Transcript 2.Refraction and Lenses

Chapter 17/18
Refraction and Lenses
When a ray of light passes from one medium to
another it may be reflected, refracted or both.
Refraction
• Bending of light when it enters a new medium
is called refraction.
Refraction
• When light rays refract it will change:
1. speed
2. direction (bending of a wave) at the boundary
between two media.
Speed of a wave depends on medium
Video clip:
https://www.youtube.com/watch?v=stdi6XJX6gU
Tractor Example
What happens when you pull your toy tractor from smooth
(less dense) pavement to a rough patch (more dense) of
grass?
Path of Tractor through Grass (no angle)
Grass
The truck will slow
down
Tractor Example Continued…
How is it different if the tractor is at an angle so
only one wheel hits the grass at a time?
Changes Direction
(at an
angle)
How does light bend when it encounters a new
medium at an angle?
TOWARD THE NORMAL
or
less dense
medium
more dense
medium
AWAY FROM THE NORMAL
How does light bend when it encounters a new medium at
an angle?
TOWARD THE NORMAL
or
AWAY FROM THE NORMAL
more dense medium
less dense medium
Least Time Principle
• Light follows the “Least Time
Principle” which can be illustrated
by a lifeguard saving a swimmer
who needs help. He would travel
the farthest distance possible on
the sand (where he can run fast)
and swim the shortest distance
possible in the water (where he
swims slow).
• Light would do the same thing! A
ray of light bends towards the
normal when crossing a boundary
from a medium in which is travels
fast into a medium in which it
travels slow.
Video…..
Optical Illusions and Refraction
• light travels only
through air (no
refraction)
• light travels
from water (to
glass) to air
(refraction, or
lots of bending,
occurs!)
Visual Distortions: objects appear to be
wider or broken
• NOTE: The image of the pencil is located
where the refracted rays intersect!
• (simulation:
http://www.physicsclassroom.com/mmedia/o
ptics/bp.cfm)
Image located where refracted rays
intersect
Quick Review:
• When light goes from a less dense to a more
dense medium, it bends ___________the
towards
normal and when light goes from a more
dense to a less dense medium, it bends
away
____________
the normal.
Index of Refraction
• Compares how fast light travels through a
material to its maximum velocity (3x108 m/s)
Index of Refraction
Material
vacuum
air
ice
water
ethyl alcohol
plexiglass
crown glass
flint glass
diamond
Index of Refraction
1.00
1.0003 (approximately
1.00)
1.309
1.33
1.36
1.51
1.52
1.61
2.42
What do you notice about
all the index of refraction
values?
Note: Higher the index of
refraction it has a HIGHER
(optical) density, and
slower speed.
Refraction
The more the bending, the more the refractive
index or the optical density of the medium.
Here the light ray travels
from a more optically dense
medium (water) to a less
optically dense medium(air)
[to enter our eyes],
-and so the ray bends
away from the normal.
Angle of Refraction
The angle the refracted
ray makes with the
normal line
Index of Refraction Example
• If the speed of light in quartz is 1.95 x 108 m/s,
what is its index of refraction?
Try it!
• If the index of refraction of water is 1.33, what
is the speed of light in water?
Snell’s Law
n1 = Index of refraction of incident medium
n2 = index of refraction “refracted medium”
q1 = angle of incidence
q2 = angle of refraction
Snell’s Law Example
If a ray of light hits the surface of a pool at an angle of
350, at what angle will it travel through the pool?
(n=1 for air, n=1.3 for water)
n1sinθ1=n2sinθ2
If the ray of refraction is bent toward the normal the medium is
more dense. If it is bent away from the normal it is less dense.
Dispersion
• Separating white light into ROYGBIV
• Each color of light has its own wavelength and
will have its own refractive index. Because of
this when light hits certain surfaces each color
will refract at a slightly different angle …
rainbow
LT Quiz #1
-What makes diamonds sparkle?
– How do fiber optics keep us communicating?
– How do those cool holiday decoration light up?
– This property of light is called total
internal reflection.
• When light passes from a more dense medium
(high n value) to a less dense medium (low n
value) at a certain angle, all light is reflected back
into the first medium and there is no refracted
ray.
Critical Angle
• any angle < critical angle refracted
• any angle = critical angle runs along the boundary
• any angle > critical angle totally reflected
total internal reflection.
• The angle at which this
will occur is called the
critical angle.
• TIR results in a refracted
angle of 90°.
• This means that Snell’s
Law for TIR is:
n1sinѲ1 = n2sinѲ2
n1sinѲc = n2sin(90°)
n1sinѲc = n2(1)
sinѲc = n2
n1
Make sure your
calculator is in
degree mode!
Examples
• What is the critical angle
for light traveling from
diamond (n = 2.42) to air?
sinѲc = n2 = 1.00
n1 2.42
Ѳc = 24.4°
• What about diamond
to water?
sinѲc = n2 = 1.33
n1 2.42
• What about water
to diamond?
– Not possible!
Ѳc = 33.3°
Demos
Total internal reflection
• There is no refracted ray
How to Find the Critical Angle for Any Material
The critical angle is when light travels along the
boundary of two mediums (900 to the normal).
To find critcal angle use θ2 = 90
n1sinθ1=n2sinθ2
sin(900) = 1 … Snell’s Law at critical angle becomes …
n1sinθ1=n2
Solving for the critical θ1 … Snell’s Law becomes …
θ1= Sin-1(n2/n1)
LT Quiz #3
Lenses
Convex and Concave Lenses
Convex Lens
Thicker in the middle than at the edges – converges rays
Produces real and virtual images depending on distance away
from the lens
Concave Lens
Thinner in the middle than at the edges – diverges rays
Produces virtual images upright and smaller
Lenses can be used to magnify of de-magnify images
How the Eye Works….
•
Video clip: https://www.youtube.com/watch?v=POLSLQrelCw
Myopia - Nearsighted
-Light rays focus too
early
-Can see things clearly
that are near
What type of lens would be used to correct this
problem?
Nearsightedness corrected using a diverging lens
Hyperopia
–Light rays focus too far
behind retina
–Farsighted
–Can see things clearly
far away
What type of lens would be used to correct this problem?
Farsightedness is corrected using a converging lens
LT Quiz #4
LT Quiz #5