Grade 10 Academic Science – Optics (Physics)
Index of Refraction
Science Perspectives 19 Section 12.4 Pages 524-525
The speed of light is different for each medium AND it is always less than the speed of light in a vacuum. For the
latter point, why?
INDEX OF REFRACTION

Ratio of the speed of light in a vacuum to the speed of light in a medium

Ratio of the sine of the Angle of Incidence in a vacuum to the sine of the Angle of Refraction

Mathematical formulas
n = c/v
Where n is the Index of Refraction, c is the speed of light in a vacuum and v in the speed of
light in a given medium
n = sin i / sin R
Where I is the Angle of Incidence and R is the Angle of Refraction
Example
Speed of light in a diamond is 1.24 X 108 m/s. Knowing that the speed of light in
air is approximately 3.00 X 108 m/s, what is the Index of Refraction for diamond?
Given

Vdiamond = Speed of light in a diamond = 1.24 X 108 m/s

c = Speed of light in air = 3.00 X 108 m/s
Required

n = Index of Refraction of diamond
Analysis
n = c/v
Solve
n = 3.00 X 108 / 1.24 X 108
= 2.42
Present

The Index of Refraction for diamond is 2.42
QUESTION
What is the Index of Refraction of 2.42 telling us?
Task

Given the Index of Refraction for various media, find the speed of light in each medium.
Medium
Index of Refraction (n)
H2O
1.33
Glass
1.52
Zircon
1.92
Quartz
1.46
Speed of light
(m/s)
Why is the Index of Refraction (n) a DIMENSIONLESS QUANTITY? Explain in your own words.
You are presented with the following problem and the two diagrams.
What is the Index of Refraction for jello (njello)?
HINT: In this case, what does the slope represent?
FORMULA nwater to jello = nwater / njello
BONUS – From the diagrams, how do you know the SPEED OF
LIGHT is faster for jello than water? NOTE: Dotted line is the Normal.

Homework
Page 525 - Questions 1-3, 5, 6, 9
Total Internal Reflection
Science Perspectives 10 Section 12.5 Pages 526-531
When light travels from one medium to another, some light is
REFLECTED and some is REFRACTED. If light slows down when it
travels through different media, the light BENDS TOWARD the
Normal in the slower media and BENDS AWAY from the media as it
speed up. If light speed ups, the ANGLE OF REFRACTION is
larger than the ANGLE OF INCIDENCE, and visa versa. What does
the diagram illustrate?
The Angle of Incidence where the Angle of Refraction is 90O
is called CRITICAL ANGLE. In the diagram, where do you
measure the 90O?
If the Angle of Incidence exceeds the Critical Angle, all light is
reflected back into the media. This is called TOTAL INTERNAL
REFLECTION.
Applications of Total Internal Reflection
Science Perspectives 10 Section 12.5 Pages 526-330
Diamond Cutting (see page 527)
 Diamonds have a very high index of refraction, and therefore, they have a small critical angle.
 Diamonds are cut so that most of the light that enters is reflected internally and leaves through the top
surface.
Fibre Optics (see page 528)
 Glass fibres that are the width of a human hair can carry
information with light signals.
 Strands of glass fibres carry light with almost no loss of
energy because light undergoes total internal reflection.
 Fibre optics cables have revolutionized
telecommunications and replaced copper wire.
 One fibre can carry tens of thousands of messages at
the same time.
 They are also used in sensors and surgical cameras
Optical Instruments (see page 529)
 The critical angle for glass is about 41o.
Using a right-angled triangular prism, the
prism can be situated so that light inside
the prism strikes the surface at 45o and
therefore undergoes total internal
reflection (see diagram).
 Good binoculars and periscopes use
glass prisms instead of mirrors to reflect
light because there is less loss of light and
they do not tarnish.
HOMEWORK
 Page 531 Questions 1-5