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## Reflection and Refraction

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### Used this particle model to explain reflection and refraction. Huygens. 1670. Explained many properties of light by proposing light was wave-like ... – PowerPoint PPT presentation

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Title: Reflection and Refraction

1
Chapter 22
• Reflection and Refraction
• of
• Light

2
22.1 The Nature of Light
• It was proposed that light consisted of tiny
particles
• Newton
• Used this particle model to explain reflection
and refraction
• Huygens
• 1670
• Explained many properties of light by proposing
light was wave-like

3
A Brief History of Light, cont.
• Young
• 1801
• Strong support for wave theory by showing
interference
• Maxwell
• 1865
• Electromagnetic waves travel at the speed of
light

4
A Brief History of Light, final
• Planck
• Implies particles
• Explained light spectrum emitted by hot objects
• Einstein
• Particle nature of light
• Explained the photoelectric effect

5
Dual Nature of Light
• Experiments can be devised that will display
either the wave nature or the particle nature of
light
• Nature prevents testing both qualities at the
same time

6
The Nature of Light, final
• Particles of light are called photons
• Each photon has a particular energy
• E h
• h is Plancks constant
• h 6.63 x 10-34 J s
• Encompasses both natures of light
• Interacts like a particle
• Has a given frequency like a wave

7
22.2 Geometrical Optics Using a Ray
Approximation
• 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

8
Wave Fronts and Rays
• (a) Near a point source, the wave fronts (i.e.,
surfaces of constant phase) are circular in two
dimensions (? stone in the water) and spherical
in three dimensions. (b) Far from a point
source, the wave fronts are approximately linear
or planar. A line perpendicular to a wave front
in direction of the waves propagation is called
a ray.

9
Wave Fronts and Rays, cont.
Far field
Near field
10
Geometrical Optics
• Plane waves are important in understanding the
properties of mirrors and lenses.
• For light waves, the ray concept is particularly
convenient for showing the path taken by the
light.
•
• Geometrical Optics
• We will make frequent use of light rays, and they
can be regarded essentially as narrow beams of
light much like those lasers produce.

11
22.3 Reflection of Light
• A ray of light, the incident ray, travels in a
medium
• When it encounters a boundary with a second
medium, part of the incident ray is reflected
back into the first medium
• This means it is directed backward into the first
medium

12
Specular Reflection
• Specular reflection is reflection from a smooth
surface
• The reflected rays are parallel to each other
• All reflection in this text is assumed to be
specular

13
Diffuse Reflection
• Diffuse reflection is reflection from a rough
surface
• The reflected rays travel in a variety of
directions
• Diffuse reflection makes the road easy to see at
night

14
Specular and Diffuse Reflection
Diffuse
Specular
15
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 ?1 with the
normal
• The reflected ray makes an angle of ?1 with the
normal

16
Law of Reflection, cont.
Incident and reflected ray are in the same plane.
• The angle of reflection is equal to the angle of
incidence
• ?1?1

?1
?1
17
Refraction of Light
• When a ray of light traveling in a transparent
medium encounters a boundary leading into a
second medium, part of the ray is reflected and
part of the ray enters the second medium
• The ray that enters the second medium is bent at
the boundary
• This bending of the ray is called refraction

18
Refraction of Light, cont.
• The incident ray, the reflected ray, the
refracted ray, and the normal all lie on the same
plane
• The angle of refraction, ?2, depends on the
properties of the medium

19
Following the Reflected and Refracted Rays
• Ray ? is the incident ray
• Ray ? is the reflected ray
• Ray ? is refracted into the lucite
• Ray ? is internally reflected in the lucite
• Ray ? is refracted as it enters the air from the
lucite

20
22.4 The Law of Refraction
• sinq1v1t/d (yellow triangle)
• sinq2v2t/d (green triangle)

The geometrical derivation of the law of
refraction (Snells law).
21
Refraction, cont.
• Speed of lightconstant?
• Yes, but only in ONE medium!

vc (light velocity)
Air
Index of refraction
vc/n Water (optically denser than air)
22
Index of refraction
• The index of refraction defines the velocity of
light in the optically denser medium ? c/n.

Speed of light in vacuum (air)
Index of refraction
Speed of light in a medium (e.g. water)
23
Index of Refraction, cont.
• For a vacuum and air, n 1
• For other media, n gt 1
• n is a unitless ratio

24
Frequency Between Media
• As light travels from one medium to another, its
frequency does not change
• Both the wave speed and the wavelength do change
• The wavefronts do not pile up, nor are created or
destroyed at the boundary, so must stay the same

25
Change of Wavelength
Wavelength of a medium with the refractive index
n
nl0/ln
Vacuum wavelength
26
Refraction Details
• Light may refract into a material where its speed
is lower
• The angle of refraction is less than the angle of
incidence
• The ray bends toward the normal

27
Refraction Details, cont.
• Light may refract into a material where its speed
is higher
• The angle of refraction is greater than the angle
of incidence
• The ray bends away from the normal

28
Index of Refraction Extended
• The frequency stays the same as the wave travels
from one medium to the other
• v ?
• The ratio of the indices of refraction of the two
media can be expressed as various ratios

29
Snells Law of Refraction
• n1sin?1n2sin?2
• ?1 is the angle of incidence
• 30.0 in this diagram
• ?2 is the angle of refraction

30
Some indices of refraction for various substances
at 590 nm
31
Example and Application
• A digital information on a DVD consists of a
series of pits that are read by a laser beam.
The surface of a DVD is shown on the right side.

32
Example and application, cont.
• The picture shows the cross section of a
cone-shaped laser beam used to read the
information on the DVD. Find the required angle
?1 at which the conical beam should enter in
order to achieve a1 mm.

33
Example and application, cont.
• The following informations are given t1.2 mm,
w0.70 mm, and the refractive index of the
plastic is 1.55

w2ba b(w-a)/2
34
Example and application, cont.
b(0.70?10-3 m - 1?10-6 m)/2699 mm/2349.5
mm tg?2b/t349.5 mm/1200 mm0.29 ?2
16.2? Snells law n1sin?1n2sin?2 sin?1n2sin?2/
n11.55sin(16.2?)/1.00 ?125.6?
Air
Plastic