Unit on Light Chapter 14 – Light and Reflection Chapter 15 – Refraction Chapter 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 10 8 m/s Visible Light – very narrow range of light that humans can 4.3 x Hz x 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)

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 E Photon = h f h = Planck’s constant = 6.63 x 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 10 8 m/s ) E = h  f COLOR λ(m) f (Hz) E(J) I.R 8.0 x _______________ Red ________ 4.6 x _______ Yellow 5.7 x _______________ Green________5.9 x _______ Blue 4.7 x _______________ Violet________7.0 x _______

Example From the equation ( E = h f ) and the equation for waves we have: λ x f = c ( speed of light = 3 x 10 8 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 x x Red 6.52 x x ________ Yellow 5.7 x x ________ Green5.1 x x ________ Blue 4.7 x x ________ Violet4.29 x x _________

MATTER and ENERGY Einstein used his famous equation E = m x c 2 (energy and mass are equivalent!) and Planck’s equation ( E = h x f )to determine that PHOTONS do have MASS! m x c 2 = h x f (substitute for f = c/λ ) m photon = 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 constant m = mass of photon v = 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 speedcaused 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 v medium Chapter 15 Refraction

Medium n Velocity Space x 10 8 m/s Air Nail Polish Remover1.36 sugar soln. (30%)1.38 sugar soln. (80%)2.01 x 10 8 m/s crown glass1.52 calcite sapphire1.77 diamond2.42 n is related to optical density of the material. Which substance is most dense?

SNELL’S LAW When 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: n 1 sin θ 1 = n 2 sin θ 2 All angles are measured to the Normal! Rays are refracted towards the normal if n i < n r n 1 faster n 2 slower θ2θ2 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). net.fsu.edu/primer/ligh tandcolor/refractionho me.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 interference o maxima o Dark – destructive interference o 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