2Demo:

Print:Supplies: pennies, set of styro-cups

AGENDA

• REMINDER: QUIZ, Monday Convex and concave lenses

• Look at money under UV light• Optic Illusions• Convex lenses• Concave lenses

REFRACTION

• The change in direction of a wave due to a change in its speed

• Most commonly observed when a wave passes from one medium to another– When does light change speed?

WHEN DOES LIGHT CHANGE SPEED?

Activities Have students in a line

bounce off a wallDemonstrate refraction

In fluidsIn lenses

Light changes speed when it travels from one optical density to another

WHAT IS A LENS?

• A transparent item used to change the direction and control the distribution of light rays

• Where are lenses used?

WARMUP

Complete the diagrams for soldiers marching into a swamp in formation. – (Use an “X” to represent a

soldier)

Swamp

Swamp

A group of soldiers in formation, march toward a marsh. Depict (using an “x” for each soldier) what happens when

they hit the marsh?

Swamp

AN ANALOGY

THE NORMAL LINE IN

MIRRORS AND LENSES

Mirror GlassAir

Reflection In a Mirror Refraction in a

LensAir

TIPS FOR DRAWING REFRACTION LINES

• In transparent media, normal lines guide transmission of ray, not reflection

• When light travels from lower to higher optical density (i.e. from air to glass) light bends toward the normal

COMPLETE THE FOLLOWING DIAGRAMSGlass Air GlassWater

Air Air AirWater

2TYPES OF LENSES

Convex• A converging lens is thickest in the middle

Concave• A diverging lens is thinnest in the middle

CONVEX• At least one side of the outer lenses

curves outwards • Are thicker in the middle and thinner

on the outer edges

• Used for magnifying an object

Types of Lenses:

HOW CONVEX LENSES WORK• Since the lens is a different medium

than air, the light rays refract• Convex lenses refract parallel light rays

so that they meet at a focal point. (“convergence”)

COMPLETE THE RAY DIAGRAM FOR A CONVEX LENS(HINT: USE A NORMAL LINE TO GUIDE TRANSMITTED RAYS)

• Using normal lines, demonstrate the light rays through the convergent lens:

CONCAVE• At least one side curves inwards. • Thinner in the middle • Make an object appear smaller

Types of Lenses:

HOW CONCAVE LENSES WORK

• Concave lenses cause light rays to “diverge”• Divergence–To separate/curve outwards

COMPLETE THE RAY DIAGRAM FOR A CONCAVE LENS(HINT: USE A NORMAL LINE TO GUIDE TRANSMITTED RAYS)

USING NORMAL LINES, DEMONSTRATE THE LIGHT RAYS THROUGH THE

CONCAVE LENS

WHAT TYPE OF LENS DOES A PROJECTOR USE? (HINT: WHAT DOES A PROJECTOR DO TO

LIGHT?)

1 2

50%50%

1. Concave2. Convex

WHAT TYPE OF LENS IS USED ON A MICROSCOPE OR TELESCOPE?

1 2

50%50%

1. Concave2. Convex

DRAW 6 RAYS TO SHOW HOW LIGHT WOULD REFRACT THROUGH THIS LENS

INDEX OF REFRACTION

• It’s a number that indicates the amount that a transparent medium decreases the speed of light.

• It can be measured as:• Index of refraction of material= speed of light in vacuum• speed of light in medium

SNELL’S LAW

• Snell was a Dutch astronomer who was credited with identifying the exact relationship between the angle of incidence and angle of refraction

• The formula is used to calculate the new angle that a ray will take as a beam of light strikes the interface.

• Indices of refraction: n1, n2

• Angle of incidence and angle of refraction ө1, ө2

SNELL’S EQUATION

n1sinө1 = n2sinө2

TOTAL INTERNAL REFLECTION

• Light reflects completely off the inside wall of a denser medium rather than passing through the wall into a less dense medium.