Early Work – Mar. 6

• List at least three places where you see or use reflections.

Labs

• Light and Color

• Law of Reflection

• Law of Refractions

• Everyone turns in a lab

• 4 in a group

Early Work – Mar. 8

• What’s the difference between reflection and refraction?

Lab

• In groups of four, with six mirrors, align the mirrors at angles so that one object can “see” another object.– First put butcher paper on top of your desks– After mirrors are aligned, mark mirror

placements (with chalk)– Draw the line of sight between each mirror– Measure the angles made with the normal

• Write a few sentences about what you discovered doing this lab.

Ch 17

Reflection and Refraction

Demo

Law of Reflection

• Angle of Incidence– The angle the incoming

beam makes with the normal

• Angle of Reflection– The angle the outgoing

beam makes with the normal

Angle of Incidence = Angle of Reflection

Regular Reflection

• When parallel rays hit a smooth surface, such as a mirror, they are reflected in parallel rays

Diffuse Reflection

• When parallel rays hit a rough surface, such as the page of a book or a wall, they are reflected in all directions

Refraction

• A waves change in direction at a boundary

• The beam in the first medium is called the incident ray

• The beam in the second medium is called the refracted ray

• The angle of refraction is the angle the refracted ray makes with the normal– If the angle of refraction is

smaller than the angle of incidence, then the new medium is more optically dense

Snell’s Law

• Willebrord Snell, in 1621, wrote– The ratio of the sine of the angle of incidence

to the sine of the angle of refraction is a constant (depending on the medium)

r

in

sin

sinBut this is only for a vacuum!

Snell’s Law in General Cases

rrii nn sinsin

“n” found on P397 in book!

Example

• A light beam in air hits a sheet of crown glass at an angle of 30.0°. At what angle is it refracted?

Index of Refraction and the Speed of Light

• The index of refraction is not only a measure of how much the light will bend, – But also of how fast light travels in the

medium

substancesubstance

v

cn

Example

• Find the speed of light in water.

Early Work – Mar. 12

• A ray of light strikes a mirror at an angle of incidence of 28°. What is the angle of reflection?

• A ray of light has angle of incidence of 25.0° upon the surface of a piece of quartz. What is the angle of refraction?

Total Internal Reflection

Critical Angle

• The critical angle, θC, of a substance is the incident angle that causes the refracted ray to lie right along the boundary of the substance.

Only occurs when going from a more optically dense medium into a lesser one

(such as air)

To find θC, use Snell’s Law

Critical Angle

rrii nn sinsin

90sinsin rii nn

rii nn sin

i

ri

n

nsini

ri

n

narcsin

Effects of Refraction

• Mirages

• Lingering sunlight

• Shift of images in liquid

Rainbows

Bkwk

• P410: 1, 2, 3, 5, 6, 10, 11, 15, 16, 21, 22, 24, 31, 32, 33, 35, 36, 38, 39, 40, 44, 48, 49, 50, 51,

• Ch. 17 Quiz Wednesday! – Must take prior to spring break