Unit 3: Light and Optical Instruments

J. PulickeelSPH3U1

November 2008

Lesson 3: Refraction and Snell’s Law

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!

We’ll get you at the lake

Wow, you guys are

slow!

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 299,792 km/sec• Air 299,704 km/sec• Water 224,900 km/sec• Alcohol 220,435 km/sec• Glass 199,861 km/sec• Salt 194,166 km/sec• Diamond 123,932 km/sec• Silicon 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.

Which One???

Normal

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

Image of light

window

light

As light travels from medium A into medium B, the angle of incidence is 36° and the angle of refraction is 21°a) Does light bend towards or away from the normal? Draw a diagram to prove this.b) What is the angle of the reflection of the partially reflected ray?

Incident rayReflected

ray

Refracted ray

36° 36°

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.

Angle of Refraction

Incident RayAngle of

incidence

Refracted ray

Angle of Refraction

Incident Ray Angle of incidence

Refracted ray

Angle of Refraction

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…

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 Q4• 9.4 Q1,2,4• 9.5 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) …