Light Waves      Chapter 11  Joseph F. Alward, PhD  Department of Physics  University of the Pacific

Thin film interference in soap film.

 

Applets used in this eLecture: Young's Double Slit 1 Young's Double Slit 2

  Oscillating Charges Emit Electromagnetic Waves

1.  Stationary charges set up static electric fields;      oscillating charges charges set up oscillating      electric fields. 2.  Charges moving with constant speed, as in      electrical currents, set up static magnetic fields;      oscillating charges set up oscillating magnetic      fields. Conclusion:  an oscillating charged wand sets up oscillating electric and magnetic fields.

 Generating Electromagnetic Waves         

 Electromagnetic Waves                         

c = 186, 000 miles/second    = 3 x 108 meters/second

 

Click here to see animation of the creation of electromagnetic wave from an oscillating charge.

  Unmodulated Carrier Wave     

  Light is a Wave-Like Disturbance                                               

  The Electromagnetic Spectrum                                               

   Wavelengths                                                                                            

   Wavelengths of Visible Light                                                   

   Transparent and Opaque Materials  

One tuning fork stimulates the vibration of another.  The second tuning fork absorbs the   sound energy from the first and begins to vibrate, sending out its own sound energy.

  Atomic Tuning Forks

Electrons are light enough to respond quickly to oscillating electric field. They receive the energy from the electromagnetic wave, then re-radiate it.

  Absorption and Emission in Solids

Light energy is absorbed by the atomic oscillators and is re-emitted by the oscillating electron.

   Transmission of Light through Glass and Air

High frequency ultraviolet (UV) is blocked by atmosphere.  Lower-frequency UV   gets through atmosphere.  Clouds will allow some UV to pass through.

Ultraviolet light oscillates at too high a frequency for electrons in glass molecules, while infrared is too low. Visble light is just right.

  Light Reflection from Metals

Free electrons in metals absorb and re-emit light at the surface. Not all light incident on metals is reflected; the light that enters the metal is absorbed and re-emitted, but the energy is mainly used to warm the metal; none of the light passes through the metal.

 The Spectrum of White Light                                           

  Reflection and Absorption

White reflects all colors equally well. Black absorbs all light. Grass absorbs which range of colors?

   Reflection  

Lipstick reflects differently in different lighting.

  Transmission of Light

When red light shines on a red rose, which becomes warmer, the rose, or the petals?

    Color Spectrum of Sunlight

Through evolution, our eyes have become most sensitive to yellow-green light.  Why? Yellow light from sodium- vapor lamps illuminates better at night than common tungsten-filament lamps of the same brightness.

   Rods and Cones 

Rods in the retina distinguish brightness. Cones in retina distinguish between colors, and require more energy to respond than do rods. Twinkling starlight consists of separated colors.  Why do we usually just see white?

    The Additive Primary Colors

Sunlight may arbitrarily divided into three man color groups spanning different ranges of wavelengths: Red     Green     Blue

    The Additive Primary Colors

They're called primary because any color can be made from a suitable combination of red, green, and blue.

   Complementary Colors

(Red + Green) + Blue = White   Red + Green = Yellow Colors which  combine to produce white are said to be complementary.  Therefore, yellow and blue are complementary colors. Red + Blue = Magenta Of what color is magenta complementary?

   Complementary Colors

Red, green, and blue light sources are in the plane of the picture at the bottom, not shown. A shadow corresponding to each light source is formed behind the golf ball. The yellow shadow is white light, minus blue. What color is the middle shadow?

  Mixing Color Pigments (Paints)

Red paint In red paint, what primary colors are absorbed from white light? What primary color is reflected?

Green Paint In green paint, what primary colors are absorbed from white light? What primary color is reflected?

  Mixing Colored Pigments

1.  Magenta:  reflects red and blue;      absorbs green. 2.  Yellow:  reflects red and green;       absorbs blue 3.  Cyan:  reflects green and blue;      absorbs red.  Consider bright red triangle of  color to the right of the boy's head. Absorbs green (1) and blue (2), reflects red.

    Mixing Color Pigments (Paints)  

Magenta reflects red and blue, absorbs green. Cyan reflects blue and green, absorbs red. Yellow reflects red and green, absorbs blue. 1.  Which colors must be subtracted (absorbed)       from white light to give reflected green light? 2.  Which pigments should be added to give       the color green? 3.  Which pigments should be added to give       red?

  Light Scattering

Electromagnetic energy is absorbed by electron in atom, then re-emitted in all directions (only one direction at a time, for any particular atom).

 Blue Sky

Small-wavelength light (blue-green) from the sun is scattered by very small particles of dust, and especially oxygen nitrogen in the atmosphere. If no light were scattered by the atmosphere, would any light from the sun reach our eyes (assuming we don't look directly at the sun)? What color is moon sky?

    White Sky

Particles in the air which are larger than oxygen and nitrogen scatter red light. Mixed with the blue light scattered by N2 and O2, the result is a nearly white sky.

  Red Sunsets 

Atmosphere at sunset provides a much longer path through which white light from the sun must travel. Atmosphere scatters a large percentage of the blue part of the white light in all directions, leaving mainly the red part to reach the eyes of the person viewing the setting sun.

  White Clouds

Small water-droplet cluster scatter red light, while larger clusters scatter blue.  Together, this scattered light is white.

 Water is Blue-Green

This wave absorbs red light, and reflects blue-green (cyan). Froth is white because tiny water-droplet clusters absorb and then re-radiate red light.  The red mixes with the blue-green to produce white.

     Water Waves Bend around Obstacles  

Oscillating board up and down creates plane waves. If the wavelength is comparable to the width of opening, significant changes in direction occur.

  Diffraction by Edges of Holes

This shows plane wave waves being bent into the "shadow" regions. Bending is greatest when the opening is small compared to the wavelength.

The smaller the wavelenth is relative to the obstacle or opening, the sharper is the shadow.

  Diffraction by the Edges of Obstacles 

Wavelength is comparable                    Wavelength is very small             to size of obstacle.                               compared to object

Smaller wavelengths create sharper shadows. Foghorns operate at low frequency (long wavelength) to fill shadow regions. FM radio at wavelengths of 2.8-3.4 meters casts sharper shadows than AM radio at 180-550 meters

   Inteference of Waves from Two Sources                                             

In some places the water wavefronts are in phase (bright spots). In other places the fronts overlap with peak and valley and interfere destructively (darker spots).

    Interference

This is a "snapshot" at some instant in time of a plane wave from a single source.

If the EM waves from two or more sources overlap, they will "interfere".

  Interference

Electric field vectors point         Electric field vectors point            Incomplete cancellation.   in the same direction at all         in opposite directions at all    points at same time. This is        points.  This is destructive              constructive interference.        interference.

  Young's Double Slit Experiment

Waves from two different sources overlap.  In some places the interference is constructive, in other places it is destructive.

  Applets

Young's Double Slit 1 Young's Double Slit 2

  Interference of Two Sources                                                       

  Interference from Thin Films

If wave reflected from water is out of step--out of phase--with the wave reflected from gasoline, there will be destructive inteference and the eye will see darkness.

   Interference

Iridescence of peacock feathers is caused by light reflected from complex layered surface.

Different thickness lead to constructive inteferences for different wavelengths.

     Inteference Projects                                                                                          

Thin film of air between slides produces  interference pattern.

  Polarization

Illustrating vertical and horizontal polarized waves.

  Plane Polarized Electromagnetic Waves

Plane-polarized electromagnetic waves caused by charge oscillating in different planes.

   Unpolarized Light

3D view of planes of oscillations             End view                              Simple      of just a few of the EM waves                                                            representation      from an incandescent bulb.

  Polarizing Materials

Horizontal oscillations are absorbed; only the vertical oscillations are transmitted. Polaroid filters transmit 50 % of unpolarized light.

  Polaroid Filters

  Crossed Polaroid Filters

The first filter absorbs and re-radiates only   vertically oscillating electric fields; the second   will pass only horizontally oscillating fields.

No energy is transmitted out of the second  filter.

  Polaroid Lenses

Glasses block horizontally polarized light.

  Polaroid Filters

Filters are parallel                          Crossed filters

  Polarization by Reflection

"Glare" is sunlight reflected off smooth surfaces into the eye. Most glare is from horizontal surfaces, so glasses are worn with polarizing direction vertical.

Polarization Applets

Polarizing Lenses Two rotatable lenses, camera; excellent Polarization Shows rotatable filter on lake; very nice