Transcript Unit 3: Light and Optical Instruments

Unit 3: Light and Optical
Instruments
Lesson 3: Refraction and
Snell’s Law
J. Pulickeel
SPH3U1
November 2008
Reflecting on Reflections
•
Reflections occur when light rays bounce off
objects.
•
We can predict the direction in which reflected
light rays travel by using the laws of reflection
1. The angle of reflection will ALWAYS equal the angle of
incidence
2. The incident ray, the normal and the reflected ray are all in
the same plane
YAY!!! MIRRORS!!!!
Mirror, Mirror on the Wall
Mirror, Mirror on the Wall
What happens in Water?
• In water, objects often don’t appear to be where
they should be.
• Refraction is the bending of light when it
travels from one medium to another.
• Light bends because it changes speed when it
moves between materials that have different
densities
• Density tells you how closely packed the
particles of a material are
Faster than a Speeding
Bullet
• Light travels at 300 000 km/s in air!!!
• It slows down to 200 000 km/s in
glass.
• It slows down to 165 000 km/s in
diamond.
• Light slows down in denser
materials!
Wow, you
guys are
slow!
We’ll
get you
at the
lake
Why does light slow down?
Imagine a car on a road. If one tire hits the gravel, it will cause that tire to slow down.
Because it is moving slower than the other tires, the other tires will cover more distance
per second. This will cause the car to turn into the gravel.
Things will turn towards the more dense medium.
Speed of Light
•
•
•
•
•
•
•
•
Vacuum
Air
Water
Alcohol
Glass
Salt
Diamond
Silicon
299,792 km/sec
299,704 km/sec
224,900 km/sec
220,435 km/sec
199,861 km/sec
194,166 km/sec
123,932 km/sec
74,761 km/sec
Not
dense
Very
Dense
Around a Bend with Light
• When light travels from one medium to a
denser one (air to water), it will bend
towards the normal.
I’m directly under the sun….. Now!
Around a Bend with Light
• When light travels from a dense medium to a lighter
one (water to air), it will bend away the normal.
Normal
Which
One???
Partial Reflection and Refraction
• When light is refracted, it can cause the ray
to split into two. Some will be refracted,
and some will be reflected back into the
medium. This called partial reflection
and refraction.
Click here for animation
(show laser in water)
Imagine you are in a darkened room looking out a window.
Someone turns on the light in the room. Will this help you see
out the window better ? Explain why using a ray diagram
When the light is turned on, it
will experience partial reflection
at the surface of the glass.
This light will illuminate other
objects in the room, so you will
see images of all of them. This
will interfere with your viewing of
objects actually outside.
Image of
light
light
window
a)
b)
As light travels from medium A into medium B, the angle of
incidence is 36° and the angle of refraction is 21°
Does light bend towards or away from the normal? Draw a
diagramto prove this.
What is the angle of the reflection of the partially reflected ray?
Incident ray
Reflected
ray
Refracted
ray
36°
21°
36°
Around a Bend with Light
When light travels from a
dense medium to a lighter
one (water to air), it will bend
away the normal.
Angle of Refraction
Angle of
Refraction
Incident Ray
Angle of
Refraction
Refracted
ray
Incident Ray
Angle of
incidence
Normals
Refracted
ray
Angle of
incidence
Light being Refracted
• Light travels fastest in a vacuum because there
are no medium to slow it down.
• Other mediums produce different speeds, it’s
always less than in a vacuum.
• The ratio of the speed of light in a vacuum (c) to
the speed of light in a given material (v) is called
the index of refraction (n)
Calculate the speed of light in zircon in (a)
m/s and (b) in terms of the speed of light.
Snell’s Law
In the 1600’s Snell determined that the angle of
incidence and the angle of refraction were directly
proportional.
By introducing a constant, we can make this an
equation. Through experimentation, it was determined
that the constant was n. Therefore
This however assumes that the incident ray began in
a vacuum…
The Law’s of Refraction
1.
2. The incident ray and the refracted ray
are on opposite sides of the normal at
the point of incidence, and all three
are in the same plane.
General Equation of Snell’s Law
If a ray is originating in medium 1 and continuing into
medium 2, the general equation of Snell’s Law can be
written as
Light travels from crown glass (g) into water (w).
The angle of incidence in crown glass is 40.0°. What
is the angle of refraction in water
Homework
• 9.3
• 9.4
• 9.5
Q4
Q1,2,4
Q2-4
Note!
• Lab Tomorrow. Read Investigation 9.5.1 (not Activity).
Submitted (?) - Hand written, as group on Mon/Tues ?
• Problem Set Posted. Due Friday December 4
• Test Dec 8th on Chapter 9-10
• Project Dec. 14 (maybe 1-2 days later) …