Unit 9: Light and Optics

The Electromagnetic Spectrum German Physicist Willhelm Rontgen’s wife’s finger (1895) (radiation dose 1,500 times greater than today’s dosage) (video) Food Irradiation – (Article) (Video) Night Vision (Video)

Why can Blu-Ray discs hold more data? The Electromagnetic Spectrum Why can Blu-Ray discs hold more data? Min. Capacity 0.7 GB 4.7 GB 15 GB 25 GB

I. Electromagnetic (EM) Waves _______________charges create __________ and ___________ fields ACCELERATING ELECTRIC MAGNETIC These fields create __________________________ ELECTROMAGNETIC WAVES Electric field Magnetic field Notes: Electric field is perpendicular to magnetic field Both fields are perpendicular to the direction of energy flow 4

I. Electromagnetic (EM) Waves Characteristics of EM Waves 1. EM waves are ______________ waves. (where the disturbance is ________________ to motion of energy. TRANSVERSE PERPENDICULAR VACUUM EM waves can travel through a __________ or a ________________ CELL PHONE VACUUM PUMP DEMO MATERIAL SUBSTANCE 3. All EM waves travel at the speed of ________ ( ) in a ____________ LIGHT c VACUUM 5

I. Electromagnetic (EM) Waves Equation (See Ref. Tabs.) f = frequency v = λ f λ = wavelength v = c = speed of light (3.00 x 10 8 m/s) (or 670,616,629 mph or 186,282 miles/s) It takes 8 minutes 17 seconds for light from the sun to reach us (91 million miles) 6

II. The Electromagnetic Spectrum EM energy (wave) is categorized by its wavelength and frequency. Visible light makes up only about _______ of the electromagnetic spectrum. 1 % 7

II. The Electromagnetic Spectrum Example: What is the wavelength of an E-M a wave traveling in a vacuum that has a frequency of 4.13x1013 Hz? How is this energy classified on the electromagnetic spectrum? 8

III. Proof Light is a Wave A. Polarization: Visual light consists of a bunch of EM waves oscillating in random directions (Un-polarized Light) Linear Polarized light is when EM waves are oriented in the same direction Polarization through Transmission: Created by sending light through certain materials. Example: ______________ SUNGLASSES 9

III. Proof Light is a Wave Polarization through Reflection: Light reflects off of surfaces polarized. The reflected polarized light is parallel to the surface Example: _____________ CAR HOODS 10

III. Proof Light is a Wave B. Diffraction Patterns (animation) INTERFERNCE LIGHT _______________ pattern of ________and ________ fringes caused by wave behavior of light DARK 11

IV. Reflection of Light Law of Reflection: angle of incidence = angle of reflection Equation θ i = θ r 12

IV. Reflection of Light Types of Reflection: A) Specular Reflection Light is reflected off of smooth, shiny surfaces. Rays are ___________ to each other. PARALLEL 13

IV. Reflection of Light Types of Reflection: B) Diffuse Reflection Reflects light in __________________ VARIOUS DIRECTIONS 14

V. Refraction Refraction: The _________ in direction of a wave as it enters a new _________ at an angle . CHANGE MEDIUM When a (light) wave strikes the interface between two different transparent media, the EM energy is ___________, ____________, and/or ____________. REFLECTED REFRACTED ABSORBED 15

V. Refraction When a (light) wave is transmitted through a new medium its ____________ changes. When the material is________________, light _____________ When the material is ________________, light ___________ VELOCITY MORE “DENSE” SLOWS DOWN LESS “DENSE” SPEEDS UP 16

VI. Index of Refraction Ratio of speed of light in a vacuum to speed of light in a specific material Equation (see ref. tabs) c n = n = index of refraction v Example: What is the speed of light in corn oil? 17

VII. Basic Refraction Diagrams Snell’s Law Animation Video Clip - Bends away from the normal - v and λ ↑ - f remains constant - Bends towards the normal - v and λ ↓ - f remains constant 18

VIII. Snell's Law of Refraction Equation (see reference tables): n1sinθ1 = n2sinθ2 19

VIII. Snell's Law of Refraction What happens to light when it passes into a new medium that has the same index of refraction as the original? Beaker Demo (1:30 min) 20

VIII. Snell's Law of Refraction Example: Light, traveling in air, strikes the surface of a diamond at an angle of incidence of 55o. What is the angle of refraction when it enters diamond? Example: What is the new speed of light when it travels from air into diamond? 21

IX. Apparent Depth One interesting consequence of refraction is that an object lying under water appears to be closer to the surface that it actually is. 22

X. Total Internal Reflection What happens to a beam of light that leaves water and enters air if the angle of incidence is increased? (animation) Air (n = 1.00) Water (n = 1.33) Critical angle when θ2 = 90o 23

X. Total Internal Reflection Critical Angle: When angle of incidence is angled so that the angle of refraction equals _______ 90o Equation: n1sinθ1 = n2sinθ2 At the critical angle, Θ2 = 90o sinθc = n2 n1 NOTE: Total Internal Reflection only occurs when light is entering a material with a lower index of refraction. Example: From water into air. 24

X. Total Internal Reflection Fiber Optic Cable Demos 25

X. Total Internal Reflection Snell’s Law Animation Incident ray is completely ___________ back into original medium. Occurs when: REFLECTED 2. Angle of incidence is __________ than ________________. θc GREATER CRITICAL ANGLE Whiteboard: Find the critical angle for diamond (use reference tables) with an air boundary. A ray of light has an angle of incidence of 30.0o on a block of quartz and an angle of refraction of 20.0o. What is the index of refraction for this block of quartz? 24.4o 1.46 26

X. Total Internal Reflection 27

XI. Dispersion 28

XI. Dispersion Definition: separating (polychromatic) light into separate colors (wavelengths/frequencies) Uses _______________ to bend light at different angles to separate visible light into various components. REFRACTION Red light (longer wavelength) travels faster than violet (shorter wavelength) through the prism, so violet refracts more and appears at the bottom. Light is dispersed in order of ______________ wavelength. DECREASING Index of refraction ultimately depends on the wavelength of light, so every color has a slightly different speed within a substance 29

XI. Dispersion Dispersion of Light to form a Rainbow The red light is refracted out of a droplet at steeper angles towards the ground than the blue light. Thus, when an observer sights at a steeper angle with respect to the ground, droplets of water within this line of sight are refracting the red light to the observer's eye. The blue light from these same droplets is directed at a less steep angle and is directed along a trajectory that passes over the observer's head. Thus, it is the red light that is seen when looking at the steeper angles relative to the ground. Similarly, when sighting at less steep angles, droplets of water within this line of sight are directing blue light to the observer's eye while the red light is directed downwards at a more steep angle towards the observer's feet. This discussion explains why it is the red light that is observed at the top and on the outer perimeter of a rainbow and the blue light that is observed on the bottom and the inner perimeter of the rainbow. (source) 30

Why are sunsets red? Why is the sky blue? Video (4 min) Video (2 min) (3 min) Why are sunsets red? Video (4 min) 31

XII. Interference of Light A. Reminders When a crest and a crest or a trough and trough meet in the same medium ________________ interference occurs. The path difference between the two sources is whole wavelength. CONSTRUCTIVE When a trough and crest meet in the same medium _______________ interference occurs. The path difference between the two sources is a half wavelength. DESTRUCTIVE The bending of wave (light) around a barrier is called ______________. Draw how the light exits the barrier. DIFFRACTION 32

XII. Interference of Light B. Young’s Double Slit Experiment Thomas Young: English Physicist (1773-1829) Proves light is a wave. Shows that light exhibits diffraction and forms an interference pattern when shown on two closely spaced slits. Suppose Monochromatic light is shown on two closely spaced slits. CREST MEETS CREST CREST MEETS TROUGH http://phet.colorado.edu/en/simulation/wave-interference 34

XII. Interference of Light B. Young’s Double Slit Experiment Relationships Wider the barrier opening, smaller the diffraction pattern (decrease in the amount of bending) 35

XII. Interference of Light D. Thin Film Interference 36

XII. Interference of Light D. Thin Film Interference Reflected light waves off of the top surface interferes with the reflected light waves that travels through the surface. 37