Chapter 7:Understanding Light

Topic 7.1: What is Light?Topic 7.2: Reflecting on

ReflectionsTopic 7.3:Refraction

Topic: What is Light?A.The First Property of

Light Light is a form

of energy. There are natural

light sources: sun,fire,stars.

A.The First Property of Light:

Energy can change forms- from natural to artificial.

Ex. Solar to thermal. Ex. Solar to chemical.

Ex. Solar to electrical

A. Light: cont’d

Artificial light sources: light bulb, match (chemical), a flashlight (electrical)

Thomas Edison (1847-1931)

B. Sources of Light:1. Incandescent

Incandescent light source: they are so hot they glow.glow. Ex. Light bulbs.

Electrical energy is converted into 95% thermal energy and 5% light energy

2. Luminescent

Luminescent light: does not need to be hot to create light.

The following, (a)-(d), are all types of luminescent light sources.

a) Fluorescent light:

Ex. Fluorescent light bulb. Advantage: does not waste energy as heat

(is more energy efficient.) Disadvantage: Filled with mercury (Hg) and

phosphorus (P) , both toxic chemicals

b) Phosphorescent light:

Ex. Glow in the dark items.

Particles absorb light (radiant) NRG and store it for a time.

c) Chemiluminescent light source:

Ex. Glow sticks

Light energy released from a reaction between two chemicals.

d) Bioluminescent light Source:

Ex. Fire flies Chemical

reactions in a living organism provide energy for light.

Microscopic organism- white spot on the left is the bioluminescence

Bioluminescent mushrooms

Examples of bioluminescence

C. Watt

The Watt: is the unit of electrical power(W). It indicates the rate at which energy is consumed.

1000 watts= a kilowatt (kW)

Kilowatt-hour is the amount of electrical NRG used in one hour.(kW•h)James Watt (1736-

1819)

Homework:

BLM7-1BLM 7-2BLM 7-5

D. Second Basic Property of Light:

Light travels in a straight line.

A ray diagram is used to show the direction in which light is traveling.

D. Light travels in a straight line

Sun rays through clouds, and shining down into a gorge.

D. Light travels in a straight line.

Light travels in a straight line until the light strikes an object or material.

The material can be opaque: that is not let any light pass through it.

Ex. wood, metal.

D. Light travels in a straight line

The material can be translucent: Some light passes through it and it is scattered.

Ex. Frosted glass, wax paper, stained glass.

D. Light travels in a straight line.

The material can be transparent: All light passes through freely.

Ex. Glass.

Homework:

BLM 7-8 P.215 #1-4, 6, 7

Topic 7-2: Reflecting on Reflections

Reflection occurs when light bounces off a surface.

If the surface is smooth, a clear image is produced.

If the surface is rough, a diffuse reflection results.

A. Laws of Reflection

First Law of Reflection:

The Angle of incidence

is equal to the angle of

reflection.

A. Laws of Reflection

Second Law of reflection:

The incident ray, thereflected ray and

thenormal are all in thesame plane. This supports the

second basic property of light.

B. Optical Illusion

Optical illusions fool our brain into making false conclusions.

And they’re lots of fun!

Man playing a horn or silhouette of a woman?

Wine glass or 2 people?

C. Predictable Behavior of Light

Light rays will either be : Absorbed Reflected Refracted

Reflection: Occurs when light hits a smooth shiny surface and “bounces” off. EX. mirror

“Smooth as glass…”

C. Predictable Behavior-Reflection

All of the light that reaches a plane(flat) mirror is reflected.

C. Predictable Behavior-Reflection

This will produce a clear image that is: Upright Same size Same shape Virtual- looks like it is

at the same distance behind the mirror as the actual object is in front of the mirror.

C. Predictable Behavior-Reflection

C. Predictable Behavior-Reflection

Any smooth shiny surface will reflect some light rays and images can be seen.

C. Predictable Behavior-Reflection

Very cool…

Homework:

P.215 #1-4, 6, 7 BLM 7-8 ,7-10, 7-11, 7-12aq Lab- next class YOU NEED A PROTRACTOR FOR

THIS UNIT.

Topic 7-3: Refraction

Light travels straight and fast.

The speed of light in air is 300,000 km/s.

Light rays travel more slowly through H2O, glass, etc.

Refraction

Topic 7-3: Refraction

Light from the sun takes about 8 minutes to reach the earth!

Refraction: cont’d

Rainbows are an example of refraction.

 

Refraction: cont’d

Light slows down as it travels through a denser medium.

The Normal is perpendicular to the edge of the material.

Refraction

Light rays bend as they move from materials of differing densities.

Ex. Air to glass Some light rays

are reflected, some refract.

Refraction: cont’d

Relationship with respect to density:

As density increases, the angle of refraction decreases.

An inverse relationship.

Refraction: cont’d

When light travels through a less dense medium, the light ray will bend away from the Normal.

Refraction: cont’d

The fish sees the bird in a refracted position.

Water to air: density decreases;angle of refraction increases.

Bends away from the Normal

Homework

BLM 7-14, 7-18 Draw diagram

7.23 (p. 230) Draw table 7.1

(p.231)