Incidence and reflection equal
Measure angles from “Normal” to surface
Light rays and angles are in one plane
Light paths are reversible
From air-to-water: angle of refraction is
less than angle of incidence
Specular
Reflection
Specular reflection is
reflection from a
smooth surface
The reflected rays
are parallel to each
other
Quick Quiz: What happens if the surface is rough?
A.Angle of incidence equals angle of reflection
B.A single beam reflects at a different angle than incidence
C.The reflection is defuse
D.How rough is rough?
E.All the light goes into the block
Experiments can be devised that will display either the
wave nature or the particle nature of light
In some experiments light acts as a wave and in
others it acts as a particle
Nature prevents testing both qualities at the same time
Light travels in a straight-line path in a
homogeneous medium until it encounters
a boundary between two different media
The ray approximation is used to
represent beams of light
A ray of light is an imaginary line drawn
along the direction of travel of the light
beams
Diffuse
Reflection
Diffuse reflection
is reflection from
a rough surface
The reflected rays
travel in a variety
of directions
Diffuse reflection
makes the dry
road easy to see
at night
Law of Reflection
The normal is a line
perpendicular to the
surface
It is at the point where
the incident ray strikes
the surface
The incident ray makes
an angle of θ1 with the
normal
The reflected ray makes
an angle of θ1’ with the
normal
The angle of reflection is equal to the
angle of incidence
θ1= θ1’
Following the Reflected and Refracted Rays
Ray 1 is the
incident ray
Ray 2 is the
reflected ray
Ray 3 is refracted
into the lucite
Ray 4 is internally
reflected in the
lucite
Ray 5 is refracted
as it enters the air
from the lucite
Refraction of Light
The incident ray, the
reflected ray, the
refracted ray, and
the normal all lie on
the same plane
The angle of
refraction, θ2,
depends on the
properties of the two
media