Index of Refraction

11.1 Refraction of Light
Learning Goals
• Learn about how light travels through
different media – refraction
• Learn how to calculate the index of
Refraction is the bending
of light as it travels into
medium with a different
index of refraction.
The direction of the ray
Why does it change?
Fermat’s Principle
A ray of light takes the path that will take the least
amount of time
It will stay in the faster medium as long as it can
A lifeguard hears cries of distress from someone in the water. To
reach the drowning victim as soon as possible, the lifeguard must
take one of three paths shown in the diagram below. What is the
fastest choice?
Refracted Rays
Refracted ray: the ray that is bent
upon entering a second medium
Angle of refraction (R): the angle
between the normal and a
refracted ray
• Not always equal to angles of
Reflection always occurs when a
ray refracts
What way does it bend?
When light goes from
FASTER  SLOWER medium: bend toward normal
SLOWER  FASTER medium: bend away from normal
How much a light ray refracts is determined by the extent
of the change in the speed of light as it travels from one
medium to another.
As the change in speed of light becomes greater, the
angle of refraction becomes greater.
Pg 454
Index of Refraction
Index of Refraction: the ratio of the speed of light in a
vacuum to the speed of light in a given medium
n : index of refraction
c : speed of light in a vacuum (3 x 108m/s)
v : speed of light in a medium
Pg 454
Sample Problem
Calculate the speed of light in fused quartz.
The separating of light into
colours by refraction
It is due to different speeds
Which coloured light
travels fastest? slowest?
• Chapter 10 Quiz Thursday
• Read 11.1
• Questions: pg 455 # 1 – 4 and pg 456
# 1 – 3, 5, 8