Light & Optics – Lesson #10
Unit #3, Lesson #10
75 min
Minds
On…
5 MIN
Action!
40 MIN
Learning Goals
-Refraction of light
-Index of Refraction
-Rules of Refraction
-Refraction ray diagrams
Whole Class Demonstration
Demonstrate refraction: Glass of water and glowstick.
We have learned that light travels in a straight line so long as it remains in one
medium. Where the medium is the material the light is transmitted through (i.e. air,
vacuum, water, etc.). We also learned that light travels at a constant speed in a
vacuum of 3 x 108 m/s, or 300 million metres per second.
So then what happens to light as it travels from one medium to another?
What occurs is something called refraction. What this means is that as light travels say
from the air into the water, the direction of the light rays changes, as well as the
speed of the light. The speed of light will slow down when going into a more dense
medium because there are more particles in which the light interacts with.
What’s happening?: Light travels from the water into the air, bending away from the
normal to the surface. Our brain assumes the light travels straight and projects a
virtual image of the straw in the water in a different location. Therefore the straw
appears displaced, when it actually is not!
Whole ClassPresentation
Present & Discuss PowerPoint presentation
Analogy: To visualize what happens when a wave front reaches the surface between
the two media, called the boundary think of a Car driving on pavement at an angle
slows down and changes direction when it crosses the boundary and enters mud.
Or, we can think of a marching band, lines of students, crossing from grass into
mud.
Fermat’s Principal: The exact path of light as it travels from one medium to another
can be found by applying Fermat’s principle, which says that when light travels from
one point to another, it will follow a path that will take the least time. In a single
medium the path that takes the least time is a straight line. When traveling from
one medium to another the path that takes the least time is not a straight line.
Ray Diagrams for Refraction:
Most of the terms used to describe refraction are the same as the terms used to
describe reflection. In addition to the incident ray and reflected ray there is now
a refracted ray. The incident ray is divided into two rays, one that reflects and one
the refracts. With this new ray we must assign a new angle, which we call the angle
of refraction, θR. The angle of refraction is the angle between the normal and the
reflected ray.
SNC 2D,
Grade 10
Materials
 Semicircles
 Water, oil,
plastic
 Rayboxes
Light & Optics – Lesson #10
Rules of Refraction:
1) When a ray of light passes into a slower, more optically dense medium, the ray of
light bends towards the normal. (example air  glass)
2) When a ray of light passes into a faster, less optically dense medium, the ray of light
bends away from the normal. (example glass  water)
3) When the angle of incidence is zero, there is no refraction. (No change in direction,
yes a change in speed)
Index of Refraction:
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. When light passes from one medium
to another the greater the change in the speed of light, the greater the angle of
refraction. The index of refraction is a constant that describes how fast light travels
in a specific medium. It is the ratio of the speed of light in a vacuum to the speed of
light in a given medium: n = c/v, where c = the speed of light in a vacuum, 3x108
m/s.
Calculating the Index of Refraction:
n=c/v
Examples:
See presentation for examples.
IndividualMini-Lab
Lab with semi-circular containers and water or just acrylic semi-circles.
Consoli
date
Debrief
IndividualCheckout Problem
1) What is refraction? Why does it occur?
2) The speed of light in a vacuum is ______, the speed of light in _____ is _____
calculate the index of refraction for the medium.
15 MIN
Home Activity or Further Classroom Consolidation
Homework: pg 456, Qs: 1-7
Light & Optics – Lesson #10
Investigating & Analyzing Refraction in Different Media
Name:__________________
Mark:___________/15
Answer the following:
1) Calculate the speed of light in water given that the index of refraction of water is
n = 1.33 [1 mark]
2) Calculate the speed of light in oil given that the index of refraction of oil is n =
1.47 [1 mark]
3) Calculate the speed of light in Plexiglass given that the index of refraction of
Plexiglass is n = 1.51 [1 mark]
Procedure: Complete the following data table using a raybox and a single ray pointed at
the semicircular containers of each material. Trace the ray of light as it passes through
each medium on polar graph paper or with a protractor on a piece of paper. Measure
and record the angles of incidence and refraction. [6 marks]
Material
Water
Oil
Plexiglass
Angle of
Incidence, θi
20
Angle of
Refraction, θR
Angle Difference:
θi - θR
Index of
Refraction
1.33
40
1.33
20
1.47
40
1.47
20
1.51
40
1.51
Light & Optics – Lesson #10
Analysis:
1) What happens when the light ray travels from air into the more dense mediums?
[1 mark]
2) What does a larger index of refraction imply about the speed of light in a
medium? [1 mark]
3) Does the angle of refraction bend toward the normal relative to the angle of
incidence? What does this imply about the speed of light in the mediums you
were using? [2 marks]
4) How do the angles of refraction and incidence change in media with different
indices of refraction? Reference your data specifically. [1 mark]
5) Does the difference in angle of incidence and angle of refraction (θi - θR) increase
or decrease with increasing index of refraction? Reference your data specifically.
[1 mark]
Light & Optics – Lesson #10