Transcript Refraction - Moline High School

Conceptual Physics: pp. 448-459; Chapter 30
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Refraction-The bending of a wave as it enters
a new medium
Medium-The material the wave
travels through
Why does light refract?
 It enters the new medium at an angle
 It changes speed
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Angle of incidence (i or θi) -The angle
between the incident ray and the normal line
Angle of refraction (r or θr) -The angle
between the refracted ray and normal line
Normal line – an imaginary line
perpendicular to where the light
strikes the interface.
Incident ray
i
r
Refracted ray
Interface – the
boundary between
substances
Imaginary line showing the
path that the light would
take if it could go straight.
(used as a reference line)
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Index of refraction (n): the ratio of the speed of light in a
vacuum to the speed of light in that medium
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Example:
nglass = speed of light in vacuum
speed of light in glass
nglass = 3.00 x 108 m/s
2.00 x 108 m/s
nglass = 1.50
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The higher the n value (index of refraction)
 The slower the light will travel through the
material
 The greater the optical density
 The more light will refract (bend)
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A few index values that we will be using often
in class: (Pg. 15 in workbook)
nglass = 1.50
nwater = 1.33
nair = 1.00
ndiamond = 2.42
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Light will NOT refract when;
1. the two materials have the same index
 If the materials have the same index, the speed of
light is the same in each, so no refraction takes
place
n=1.3
n=1.3
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Light will NOT refract when;
2. the light strikes perpendicular (normal) to
the surface
n=1.2
n=1.4
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Critical AngleThe angle of
incidence that
produces a 90º
angle of
refraction
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(can ONLY be
produced going
from more to less
optically dense)
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1. Light passing from less to greater index
 Light bends toward the normal line
Angle of incidence
(measured from
the normal)
i
n = 1.2
n = 1.5
r
Angle of
refraction
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2a. Light passing from greater to lesser index
 Light bends away from the normal line
(i is less than critical angle)
n = 1.6
n = 1.3
As the angle of incidence increases, the
angle of refraction also increases and the
refracted ray moves closer to the interface.
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2b. Light passing from greater to lesser index
 Light travels along interface
(i equals / at critical angle)
n = 1.6
n = 1.3
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2c. Light passing from greater to lesser index
 Total internal reflection
(i is greater than critical angle)
n = 1.6
n = 1.3
The light doesn’t refract.
Instead, it reflects back
into the first substance.
This is how fiber optics
work.
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Two Types of Lenses
1. Convex (converging)
2. Concave (diverging)
Lenses are simply rectangular and triangular
prisms put together.
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When light passes through a triangular prism,
it bends toward the thicker portion of the
prism.
n = 1.0
n = 1.0
n = 1.5
n = 1.5
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Light bends when it enters
and unbends by the same
amount when it exits the
prism
The path is unchanged- it is
simply displaced sideways.
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Putting the triangular and rectangular prisms
produce the 2 types of lenses (double
concave and double convex)
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Focal point-The point where refracted rays
from parallel incident rays intersect
Focal length-The distance between the focal
point and the lens
Real image-An image formed from the
intersection of actual light rays
Virtual image-An image that is NOT formed
by the intersection of actual light rays
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Have real foci
Two focal points at equal distances on each
side of the lens
F
F
F
F
1. Draw the ray parallel to the axis, hit the lens, and
refract through the focal point on the other side.
 2. Draw the ray through the center of the lens
without bending.
 3. Draw the ray through the first focal point, hit the
lens, and refract parallel to the axis.
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F
F
The image is
real, inverted
and smaller
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Have virtual foci.
F
F
F
F
1. Draw the ray parallel to the axis, hit the lens, and
refract away from the first focal point.
 2. Draw the ray through the center of the lens
without refracting.
 3. Draw the ray toward the focal point on the other
side of the lens, hit the lens, and refract parallel to
the axis.
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The image with a concave lens will always be
smaller, upright, and virtual
It will also always be found between the
focal point and the lens. (similar to a convex
mirror)
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Rules to determine whether the image is real
or virtual:
 1. Real images are always inverted
 2. Virtual images are upright.
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(Except for combinations of lenses)
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2 Types
 1. Double convex
 2. Double concave
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Converging lens
Examples:
 Magnifying glasses
 Microscopes
 Correct farsightedness (hyperopia)
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Diverging Lens
Examples:
 Peep hole
 Correct nearsightedness (myopia)
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Light traveling through air hits glass at a 28
degree angle. What is the refracted angle?
1. Draw a picture
2. List variables
3. Write formula
4. Substitute
5. Show some work
6. Solve and label
7. Check if the answer is reasonable!!!
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1. Light strikes the surface of a piece of glass
from air with an incident angle of 70◦. What
is the angle of refraction?
Person #1: Draw a diagram
Person #2:List the variables
Person #1: Write the formula and
substitute
 Person #2: Solve
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2. Calculate the critical angle for the
substances water and glass.
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Person #2: Draw a diagram
Person #1: List the variables
Person #2: Write the formula and substitute
Person #1: Solve
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Object
Distance
Image
Distance
Larger/sma Erect/Inver Real/Virtua
ller/same
ted
l