Total Internal Reflection: _________________________________________________
________________________________________________________________________
________________________________________________________________________
_______________________________________________________________________.
Total Internal Reflection only takes place when both of the following two conditions
are met:


__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
For a person to see an object underwater, light must hit the object, reflect off it, and travel
to your eyes.
When light is going from a less dense to a more dense medium, there is always some
light refracted and some light reflected. But when light is going from a more dense to a
less dense medium, there is no refracted light and all light is reflected if the angle of
incidence is greater than a certain value.
This "special" angle of incidence is called the critical angle of incidence and its size
depends on the refractive index of the two media.
http://www.youtube.com/watch?v=8VZHym6HqVU&NR=1&feature=endscreen
http://www.youtube.com/watch?v=2kBOqfS0nmE
Check Your Understanding
For each combination of media, which light ray (A or B) will undergo total internal
reflection if the incident angle is gradually increased?
REMEMBER: Total Internal Reflection only takes place when both of the following two
conditions are met:


A light ray is in the more dense medium and approaching the less dense medium.
The angle of incidence for the light ray is greater than the so-called critical angle.
Answers:
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.
If you want to change the direction of light, you can use a glass prism because it creates
the conditions for total internal reflection. The critical angle between glass and air is less
than 45 degrees. Therefore, light hitting an inner surface at exactly 45 degrees will be
totally reflected inside the glass. The smaller the index of refraction, the more TIR will
happen.
Sparkle of Diamonds
The critical angle for the diamond-air boundary is an extremely small number. The combination of
diamond and air provides one of the largest differences in the index of refraction values. Having a small
critical angle, light has the tendency to become "trapped" inside of a diamond once it enters. A light ray
will typically undergo TIR several times before finally refracting out of the diamond. Because the
diamond-air boundary has such a small critical angle (due to diamond's large index of refraction), most
rays approach the diamond at angles of incidence greater than the critical angle. This gives diamonds a
tendency to sparkle. The effect can be enhanced by the cutting of a diamond gemstone with a
strategically planned shape. The diagram below depicts the total internal reflection within a diamond
gemstone with a strategic and a non-strategic cut.
Optical Fibres
The main function for Optical Fibres is to send information through it by transmitting a beam of light
from one end to another, trying to have as little quality loss as possible.
Optical Fibres are more commonly used in surgery as a means of a tiny camera for surgeons in a
patient's body and used together in their thousands in Fibre Optic communication cables for telephones
and high-speed Internet connections. Each Optical Fibres has a core of thin glass, which is wrapped then
by a cladding (usually a plastic with a lower refractive index than glass) and is then finally all wrapped
in a harder plastic buffer to prevent damage.
Homework Questions:
1. Define the terms critical angle, and total internal reflection.
2. Under what conditions will nearly all the light that reaches a boundary between two different
media be refracted?
3. What two conditions must exist for total internal reflection?
4. Evaluate the impact of the development of optical fibres. What type of application do you think
has the greatest impact on our lives? (Fibre optics in telecommunications or in Medicine).
5. Diamonds have a large index of refraction of 2.42, and they are cut with many facets. Based on
what you know about refraction, explain why cut diamonds sparkle more than any other cut
stones. (A facet of a diamond is a small, flat area)
QUESTION:
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?
Answer:
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.