Unit on LightChapter 14 – Light and ReflectionChapter 15 – RefractionChapter 16 –Interference and Diffraction

Chapter 14 The Nature of Light“Light” comes in a wide variety of frequencies and wavelengths. What is constant? Speed – all form of EM Radiation travel at the same speed – 3.0 x 108 m/sVisible Light – very narrow range of light that humans can

4.3 x 10 14 Hz -- 7.5 x 10 14 Hz

EM video

Electromagnetic Waves (EM) – Transverse Waves - Oscillating electric and magnetic fields moving at right angles to each other – perpendicular to motion of wave

Electromagnetic Spectrum (EM)Long wave lengths (low frequencies) Short wave lengths (high frequencies)

ELECTROMAGNETIC SPECTRUM

ENERGY TYPE PROPERTIES

Higher X-Rays/ Go through everything Gamma Rays except lead.

Ultra Violet - Penetrates several layers of skin.

Visible - The Rainbow- can be ROYGBIV can be seen, no penetrating power

Infrared - it is absorbed and ( heat ) makes the molecules get hotter.

Microwaves - can either become heat, or travel through with no effect.

Lower Radio - go through everything but metal

Dual Nature of Light (Electromagnetic Energy) behaves as both a wave and a particle EM radiation travel as waves and move energy EM radiation is also a

stream of particles called photons – bundles of energy. MAX PLANCK – (1900) Black Body Radiation - If you heat an object it

will begin to give off light. Planck concluded that light is ‘quantized’. This means that it gives off energy that is proportional to its frequency.

As temp increases total energy given off an object increases and the peak shifts to shorter wavelengths

EPhoton = h • f

h = Planck’s constant = 6.63 x 10 -34 J x s

Energy of a Photon

Einstein used Planck’s idea to explain the PHOTO ELECTRIC EFFECT

When light hits a metal, electrons ( e- ) are given off.

The Kinetic Energy of the electron depends on the COLOR of the light, NOT its BRIGHTNESS.

EINSTEIN There is a Threshold Energy which is the minimum energy needed to knock the electrons off. Any extra energy became the K.E. of the e-

Example From the equation ( E = h f ) and the equation for waves we have:

λ x f = c ( speed of light = 3 x 108 m/s )E = h f

COLOR λ(m) f (Hz) E(J) I.R 8.0 x 10-7 ________ _______ Red ________ 4.6 x 1014 _______ Yellow 5.7 x 10-7 ________ _______ Green ________ 5.9 x 1014 _______ Blue 4.7 x 10-7 ________ _______ Violet ________ 7.0 x 1014 _______

Example From the equation ( E = h f ) and the equation for waves we have:

λ x f = c ( speed of light = 3 x 108 m/s )(in a vacuum) but as light passes through a medium it slows down

COLOR λ(m) f (Hz) E(J) = h x f I.R 8.0 x 10-7 3.75 x 10 14

2.49 x 10 -19

Red 6.52 x 10 -7 4.6 x 1014 ________ Yellow 5.7 x 10-7 5.26 x 10 14 ________ Green 5.1 x 10 -7 5.9 x 1014 ________ Blue 4.7 x 10-7 6.38 x 10 14 ________ Violet 4.29 x 10-7 7.0 x 1014 _________

MATTER and ENERGY

Einstein used his famous equationE = m x c2

(energy and mass are equivalent!)

and Planck’s equation ( E = h x f )to determine that PHOTONS do have MASS!

m x c2 = h x f

(substitute for f = c/λ )mphoton = h

c x λ

LOUIS de BROGLIE – p. 466

Took Einstein’s and Plank’s ideas and said that maybe ALL matter has wavelike properties not just particle like properties.

If matter has a velocity, then it has a wavelength.

de Broglie’s wavelength:Formula: λ = h = h

p m x v

p = momentum h = Planck’s constantm = mass of photonv = velocity

Photoelectric Effect light as particle and wave

Lower frequencies travel ___________ Faster (red light)

Higher frequencies travel ___________ Slower (blue light)

WHY?

Lower frequencies don’t make the e- “jump” as far.

Higher frequencies make the e- “jump” to higher levels – but they take more time to do this, or they may cause ionization (e- jumps off the atom altogether ).

Electron Dance – light propagating through a medium

Seeing Colors When you see a wavelength of 550 nm your brain interprets green. Objects reflect certain wavelengths and absorb others. “White” light passed through a prism – or a diffraction grating will

separate in different colors.

When white light strikes a leaf, the leaf reflects light with a wavelength of 550 nm – green The leaf absorbs all the other wavelengths

Colors may add or subtract Additive primary colors = red, green and blue. If mixed together they make white. Subtracted primary colors - Reverse process

All three subtracted primary colors mixed together make black – the absence of color.

Refraction is the bending of the path of a light wave as it passes across the boundary separating two media.

Refraction is caused by the change in speed

Index of Refraction:This is a ratio of how much faster light is in space compared to a specific medium.

Index of Refraction (n) = speed of light in vacuum (air) speed of light in medium

Formula: n = c vmedium

Chapter 15 Refraction

Medium n Velocity

Space 1.00000 3.00 x 108 m/s

Air 1.00029

Nail Polish Remover 1.36

sugar soln. (30%) 1.38

sugar soln. (80%) 2.01 x 108m/s

crown glass 1.52

calcite 1.6583

sapphire 1.77

diamond 2.42

n is related to optical density of the material. Which substance is most dense?

SNELL’S LAWWhen waves travel from one medium to another that has a different velocity, they refract, or change direction. The amount of that change is given by the equation:

n1 sin θ1 = n2 sin θ2

All angles are measured to the Normal!Rays are refracted towards the normal if ni < nr

n1

faster

n2

slower

θ1

θ2

Air = n = 1.00

Water = n = 1.33

Draw and label Diagram

If light enters a medium that is more at a perpendicular angle the light ray will not refract – only slow down or speed up

Fish appears closer to the cat than it really is

Double refraction of

Calcite

p. 583 Dispersion – incoming ‘white’ light of different wavelength is bent at different angles as it moves into a refracting material.

Index of refraction (n) is dependent on the frequency and wavelength – value for n decreases with increasing wavelength

Blue light (λ = 470 nm) bends more than red light (λ = 650 nm) Shorter wavelengths of light (violet and blue) are slowed more

and consequently experience more bending than red wavelengths (orange and red).

http://www.micro.magnet.fsu.edu/primer/lightandcolor/refractionhome.html

Prisms disperse ‘white’ light into the ‘Visible Spectrum’ by refraction and dispersion of light

Rainbows are created by dispersion and refraction of light in water droplets

Red on top of Rainbows

p. 604 Diffraction

Bending of waves around obstacles – or the spreading of waves by passing them through an aperture, or opening light deviates or bends from a straight line path Supports Wave Theory of light Diffraction Animation

Interference of Light – p. 598

Light passed trough narrow slits produce a series of bright and dark parallel bands

o Bright – constructive interferenceo maximao Dark – destructive interferenceo minima

CD’s – example of

interference and diffraction

- Also the reflection of

colors

Interference Phenomena in Soap, plastics - Iridescence

Diffraction grating –Consists of many closely spaced narrow slits Diffraction and interference to disperse light into its component

colors – Used in spectroscopes - chemical analysis of stars, and interstellar

gases, structure of atoms and molecules. Holograms – 3D impression of an object

Polarization – p 546 Light is a transverse wave vibrating in and out Normal light vibrates in all directions Polarization - the vibrations of light occur in a single plane

Propagation of EM Waves

Polarization of Light

Light becomes polarized when it interacts with a substance that only allows it to vibrate in one plane. Certain substances only allow light to pass through if it is vibrating in the proper direction. The ability to polarize light depends on the molecular structure

Polaroid lenses reduce glare Sunglasses Polarize Light vertically – Reduces the glare given off of lakes/ocean/cars Polarized lenses cut out the horizontal component.

Polarized filters on cameras – better, clearer image

Polarization can be used to analyze substances, check for stress, and find weaknesses.

EM Waves are Transverse Waves

Dispersion of light