Total Internal Reflection
Light at the Bottom of Lake
• Recall:
• Light bends towards the normal when it slows
down (air into any other substance).
• Light bends away from the normal when it
speeds up (water into air)
• University of Toronto: Reflection and
Refraction with Total Internal Reflection
• Reflection and Refraction
• -If you continue to increase the angle of incidence,
angle of refraction increases as well.
• -it will eventually get to a point where the angle of
refraction becomes 90o
• -The angle of incidence at this point is called the
critical angle.
• At this point, if you increase angle of incidence,
there will be no more refraction, but reflection back
into the medium.
Total Internal Reflection occurs when
two conditions are met:
• Total internal reflection occurs when:
• 1) Light is travelling more slowly in the first
medium than in the second.
• 2) The angle of incidence is large enough that
no refraction occurs in the second medium.
Instead the ray is reflected back into the first
medium.
• http://www.gcse.com/waves/tir.htm
• For example, when you reach for a rock you see
at the bottom of a shallow pond, you may come
up empty handed unless you take refraction
into account. You automatically extend the light
rays that reach your eyes in a straight line to the
place on the bottom of the pond where the
rock appears to be.
• When the refracted ray is gone, and only the
reflected ray is visible it is called total internal
reflection.
• Laser in milk-water combo.
• If you continue to increase the angle of incidence, angle
of refraction increases as well.
• -it will eventually get to a point where the angle of
refraction becomes 90o
• -The angle of incidence at this point is called the critical
angle. (critical angle is unique for each substance)
• At this point, if you increase angle of incidence, there will
be no more refraction, but reflection back into the
medium.
Diamonds are a girl’s best friend….
• Diamond has a critical angle of 24.4o.
(index of refraction = 2.42, slows down light a lot)
• A badly cut diamond will not allow light to internally
reflect a few times.
• Usually good diamonds have a high refractive index
which means that diamonds have a very small
critical angle. So a great deal of incident light
undergoes total internal reflection inside the
diamond.
• A light ray can bounce around several times inside
the diamond before eventually exiting through a top
face of the gemstone
• Place a penny on the bottom of a
large glass and look at it from above
the glass. ... It disappears if you look
at it with an angle greater than 40
degrees. . This angle corresponds to
the critical angle of for light passing
from water to air. Light that strikes
this interface at an angle greater than
this angle will not reach your eyes.
• Fiber Optics
• Critical angle of pure glass and special plastics
(Lucite) have small critical angles. Light gets
trapped within fiber optics because critical
angle is small. Used in phones, computers
(internet), TV’s (HD, cable, etc.) medicine,
Christmas lights, etc.
• Triangular Prism
• Critical angle for glass is 41.1o.
• Can use triangular prisms to reflect
light.
• Better than mirrors because mirrors
lose some light through
absorptionand silver surface
deteriorates over time.
• Used in cameras, binoculars,
periscopes etc.. Can reflect rays 90o
or 180o.
Retro-Reflectors and Prisms
• A retro-reflector is an optical device that
returns any incident light back in exactly the
same direction from which it came
Prism annnnnnd retroreflector!
They are built into bike reflectors
and the reflective strips on clothing
and helmets. Road signs also
contain tiny retro-reflectors in the
paint so you can see the signs at
night!
Diagram for phenomenon of refraction.
• Refraction and Images video from media on
demand to conclude lesson.
-Homework page 531 1-5, 8