Ch. 27 – Introduction to Light

I. EM Spectrum

a. Demo – Show red filter and colored pipe cleaners

b. What if you could only see red?! You would have a very limited view of the world. This is the way it is though. We only see a very small part of the Electromagnetic spectrum

c. ------------------------------------------Increasing frequency -----------------à

d.

e. Where does sound fit in? (Sound is vibration of material, EM is vibration of PURE energy)

II. Transparent vs Opaque

a. DEMO – match and mirror

b. If light hits an object it will do a combination of three things

i. Rebound = reflect

ii. Go through = transmit

iii. Stay inside the material = absorbed

c. If most of the light is transmitted, the material is called transparent.

i. Light will take LONGER to go through material than air

ii. Picture:

iii. Each atom most absorb and transmit the wave of light energy

iv. Each one gets excited then readmits the wave

v. Since glass and water are more dense than air, there are more molecules for the light to travel through.

d. If most of the light is absorbed or reflected object = opaque

i. Trapped light makes atoms vibrate

ii. Energy transformed into heat

iii. Black is warmer than white (absorbs more light)

iv. Glass is warmer in sunlight (absorbs UV)

e. Objects REFLECT the color you see

i. Reflects all colors = white

ii. Absorbs all colors = black

f. DEMO – color filter mixing with shadows

g. DEMO – DG – separates white light

III. Shadows

a. DEMO – Hold hand above desk---why no shadow??

b. Solar vs lunar eclipses:

i.

shadow

Solar eclipse – you can’t see the sun. The moon’s shadow falls on the Earth

ii. Lunar eclipse – you can’t see the moon. The Earth shadow falls on the moon

IV. Polarization

a. DEMO – Polarizers on overhead – ohh and ahh J

b. Polarization only allows ONE direction of vibration through filter

c. Many different directions to vibrate perpendicular

d. = 50% of light in ONE direction

e. if crossed = 0% light goes through

f. if angled > 0 but < 50% goes through

g. Reflection can cause polarization = Brewster’s angle.

i. Reflected and transmitted light – 90 degrees to each other

ii. Like skimming rock off water

iii. Light polarized PARALLEL TO SURFACE

h. Which pair of sunglasses would be best for reducing road glare?

Chapter 29 – Reflection and Refraction

I. Reflection

a. Reflection is when light bounces back off the object

b. Law of Reflection – The angle of incidence is always equal to the angle of reflection. Just like billiard balls

c. DEMO – small mirror – where can you stand and see your whole body? Have student try it!

d. What is the smallest mirror that you can see your whole body?

e. Because of finite speed of light, your image in the mirror is always younger than you!

f. Diffuse reflection – rough surface gives blurry reflection, like off a shiny plate or desk

i. Would your book be easier to read if the pages were shinier?

g. Sound reflects the same way

i. Why does your voice sound fuller in the shower?

ii. Reverberation = multiple reflections that ADD together for a more full sound

iii. Amphitheaters are built so that reflections are focused back to where people are sitting

II. Refraction

a. DEMO – fish tank and laser

b. Refraction = bending of wave due to change in medium (change in speed).

c. DEMO – Human wave with arms extended

d. One part of wave slows down before the other causing turning or bending

i. If goes from LESS dense (air) to MORE dense (water) = bends TOWARDS the normal

ii. If goes from MORE dense (water) to LESS dense (air) = bends AWAY from the normal

e. Snell’s Law

i. n₁sinq₁ = n₂sinq₂

1. q₁ = angle of incidence

2. q₂ = angle of refraction

3. n = index of refraction = ratio of speed of wave in medium compared to speed of wave in vacuum. (For air n =1)

a. n = c/v

i. c = speed of light in vacuum = 3 x 10⁸ m/s

ii. v = speed of light in other medium

iii. n is ALWAYS > or = to 1

ii. Different frequencies of light travel at different speeds through same medium

1. So different frequencies have different n’s

2. Cause dispersion = separation of light

3. Violet = highest frequency = bends the most, red = lowest frequency = bends the least

4. Ex. rainbows and prisms (need at least 42^o angle of incidence to notice color difference)

a. so prisms are triangular to give angle

b. rainbows only occur in morning or evening, not at noon

iii. DEMO – fish tank , total internal reflection

iv. Total Internal Reflection = no refraction, all light is reflected back into the medium

a. q_r = 90^o

b. Only works when go from MORE dense to LESS dense

c. Incident angle = CRITICAL angle

v. DEMO – waterfall of fire

vi. DEMO – plastic squares

vii. Fiber optics = use total internal reflection

1. Polar bears hair is like fiber-optics.

viii. Ultrasound = uses refraction of sound to show different densities of liquids and solids

ix. If a bullet were fired through a thick board, it would emerge with a speed that is LESS than its incident speed. It loses energy

x. LIGHT however emerges with the SAME speed. No energy loss.

III. Sample Problem

a. Light is incident upon a piece of quarts at an angle of 45^o. If the index of refraction of quartz is 1.54, what is the angle of refraction of the light?

i. G: n₁ =1, n₂ = 1.54, q₁ = 45

ii. F: q₂ = ?

iii. R: n₁sinq₁ = n₂sinq₂

iv. S: q₂ = sin^-1(n₁sinq₁/n₂)

v. C: q₂ = sin^-1(sin(45)/1.54) = 27.3^o

b. Red light travels at 1.5 x 108 m/s in a piece of plastic. What is the index of refraction of the plastic?

i. G: v = 1.5 x 10⁸ m/s, c = 3 x 10⁸ m/s

ii. F: n =?

iii. R: n = c/v

iv. S: “ ”

v. C: n = (3 x 10⁸ m/s)/( 1.5 x 10⁸ m/s) = 2

c. What is the critical angle of oil if its index of refraction is 1.4?

i. G: n₁ =1.4, n₂ = 1, q₂ = 90

ii. F: q₁ = ?

iii. R: n₁sinq₁ = n₂sinq₂

iv. S: q₁ = sin^-1(n₂sinq₂/n₁)

v. C: q₂ = sin^-1(sin(90)/1.4) = 43.9^o