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

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 = 3.00 108 m/s

• Wave Speed Equationc = fl

speed of light = frequency wavelength

Different Views of Light over Time

Corpuscular Theory (Newton)particle explanations

Wave Theory (Huygens)wave explanations

Electromagnetic Theoryenergy transfer by waves

Quantum Theoryenergy transfer in “packages”

LIGHT: Wave or Particle?

Light PROPERTYNewton’s Particles

Huygen’s Waves

Rectilinear Propagation explained explained

Reflection explained explained

Refraction explained if light travels faster in water that in air

explained if light travels faster in air that in water

Diffraction (~1800) not explained explained

Interference not explained explained

Photoelectric effect explained not 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 more

JAVA APPLET: http://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, 11/18What are some

characteristics of a reflected image?

Reflection of Lightthe turning back of a wave meeting the boundary of a medium

Reflectanceis 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 negligibleDiffuse reflection – scattering of light is significant

(rays not parallel)

Laws of Reflection1st – angle of incidence equals angle of reflection, i = r2nd – incident & reflected rays & normal are all in a plane

Mirror Terminology

C = center of curvature R = radius of curvaturef = focal length red dot = principal focus

R = 2fPrincipal axis = goes through C & principal focus

Reflected ImagesReal images formed by converging rays of light passing through a

real image pointappear 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 rightproduced 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 MirrorsConcave 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 MirrorsConvex Mirrors• always virtual, erect images of reduced size

Object-Image RelationshipsMirror Equation 1 / f = 1 / p + 1 / q

Heights & magnification M = h’ / h = q / pwhere f = focal length of mirror

p = distance of the object from the mirrorq = distance of the image from the mirror

(negative value means image is virtual)M = magnification (# times bigger image)h’ = height of the imageh = height of the object

Sign conventions + f = concave mirror- f = convex mirrorp = 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, 11/19How 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, 11/20All 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 Lightred, green, bluemixing all 3 makes white light

Complimentary Colors of Lightany two colors that form white light when

combined (cyan-red, yellow-blue, green-magenta)

Primary Pigments (reflect light)cyan, magenta, yellowcompliments of primary light colors

Dispersion & the Colors of Light

DispersionWhite light passing through a prism is

separated into a visible solar spectrum consisting of red, orange, yellow, green, blue, and violet (elementary) colors.

Object Coloropaque – 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

Shadows

Umbra – full shadow

Penumbra – gradational partial shadow

http://www.schorsch.com/kbase/glossary/penumbra.html

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, 11/24How 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, 12/1What’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.

Photometrythe 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 1014 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/meter2.

E = F / A = I / r2 (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 / r2

Photometry Mini-LabUse 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?