# Daily Challenge, 11/17 What is LIGHT?. The Electromagnetic Spectrum.

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Daily Challenge, 11/17 What is LIGHT?

http://www.the-wombat.com/images/emspeg.jpg The Electromagnetic Spectrum

Electromagnetic Waves Electromagnetic waves vary depending on frequency and wavelength. All electromagnetic waves move at the speed of light. The speed of light, c, in a vacuum equals c = 3.00  10 8 m/s Wave Speed Equation c = f speed of light = frequency  wavelength

Different Views of Light over Time Corpuscular Theory (Newton) particle explanations Wave Theory (Huygens) wave explanations Electromagnetic Theory energy transfer by waves Quantum Theory energy transfer in “packages”

LIGHT: Wave or Particle? Light PROPERTY Newton’s Particles Huygen’s Waves Rectilinear Propagation explained Reflectionexplained Refraction explained if light travels faster in water that in air explained if light travels faster in air that in water Diffraction (~1800)not explainedexplained Interferencenot explainedexplained Photoelectric effectexplainednot explained

The Photoelectric Effect When a metallic surface is exposed to electromagnetic radiation that is above a threshold frequency (which is specific to the type of surface and material), electrons are “kicked off” the metal and current is produced. No electrons are emitted for radiation with a frequency below that of the threshold frequency. See http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html for morehttp://www.colorado.edu/physics/2000/quantumzone/photoelectric.html JAVA APPLET: http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htmhttp://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htm

HOMEWORK: What happens when you viewed yourself at different distances on either side of a spoon? Why?

Daily Challenge, 11/18 What are some characteristics of a reflected image?

Reflection of Light the turning back of a wave meeting the boundary of a medium Reflectance is the ratio of the light reflected from a surface to the light falling on a surface, commonly expressed as a percentage Regular, specular reflection – scattering is negligible Diffuse reflection – scattering of light is significant (rays not parallel) Laws of Reflection 1st – angle of incidence equals angle of reflection, i = r 2nd – incident & reflected rays & normal are all in a plane

Mirror Terminology C = center of curvatureR = radius of curvature f = focal lengthred dot = principal focus R = 2f Principal axis = goes through C & principal focus

Reflected Images Real images formed by converging rays of light passing through a real image point appear upside-down produced by concave mirrors when object is further away than F Virtual images formed by rays of light appearing to diverge from unreal image point appear right-side-up, but are inverted left to right produced by plane & convex mirrors, concave mirrors when object is closer than F

Geometric Image Construction http://phoenix.phys.clemson.edu/labs/224/optics/mirrorray.gif

Images Formed by Mirrors Concave Mirrors virtual or real images, depends on object location with respect to F object at infinite distance “image” is a point at F object at finite distance beyond C image is real, inverted, reduced, between C and F object at C image is real, inverted, at C object between C and F image is real, inverted, enlarged, beyond C object at F image is not formed, reflected rays are all parallel object between F and mirror image is virtual, erect, enlarged See Page 460 in the text for pictures of these situations

Images Formed by Mirrors Convex Mirrors always virtual, erect images of reduced size Object-Image Relationships Mirror Equation 1 / f = 1 / p + 1 / q Heights & magnification M = h’ / h = q / p wheref = focal length of mirror p = distance of the object from the mirror q = distance of the image from the mirror (negative value means image is virtual) M = magnification (# times bigger image) h’ = height of the image h = height of the object Sign conventions+ f = concave mirror - f = convex mirror p = always positive +q = real image - q = virtual image

A 5.00 cm arrow stands at the 0.0-cm mark of a meter stick. At the 50.0-cm mark is a convex mirror whose radius of curvature is 45.0 cm. How far from the mirror is the image? How tall is it? Example Problem

MiniLab, 11/19 How far away would a 50-cm tall mirror have to be before a 2-m tall person could see themselves in it? (This challenging question can be solved with “thought” experiments, real experiments, ray diagrams, or the mirror equation!)

Daily Challenge, 11/20 All electromagnetic energy travels at the speed of light. WHY is short-wavelength electromagnetic radiation “high energy” and long wavelength electromagnetic radiation “low energy”?

Light Colors Primary Colors of Light red, green, blue mixing all 3 makes white light Complimentary Colors of Light any two colors that form white light when combined (cyan-red, yellow-blue, green-magenta) Primary Pigments (reflect light) cyan, magenta, yellow compliments of primary light colors

Dispersion & the Colors of Light Dispersion White light passing through a prism is separated into a visible solar spectrum consisting of red, orange, yellow, green, blue, and violet (elementary) colors. Object Color opaque – color seen depends on the frequency of the light reflected (white reflects all) transparent/translucent – color seen depends on the frequency of the light transmitted

MINILAB: Use the optical bench to create and view real images with a concave mirror. Check your observations to be sure that they verify the “6 cases” of images produced by concave mirrors. Compare these images to those created by a convex mirror. For credit, each person must make some detailed notes and/or sketches of your setup and observations. Daily Challenge, 11/23

Daily Challenge, 11/24 How far from a concave mirror, of focal length 6.0 cm, does a candle have to be placed to look like it is burning on both ends?

An Electrician’s Nightmare Five wires appear the following colors under sunlight: 1 white 2 black 3 red 4 green 5 yellow If an electrician must work under a cyan light, what color will each wire appear to be? If the electrician works in sunlight, but wears sunglasses with a magenta tint, what color will each wire appear to be? Daily Challenge, 11/30

Daily Challenge, 12/1 What’s the difference between a luminous object and an illuminated object?

Daily Challenge, 12/2 White spotlights often show thin colored fringes around the edge of the white light. Why? Explain what is happening and the pattern of light you would expect to see.

Photometry the quantitative study of light Luminous Intensity, I ► is the light energy produced per time per area ► measured with a photometer (Bunsen, Joly, Photoelectric, Spherical) ►measured in candelas (cd) The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.40 x 10 14 Hz and that has an intensity in that direction of 1/683 watt per steradian (sr). sr =angle intercepting a unit surface area on a unit sphere

Luminous Flux, F ► is that part of the total energy radiated per unit of time from a luminous source that is capable of producing the sensation of light (notice, it’s a rate) ► measured in lumens (lm) The lumen is the luminous flux on a unit surface all points of which are at a unit distance from a point source of one candela. F = 4  I

Illuminance, E ► is the density of a luminous flux on a surface ► measured in lux (lx) The lux is the lumens/meter 2. E = F / A = I / r 2 (assumes surface perpendicular to flux) Illuminance on a surface area varies inversely with the square of the distance from the luminous source and directly with the cosine of the angle between the luminous flux and the normal to the surface. E = I cos  / r 2

Photometry Mini-Lab Use a bunsen “grease spot” photometer to make photometry measurements as instructed. Quantitatively compare these “grease spot” measurements to the light meter readings. Daily Challenge, 12/3

Daily Challenge, 12/4 What are some practical applications of LASERS?