Transcript 4.1 Refraction Index

Refraction
The Optical Density of
a Medium
• The better a medium
transmits light, the lower
its optical density.
• The slower light is
transmitted by a medium,
the higher its optical
density is.
• H
Law of Refraction
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1. When a ray of light travels
obliquely from a medium of
lesser optical density into a
medium of greater optical
density, it will bend towards
the normal.
2. When a ray of light travels
obliquely from a medium of
greater optical density into a
medium of lesser optical
density, it will bend away from
the normal.
Law of Refraction
3. The incident and refracted
rays will be in the same
plane.
Index of Refraction
• The refractive index (or index of refraction) of a
medium is a measure of how much the speed of
light (or other waves such as sound waves) is
reduced inside the medium. For example, typical
soda-lime glass has a refractive index close to 1.5,
which means that in glass, light travels at
1 / 1.5 = 2/3 the speed of light in a vacuum. The
formula for the index of refraction is:
•N = vvacuum / vmedium
• The index of refraction has no units
The Speed of Light
• The speed of light in a vacuum is designated by the small letter, c.
• Light has a speed of 2.9979 x 108 m/s in a vacuum.
• Light has a speed of 2.9970 x 108 m/s in air.
• The index of refraction for light in air is 1.0003
• Unless special accuracty is required, the speed
of light in a vacuum or in air is normally given
as 3.0 x 108 m/s
Snell’s Law
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Snell’s Law Derivation
Objects appear shallower in water due to refraction
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The Critical Angle
• The critical angle is the minimum angle at which light
begins to internally reflect as it passes from a medium of
higher refractive index into a medium of lower refractive
index.
The Critical Angle
• Going from a medium of higher to lower refractive index,
when the incident angle passes a certain minimum critical
angle it will reflect back at the medium boundary.
Calculating the Critical Angle
• Using Snell’s Law, the Critical Angle can be calculated if
the angle of refraction is taken to be 90 degrees (In
experiments, a refraction of 90 degrees and internal
reflection happen at the same time). In Snell’s Law
Formula, the sine of 90 degrees is 1 which allows for the
Critical Angle (the angle of incidence) to be calculated,
given the indices of refraction of the two media.
Applications of the Critical Angle
• Since the critical
angle for glass is
about 42 degrees
(depending on
the kind of glass),
an incident angle
of 45 degrees
(going from
greater to lesser
refractive index
media) causes
total internal
reflection.
Prisms are used
to reflect light in
many optical
devices.
Applications of the Critical Angle
• Prisms can reflect light at various angles as long as the
incident angle is over the critical angle.
Applications of the Critical Angle
• Periscopes can be made with mirrors or prisms. Since
mirrors absorb some of the light they reflect, passing on
less than 100 percent, they are not as desirable as prisms
which reflect 100 percent.
Applications of the Critical Angle
• Binoculars use various arrangements of prisms to shorten
the length of the optical system.
Total Internal Reflection as Experienced in Water
• Aquatic organisms see many internal reflections when they
look upward at the water’s surface.
Why Do Diamonds Sparkle So Much?
• Diamond has a high refractive index which means it has a
lower critical angle also. Diamonds are specially cut to
produce as much internal reflections as possible. Many
internal reflections redirect light rays out of the top as
flashes of light to the eye.
Applications of the Critical Angle
• Total Internal Reflection means that light can be sent inside
fibers, always relecting since the incident angle is over the
critical angle.
Total Internal Reflection
• In the picture below,
a laser is off-camera
to the right, shining
light into the plastic
tube. This light can
be “piped” in circles
and around a corner
until it comes out
towards your eye.
All along the way,
light reflects
internally since it is
always at angles
greater than the
critical angle.
Total Internal Reflection
• Light can be piped into hard to illuminate areas like places
in the human body and lenses to take pictures can likewise
be inserted in hard to get at places.
Fiber Optics
• Light-conducting fibers are referred to as fiber optics.
• Fiber optic systems can carry signals faster than electricity
and do not lose energy like electrical system do. (Electrical
systems have internal resistance in wires which generates
energy loss as heat)
• Fiber optic cables can carry many more messages at a
much faster rate than conventional electric systems.
Refraction Effects in Nature
• As light enters earth’s atmosphere it is refracted. This
causes the eye to form images of objects which are not
really where they appear to be.
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The Formation of Mirages
• The hot air above the sand causes a refraction of light rays
from the sky. Thus what looks like blue water is actually
refracted rays from the blue sky. (See next slide)
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Shimmer Effects
• Hot air rising and mixing through cooler air (convection
currents) causes vibrating refraction of light. This
produces a shimmer effect or at night, a twinkling effect
when viewing stars.
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