Transcript Total internal reflection and refractive index recap
Total internal reflection and refractive index recap
Starter – draw these diagrams and describe which one shows:
i=C, i
28 April 2020
TIR and Refractive Index recap
Objectives Be able to describe what Total Internal Reflection is and use equations including the refractive index.
OUTCOMES
• • • • • •
ALL WILL
Be able to define Refractive Index and Total Internal Reflection and use other important keywords correctly
MOST SHOULD
Explain in detail how TIR is used in medicine.
SOME COULD
Apply and manipulate Snell’s Law to show how the critical angle is related to the refractive index.
•
Word loop
Look these up in the glossary
Definition Key Term
Refractive Index Total Internal Reflection Critical Angle Refraction Frequency Wavelength Lens Optical Fibre Endoscope
Endoscopes
Endoscopes
Flexible fibre bundles Light source Steering controls Eyepiece Cleaning water Objective lens
How do we get a clear picture?
Use a “coherent bundle” (light all going down and back at the same time) Relative position of fibres is the same at each end Each fibre is one pixel
How they work
• • Coherent light is shone down the cable At the surface of the cable Total Internal Reflection occurs • 2 factors need to be ensured for Total Internal Reflection to occur: (i) the angle of incidence is bigger than the Critical Angle and (ii) the cable’s refractive index is bigger that what is around it Total internal Reflection at surface
NOW: Peer assess: Make a statement that says how well they seem to have answered the criteria And then do WWW / EBI. Make comments related to the questions rather than their presentation or effort
Optical Fibre Demonstration
• Example of peer assessment: http://www.youtube.com
/watch?v=GWkYo9SxtLo WWW: You have described how a fibre optic cable works – AND ext: how it can be made!! You have also described how they can be used in endoscopes.
EBI: Make sure you include arrows on ray diagrams to show the direction of light and label it so it shows where the TOTAL INTERNAL REFLECTION OCCURS • • • 1 Describe in your own words how a fibre optic cable works (extension – explain how they can be made) 2 Draw a diagram and show where Total Internal Reflection occurs and what 2 factors are needed for this 3 and how they are useful in diagnosis in medicine.
Now reflect on their marking and correct your work or add to it
The refraction equation
When a light ray passes from one medium to another:
i n = sin i sin r where: i
is the angle of incidence in the first medium
r
is the angle of refraction in the second medium
r n
is a constant number called the refractive index .
So n, the refractive index is a way to compare the amount of refraction between 2 different mediums.
Critical angle equation
The critical angle is the angle of incidence in the that results in an angle of refraction of 90º denser medium critical angle
What is sin
90º?
n = 1 sin c
GLASS AIR NORMAL
where:
angle of refraction = 90º
n
is the refractive index of the denser medium (glass in the example opposite).
c
is the critical angle .
Important Notes:
•
The refractive Index equals:
n
=
sin i sin r
•
When the critical angle is reached and i=90,
n
=
1 sin c
C is the critical angle for that medium
Refractive index questions
medium 1 air glass water air air medium 2 water air glass diamond unknown
n
1.33
0.67
1.13
2.40
1.53
i
50 o 30 o 59.8
o 50 o 50 o
r
35.2
o 48.6
o 50 o 18.6
o 30 o
2 Calculate the critical angle of glass to air if the refractive index of glass is 1.5
3 Calculate the critical angle of water to air if the refractive index of glass is 1.3
4 Calculate the maximum refractive index of a medium if light is to escape from it into water (refractive index = 1.3) at all angles below 30 ° .
Question 2
Calculate the critical angle of glass to air if the refractive index of glass is 1.5
n = 1 / sin c
= 1.0 / 1.5
= 0.67
critical angle for glass, c = 42 °
Question 3
Calculate the critical angle of water to air if the refractive index of glass is 1.3
n = 1 / sin c
= 1.0 / 1.3
= 0.75
critical angle for water, c = 49 °
Question 4
Calculate the maximum refractive index of a medium if light is to escape from it into water (refractive index = 1.3) at all angles below 30 ° .
n = 1 / sin c
becomes:
sin c = 1 / n sin 30 °
= 1.3 /
n
0.5 = 1.3 /
n
= 1.3 / 0.5
maximum refractive index, n = 2.6
• wordloop • • • HW: Take your books home and read through them and ask your parents to test you on it all. Ask them to write your score in your book (or you do it) Write a comment at the end. Bits that you understand fully and parts of this half term’s work you are not sure about?