Transcript Geometric Optics

CHAPTER 14
REFRACTION
Ms. Hanan
Chapter 14 Overview
14-1 Refraction
Investigate which direction light will bend when it enters another
medium and uses Snell’s law to solve problems.
14-2 Thin lenses
Solves problems involving image formation by converging and
diverging lenses using ray diagrams and the thin-lens equation,
explores eye disorders and eyeglasses, and examines the positioning
of lenses in microscopes and refracting telescopes.
14-3 Optical phenomena
Calculates critical angle; predicts when total internal reflection will
occur; explains atmospheric phenomena, including mirages and
rainbows; and briefly describes lens aberration.
14-3 Optical phenomena
Objectives:
• Predict whether light will be refracted or
undergo total internal reflection.
• Recognize atmospheric conditions that cause
refraction.
• Explain dispersion and phenomena such as
rainbows in terms of the relationship between
the index of refraction and the wavelength.
Optical phenomena
• Total Internal Reflection.
• Atmospheric Refraction.
• Dispersion.
• Lens Aberration.
Total Internal Reflection
The complete reflection of light at the boundary between
two transparent mediums when the angle of incident
exceeds the critical angle.
So the 60* angle has total internal reflection
http://phet.colorado.edu/en/simulation/bending-light
At the Critical Angle
The refracted ray
skims along the
boundary
(in more technical
language)
The ray is refracted
parallel to the edge of
the block
Total Internal Reflection only happens
when………
• The ray is travelling through a medium of
higher refractive index and meets a
boundary with a medium of lower
refractive index (ie the light is slow moving
and meets a medium where it would be
fast moving)
Critical angle and refractive index
critical angles of different materials
Medium
Glass
Refractive index
Critical angle
1.50–1.70
30–42
Water
1.33
49
Perspex
1.5
42
Diamond
2.42
24
Applications of Total Internal Reflection
Fiber Optics…light signal is internally reflected.
OPTICAL FIBRES
Fiber optics are used in
communications. In many places,
thinner optical fibers have replaced
thicker copper cables to carry
telephone/internet communication…
more efficient…
The 2009 Nobel Prize in Physics was
awarded for the development of fiber
optic cable…
Fiber optic cables are used in medicine…
-exploratory surgeries:
bronchoscopes and colonoscopes
consist of two cables; one provides
light and the other sends back an
image for viewing.
Arthroscopic knee surgery
Arthroscopic knee
surgery
Bronchoscope
-
Colonoscope
Examples of total internal reflection
Mirage
In a desert.
On a dry road.
Mirage
•
On a hot day, you may see a pool of water at a distance in a
desert.
Hurray!
Sorry! What you see is
just a MIRAGE.
•
It happens due to total internal reflections.
Image of
the sky
layers of air near the ground
– hot – less dense than upper air – lower refractive indices
•
light refracted more and more towards the horizontal
•
When light meets a layer of air near the ground at an angle
greater than C,
Image of
the sky
Total internal reflections occurs.
Example 1
A ray of light travelling in the direction EO in air enters a rectangular block.
E
angle of incidence = 30
30
angle of refraction = 18
O
18
(a)
Find the refractive index n of the block.
(b)
Find the critical angle C of the ray for the block.
Solution 1
(a)
By Snell’s law,
n =
(b)
C = sin1 1
n
n sin 18 = 1  sin 30
sin 30
sin 18
= sin1
1
1.62
= 1.62
= 38.1
Example 2
(c)
If the ray is incident on surface BC of the same block in example 1,
from which surface and at what angle will the ray leave the block?
D
C
30
A
B
Solution 2
(c)
The ray comes out from surface AD.
The outgoing angle from normal is 60.
C
D
30
60
32.3
57.7
32.3
A
B
Question…….
Which of the following angles is the critical angle of glass?
A
B
C
D
Prisms totally internally reflect
light in optical instruments such
as binoculars
Diamonds “sparkle” because the
index of refraction is so large
(n = 2.42), so the critical angle is
relatively small
_______. So light inside a
diamond is more likely to be totally
internally reflected than to escape….
numerous reflections before light
exits.
Dispersion
• Prism is a transparent block with a triangular
cross section.
• Dispersion is an effect in which white light
separates into its component colors.
DISPERSION OF WHITE LIGHT THROUGH A PRISM
The phenomenon of splitting a ray of white light into its constituent colours
(wavelengths) is called dispersion and the band of colours from violet to red
is called spectrum (VIBGYOR).
A
N
White
light
B
C
Screen
So, the colours are refracted at different angles and
hence get separated.
are a product of refraction
and total internal reflection…
Indexes of refraction actually vary slightly
with different wavelengths/frequencies of
light, so light of different frequencies refracts
angles
at different _______.
Since “white” light is
a mixture of all
colors, the colors
are spread out when
refracted through a
medium such as
glass
DEMO
are typically seen as a storm is leaving, if you
look in the direction of the departing rain with the sun at your
back. Why?
Refraction of the white light occurs when the light enters a
raindrop. The light then undergoes total internal reflection at
the back of the drop, before refracting back out into the air.
Each drop disperses a full spectrum of light, but
a person only sees one color from an individual
drop. When we see a rainbow, we are seeing
the refraction of light from many different drops
at different elevations.
• Sunlight is dispersed and internally
reflected by water droplets to form a
rainbow.
Lens Aberration
Departures of real images from the ideal predicted by simple theory are
called aberrations.
Types of aberrations
To reduce spherical aberration, parabolic mirrors are used instead of
spherical.
To reduce chromatic aberration, combinations of converging and diverging
lenses are used.