Download Chapters 25

The Nature of Light
• Because of the way we see everyday objects, Newton and
many others always believed light was a stream of particles
(Chapters 25 and 26).
• Diffraction, when light bends around the edges of objects,
countered the particle theory and lead to the theory that
light acts as a wave (Chapter 27).
• It was the Photoelectric Effect that led to the quantization
of the energy of a light wave. These packets of light are
called photons and they carry the energy: E = hf
• h is Planck’s constant, h=6.63x10-34 Js
• Light exhibits the characteristics of a wave in some
situations and the characteristics of a particle in other
situations
– This is now referred to as wave-particle duality
The Ray Model
• For chapters 25 and 26, we will model light
as a stream of particles.
• By doing so, we make it possible to sketch
out ray diagrams.
• These diagrams illustrate how light waves
(drawn as rays of light) travel between
objects and observers.
Specular vs. Diffuse Reflection
• For simplicity, we will assume all surfaces are flawless
and flat, as in example (a).
• This is called specular reflection.
Law of Reflection
The angle of reflection
is equal to the angle
of incidence
1  1
'
These angles are
always measured
from the normal line.
Refraction
• When light traveling
through a transparent
medium encounters a
boundary for another
transparent medium,
refraction occurs.
sin  2 v2

 const.
sin 1 v1
The Index of Refraction
• Light passing from one medium to another is
refracted because the speed of light is different
in the two media.
• Light travels at its fastest speed in a vacuum.
c  3.0 x10 m / s
8
• The index of refraction is defined as the ratio:
c
n
v
Refraction Scenarios
• (a) As light moves from air into glass, the ray is refracted towards
the normal.
• (b) As light moves from glass into air, the ray is refracted away from
the normal.
Working to the Law of Refraction
• As light travels from
one medium into
another, the
frequency of the light
does not change.
v1  f1 ,v 2  f2
f 
v1
1

v2
2
Working to the Law of Refraction
1 v1 c n1 n2



2 v2 c n2 n1
1n1  2 n2
air
n
n
The Law of Refraction (Snell’s Law)
• Combine:
sin  2 v2

sin 1 v1
v1 n2

v2 n1
sin 1 n2

sin  2 n1
n1 sin 1  n2 sin  2
The Spectrum of Electromagnetic
Waves (24.7)
• Wave Types
– Radio waves
– Microwaves (1 mm – 30 cm)
– Infrared waves (1 mm – 700 nm)
– Visible light (700 nm – 400 nm)
– Ultraviolet light (400 nm – 0.6 nm)
– X-rays (10 nm – 0.1 pm)
– Gamma rays (0.1 nm – 10 fm)
Dispersion and Prisms
• The index of
refraction depends
on the wavelength of
the light
• Therefore, the angle
of refraction depends
on the wavelength of
the light.
• This dependence is
called dispersion.
Dispersion and Prisms
Dispersion and Prisms
• Dispersion of light into
a spectrum is
demonstrated through
the formation of a
rainbow.
• Research has
indicated that the
angles of highest
intensity (brightest
rainbows) are 40o and
42o
Total Internal
Reflection
• This occurs when light travels
from a medium with a high
index of refraction to a
medium with a lower index of
refraction.
• There is a particular angle,
called the critical angle, at
which the refracted light ray
moves parallel to the
boundary (ray 4, in picture).
• At angles greater than the
critical angle, no ray is
refracted and total internal
reflection occurs (ray 5)
sin  c 
n2
n1
Fiber Optics – Total Internal
Reflection