Download presentation

Presentation is loading. Please wait.

1
Light Physics 202 Professor Vogel (Professor Carkner’s & CJV notes, ed) Lecture 10

2
Light Electromagnetic wave oscillating electric and magnetic fields – no material medium that is moving! energy transfer at speed v (c=3X10 8 m/s in vacuum) wavelength = distance between repeats frequency = # repeats per second f =v v=c in vacuum c=3X10 8 m/s f

3
EM Spectrum

4
The EM Spectrum Radio > 1 meter penetrates solid objects easily Millimeter (microwave) 1 m - 1 mm used for communication Infrared 1 mm - 700 nm we feel as heat Visible 700-400 nm eyes evolved to see Ultraviolet 400 nm - 100 A higher energy, causes sunburn X-ray 100 A - 0.01 A penetrates soft things but not hard Gamma Ray < 0.01 A hard to produce and dangerous

5
The EM Wave Lets consider light as a wave What kind of wave is it? What is oscillating? An EM wave consists of an electric field wave (E) and a magnetic field wave (B) traveling together The 2 fields are perpendicular to each other and to the direction of travel An EM wave is transverse (like string waves) The field waves are sinusoidal and in phase

6
Wave Equations We can generalize the waves as: E = E m sin (kx - t) B = B m sin (kx - t) Nothing is actually moving There is no string A changing E field induces a B field A changing B field induces an E field The two fields continuously create each other The speed of the wave is related to the fields: c = E/B

7
Traveling EM Wave

8
Key Constants Two important constants in E and M are the permittivity constant 0 and the permeability constant 0 Permittivity is the electric force constant: 0 = 8.85 X 10 -12 F/m In farads per meter Measure of how electric fields propagate through space Permeability is the magnetic force constant: 0 = 1.26 X 10 -6 H/m In henrys per meter Measure of how magnetic fields propagate through space The wave speed depends on these constants: c = 1/( 0 0 ) ½

9
Poynting Vector EM waves transport energy The amount of energy delivered per unit area per unit time is given as flux: flux = W/m 2 = J/s/m 2 Flux for an EM wave can be given by the Poynting vector: S = (1/ 0 ) EB However, E and B are related by E/B = c so we can rewrite S as: S = (1/c 0 ) E 2

10
Intensity The value of S depends on where the EM wave is in its cycle We generally are interested in the time averaged value of S, known as the intensity I = (1/c 0 ) E rms 2 Where E rms is the root-mean-square value of the electric field

11
Radiation Pressure EM waves exert a pressure on objects If someone shines a flashlight on you, the light is trying to push you away like ball bouncing off object pushes object back The force is very small in most cases EM pressure is due to the fact that light has momentum which can be transmitted to an object through absorption or reflection

12
Momentum Transfer The change in momentum due to light is given by: p = U/c Where p is the momentum change and U is the energy change The above equation is for absorption For reflection the momentum change is twice as much: p = 2 U/c

13
Light Pressure From Newton’s second law F = p/ t The amount of energy delivered in time t is: U = I A t where I is the intensity and A is the area Since pressure (p r ) is force per unit area the pressure becomes: p r = I/c (total absorption) p r = 2I /c (total reflection)

14
Comet Hale- Bopp

15
Comet Tails

16
Light Sail

17
Color Vision Rods and cones one type of cone responds to long ’s: “R” one type of cone responds to mid wavelengths: “G” one type of cone responds to short ’s: “B” How our eyes view pure waves: red : R-type responds green : G-type responds blue : B-type responds yellow : R- and G-types respond Cyan : G- and B-types respond

18
Color Addition How our eyes view mixtures : blue + red: R- and B-types respond magenta green + blue : G- and B-types respond indistinguishable from cyan red + green : R- and G-types respond indistinguishable from yellow Demo of color addition -- HELP (Like no pure color)

19
Color Addition How our eyes view mixtures : red + green : R- and G-types respond indistinguishable from yellow red + green + blue : R-, G-, and B-types respond white yellow + blue : R-, G-, and B-types respond white

20
Color Subtraction How our eyes view pigments (absorb light) white - blue: R- and G-types respond pigment that absorbs blue looks yellow white - red : G- and B-types respond pigment that absorbs red looks cyan white - (blue + red): G-type responds pigment that absorbs blue and red looks green

21
Color Subtraction How our eyes view pigments: white - (blue + red): pigment that absorbs blue and red looks green Pigment: yellow + cyan: pigments that absorb blue and red look green A demo of subtractiondemo usflag-neg.gif

22
Complementary color = white - color primary color red green blue Complementary color cyan magenta yellow

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google