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.
We’ll get you at the lake Wow, you guys are slow!
• It slows down to 165 000 km/s in diamond.
•
Light slows down in denser materials!
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.
light window Image of light
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?
Reflected ray
36 ° 36 ° Incident ray
Refracted ray
21 °
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.
Incident Ray
Angle of Refraction
Angle of Refraction Incident Ray Angle of incidence Refracted ray Angle of Refraction Refracted ray Angle of incidence Normals
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…
1.
The Law’s of Refraction
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
• 9.3 • 9.4 • 9.5
Homework
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 8 th
on Chapter 9-10
Project Dec. 14
(maybe 1 2 days later) …