Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F-521 Email: ivan.smalyukh@colorado.edu Phone: 303-492-7277 Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang jhihan.yang@colorado.edu  Class # 24 Chapter #13 Reminder: HW #7 is due on Thursday Today: Finish Ch.13 material & Review Thursday: Exam #3

Physics 1230: Light and Color Chapter 13 Scattering and Polarization Light waves can scatter off water droplets, dust particles and other molecules in air Some wavelengths in white light scatter more than others (Rayleigh scattering) This is why the sky is blue The polarization of light refers to the plane in which the waveform and electric force field lies Most light is unpolarized The waveforms and electric force field on a given ray keep jumping around in orientation while always perpendicur to the ray Scattered or reflected light is polarized in particular planes Polaroid sunglasses can block out polarized light from sky and ground 3D movies & other applications of polarizers;

How do 3D movies use polaroid filters (polarizers)?

What can a light wave do when it encounters matter? Be TRANSMITTED laser aimed at water or glass Be REFLECTED specular reflection of light by a mirror diffuse reflection of the light in this room off all the other students reflection is re-radiation of light by the electrons in the reflecting material Be ABSORBED Cyan light shining on a red apple is absorbed by electrons in the apple Something new!! A light wave shining on molecules in the air or plastic or other “transparent” materials can be SCATTERED Light ray moves over to the side in all directions rather than forward, backward or being absorbed. Intensity of the scattered light can depend on wavelength

What is Rayleigh scattering? (or why is the sky blue) The shorter the wavelength, the more light is scattered blue is scattered more than red. this is why the sky is blue and sunsets are red. (Fig. 13.1) Think of white light from sun as a mixture of R, G and B Blue is scattered the most so sky looks blue when we look away from the sun For same reason sun looks yellow (red + green) More atmosphere allows next shortest wavelengths (green) to scatter so sunset looks red

Rayleigh scattering But why blue and not violet? Intensity inversely proportional to λ4 Light waves with longer wavelength scatter more But why blue and not violet?

Why blue and not violet? Spectral response of cones in typical human eye relative response Our eye sensitivity to the blue color is much stronger than that to violet! We perceive the deep waters being of blue color for the same reason!

Color of Day-time sun vs. Sunset Rays from Sun (not scattered) White or yellow Earth Atmosphere

Red Sun/sky during the Sunset

Why the sky is blue? A. Because of scattering of short-wavelength blue light coming from the Sun; B. Because of light absorption similar to that in color filters; C. A & B D. Because of dispersion and total internal reflection; E. None of the above;

What color of the sky one can see from Jupiter? D. Black A. Red; B. White; C. Blue; E. Yellow;

Without Rayleigh scattering the Sky would be A. Red; B. Green; C. Blue D. Black; E. White;

Light scattering in milk and smoke? Larger particles scatter red as well as blue and hence look white. Dust or smoke Clouds; Milk; Colloidal suspension There are many other types of scattering…

Why the sunset is red? A. Because of light absorption similar to that in color filters; B. Because of scattering of blue and green light and only red light coming to our eyes; C. Because of its polarized nature; D. Because of chromatic & spherical aberrations; E. None of the above;

Polarized Light due to reflection Secularly reflected light blocked using a polarizer (Polaroid film) http://en.wikipedia.org/wiki/File:Mudflats-polariser.jpg

Amplitude (maximum height) Review: What is the wave of a mono-chromatic (single wavelength) wave along a ray? Speed of light in empty space is c = 186,000 miles/sec = 3 x 108 meters/sec Amplitude (maximum height) The intensity of the light is proportional to the amplitude squared. Large amplitude means bright light. Low amplitude means dim light Ray Wavelength Waveform

Review: What is light "made of?" Light consists of electric (and magnetic) fields moving through space at the speed of light The waveform can be replaced by a set of arrows perpendicular to the ray whose tips lie along the waveform

Crossed polarizers: block propagation of electromagnetic waves Waves on a rope Think about the waves on a rope:

For light: Polarizers (Polaroid films) Same story:

What is polarized light? Light is polarized if the waveform and electric force field arrows remains in the same plane The (green) electric force arrows must always be perpendicular to the ray This is a light ray traveling in the z-direction and polarized in the y-direction Here is a light ray traveling in the same direction but polarized in the x-direction We will visualize the polarization in the x-y plane, looking at rays head-on The green force arrows point up and down or left and right, stacked up behind one-another. Here is the convention for visualizing vertical and horizontal polarization Looking at ray "head-on" see green arrows up & down y z x y z x y x

In which direction is this light ray polarized? A. Vertical; B. Horizontal; C. Along the light propagation direction (yellow arrow); D. None of the above;

What is unpolarized light? For unpolarized light the plane of polarization keeps jumping around But the electric field arrows remain perpendicular to the ray (direction of travel of the wave) We visualize this in the x-y plane (looking into the ray) as shown at right The many crossed double sided arrows are the symbol for unpolarized light See Physics 2000 electric force arrows jump around while remaining perpen-dicular to the ray x y z wave travels in z-direction y x http://www.colorado.edu/physics/2000/index.pl

When unpolarized light reflects off a horizontal surface (such as water or beach) near a special angle, the reflected light is polarized in the horizontal direction The special angle of incidence is where the refracted ray and reflected ray are perpendicular to each other This is called Brewster's angle To understand, imagine the electric force arrows of the incident unpolarized light to be decomposed into two perpendicular polariza-tions the first polarization is horizontal (force arrows are parallel to the flat reflecting horizontal surface and perpendicular to the ray) in the 2nd (Fig. 13.5), the arrows are perpendicular to both the ray and the horizontal force arrows The second polarization cannot be sustained in the reflected ray because the force arrows would be parallel to that ray (impossible for a light ray) Hence, only the horizontal polarization survives in the reflected ray

Polaroid sunglasses (and camera filters) take advantage of this effect to block bright reflected light from the snow, beach, road, water, sky, etc. When you wear polaroid sunglasses they block light polarized in the plane of your two eyes When your head is vertical this is the horizontal plane Since reflected light from the ground or scattered light from the sun are mostly polarized in the horizontal plane such light is mostly blocked Demo with polaroid filters The first filter blocks one polarization and lets through the other (perpendicular) one The second blocks more and more of the remaining polarized light as its axis is rotated so as to permit only the polarization blocked by the first When the axes of the two polaroid filters at at right angles to each other no light comes through Physics 2000 polarization applets The polaroid filter works because it has long molecules lined up parallel to each other like hairs Components of light with polarization parallel to the molecules are absorbed and components of light polarized perpendicular to the molecules are absorbed and don't go through Other demos with crossed polarizers Third polarizer at angle in between lets light through again Stressed plastic between polarizers shows colors which reveal the stress patterns

Some material from Chapter #8

How do 3D movies use polaroid filters?

A different approach: Use color filters to make the left and right eyes perceiving slightly differen images http://www.3dmovies.com/

Clicker Question: In 3D movies: Left eye sees the same as the right eye; Left eye sees the inversed image compared to that seen by the right eye; The images seen by the two eyes are slightly different and taken from different angles.