Transcript refraction ppt_2010

Optics
Reflection, Refraction &
Total Internal Reflection
Revision on Reflection
• Quiz No 1
Six students are arranged in front of a mirror. Their positions are shown
below. The image of each student is also drawn on the diagram. Make the
appropriate line of sight constructions to determine which students each
individual student can see.
Ans
•
•
•
•
•
•
Al can see Ed and Fred
Bo can see Ed, and Fred
Cy can see Cy, Di, Ed, and Fred
Di can see Cy, Di, and Ed,
Ed can see Al, Bo, Cy, Di, and Ed
Fred can see Bo, Cy, Di, and Ed
Quiz No. 2
A man 2.0 m tall wishes to see his full length in a plane
mirror. The minimum length of mirror that he must buy is
•
•
•
•
A
B
C
D
2.5 m
2.0 m
1.25 m
1.0 m
Ans
• D. 1:2 ratio between portion of mirror required to view
the image and the height of the object is observed.
What’s Wrong Here?
• Can you spot 3 subtle distortions of reality in this painting?
(Painting: A Bar at the Folies-Bergere)
3 Subtle Distortions
1)
Note the bottles at the left. Manet painted their reflections in the
mirror but misplaced them, painting them farther toward the front of
the bar than they should be.
2)
Note the reflection of the women. Since your view is from directly in
front of the woman, her reflection should be behind her, with only a
little of it ( if any) visible to you. Yet Manet painted her reflection
well off to the right.
3)
Note the reflection of the man facing her. He must be you! Because
reflection shows that he is directly in front of the woman, and thus
he must be the viewer of the painter. You are looking into Manet’s
work and seeing your reflection well off to your right. The effect is
errie because it is not what we expect from a painting or from a
mirror.
Refraction
Magic or Physics?
Revision on Refraction
• Quiz No. 2
• When light passes from an optically denser medium into
an optically less dense medium, it will bend
______________ the normal.
• When light passes from a medium with a low refractive
index into a medium with high refractive index, it will
bend ____________ the normal.
Quiz No. 3
• Complete the diagram til the light ray enters air again.
• Label the
normal / incident ray / refracted ray / angle of incidence / angle of refraction
air
glass
Quiz No. 4
• Indicate on the diagram the approximate location where Arthur
observes the fish to be. Must Arthur aim above or below where the
fish appears to be in order to strike the fish?
The story of the Archer Fish
If you were that “disabled” Archer Fish
• If you are an Archer Fish, born without the
natural talent of “solving the physics of
refraction”, what would you do to
compensate your lack of talents?
• Hint…think physics….you do have a way
out…
Just Apply Power of Physics!
Optical Density
• Like any wave, the speed of a light wave is dependent
upon the properties of the medium.
• In the case of an electromagnetic wave, the speed of the
wave depends upon the optical density of that material.
• The optical density of a medium is not the same as its
physical density.
• The more optically dense which a material is, the slower
that a wave will move through the material.
Refractive Index (η)
• One indicator of the optical density of a material is the
refractive index (η) of the material.
• It is a number which indicates the number of times
slower that a light wave would be in that material than it
is in a vacuum.
• A vacuum is given an η value of 1.0000.
Refractive Index
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Material
Index of Refraction
Vacuum 1.0000
Air 1.0003
Ice 1.31
Water 1.333
Ethyl Alcohol 1.36
Plexiglas 1.51
Crown Glass 1.52
Light Flint Glass 1.58
Dense Flint Glass 1.66
Zircon 1.923
Diamond 2.417
Rutile 2.907
Gallium phosphide 3.50
<--lowest optical density
<--highest optical density
Snell’s Law
• There exist a relationship between the
angle of incidence and refraction and the
refractive indexes of the two mediums:
Quiz No. 5
• Determine the angle of refraction for the following problems.
Ans
•A :
o
• B: 28.4
o
53.9
• We usually leave angles to 1 d.p
Quiz No. 6
• Perform the necessary calculations at each boundary in order to
trace the path of the light ray through the following series of layers.
Show all the calculated angles.
Ans
• air - flint glass: 18 degrees
• flint glass - water: 22 degrees
• water - diamond: 12 degrees
• diamond - zirconium: 13 degrees
• cubic zirconium - air: 30 degrees
Quiz No. 7
• A ray of light is traveling through air (n = 1.00) towards a lucite block
(n = 1.40) in the shape of a 30-60-90 triangle. Trace the path of the
light ray through the lucite block shown in the diagram below.
Ans
Refractive Index
Absolute Refractive Index
Relative Refractive Index
Relative Refractive Index
Quiz No. 8
Quiz No. 9
Real and Apparent Depth
Quiz No. 10
What is the refractive index of the
pond water?
World Above from a fish’s eyes
Total Internal Reflection
Total Internal Reflection
• When light ray hits the glass block at a certain angle of
incidence, the eye cannot see the light ray.
Total Internal Reflection
Total Internal Reflection
• The maximum possible angle of refraction is 90-degrees.
• Total internal reflection (TIR) is the phenomenon which
involves the reflection of all the incident light off the
boundary. TIR only takes place when both of the
following two conditions are met:
1. the light is in the more dense medium and approaching
the less dense medium.
2. the angle of incidence is greater than the so-called
critical angle.
Quiz No. 11
• For each combination of media, which light ray (A or B) will undergo
total internal reflection if the incident angle is gradually increased?
Ans
• Practice A: Light ray A is in the more
dense medium and it will be the one which
will undergo TIR.
• Practice B: Light ray A is in the more
dense medium and it will be the one which
will undergo TIR.
Critical Angle (c)
Quiz No. 12
• Suppose that the angle of incidence of a laser beam in
water and heading towards air is adjusted to 50-degrees.
• Use Snell's law to calculate the angle of refraction?
Ans
• Good luck!
• This problem has no solution. The angle of
incidence is greater than the critical angle, so
TIR occurs. There is no angle of refraction.
Quiz No. 13
• Some optical instruments, such as periscopes and binoculars use
trigonal prisms instead of mirrors to reflect light around corners.
Light typically enters perpendicular to the face of the prism,
undergoes TIR off the opposite face and then exits out the third
face.
• Why do you suppose the manufacturer prefers the use of prisms
instead of mirrors?
Ans
• A prism will allow light to undergo total internal reflection
whereas a mirror allows light to both reflect and refract.
• So for a prism, 100 percent of the light is reflected. But
for a mirror, only about 95 percent of the light is
reflected. For these reasons, a prism will produce a
brighter image due to the greater percent of light being
reflected.
Applications of T.I.R
• Optical fibres
applet
Optical Fiber
• Used T.I.R to transmit light rays/information
• Telephone signals, internet, cable television
Rainbow Formation –ACE (3 points)
• Homework groups of two
• Presentation in class
• Questions:
Extra Challenge:
Make a rainbow,
either in class or on
video!
1. What are the conditions for a rainbow to be
formed?
2. With the conditions existing, how does a rainbow
forms?
3. Why is the top of a rainbow always red and
bottom violet?
Rare: Full-circle rainbow