Light

Light

What you see (and don’t see) is what you get.

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Light Assignments

36: 16/25,26,28,29,31,

37: 16/33-36,56,59 38: 17/2,3,13-16 reflection/mirrors

39: 18/2,3,9,11,15-18 refraction/lenses

40: 19/49,52,53,58 interference

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Theories of Light Wave Theory Particle Theory Huygens Newton Properties that support each theory

Rectilinear PropagationReflectionRefractionInterference

Diffraction Photoelectric Effect

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http://www.electro-optical.com/html/images/em_spect.gif

http://www.electro-optical.com/html/images/em_spect.gif

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Important Info re EM Spectrum

V = f c = f c = 3 x 10 8 m/s in vacuum or air

decreases to the right

f, E increase to the right

E = hf h = 6.63 x 10-34 js

1 angstrom A = 10-10 m

1 nanometer, nm = 10-9 m

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The Photoelectric Effect

Heinrich Hertz first observed this photoelectric effect in 1887. Hertz had observed that, under the right conditions, when light is shined on a metal, electrons are released.

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In 1905 Albert Einstein provided a daring extension of Planck's quantum hypothesis and was able to explain the photoelectric effect in detail. It was officially for this explanation of the photoelectric effect that Einstein received the Nobel Prize in 1921. The figure below shows a circuit that can be used to analyze the photoelectric effect.

Expanding on Planck's quantum idea, Einstein proposed that the energy in the light was not spread uniformly throughout the beam of light. Rather, the

energy of the light is contained in "packets" or quanta (the plural of quantum, a single "packet") each with energy of

E = h f

LASER

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Light amplification by stimulated emission of radiation

1. The laser in its non-lasing state

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2. The flash tube fires and injects light into the ruby rod. The light excites atoms in the ruby.

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3. Some of these atoms emit photons.

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4. Some of these photons run in a direction parallel to the ruby's axis, so they bounce back and forth off the mirrors. As they pass through

the crystal, they stimulate emission in other atoms.

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5. Monochromatic, single-phase, columnated light leaves the ruby

through the half-silvered mirror -- laser light!

AKA: coherent light

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Refraction The bending of light as it passes from one

substance into another

: www.mysundial.ca/tsp/refraction_of_light.html

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Index of Refraction, nn = speed of light in vacuum n = c v

speed of light in substance c sub

n = sin I

sin f

nI sin1 = n2 sin2

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The concept of refractive index is illustrated in Figure 1 below, focusing on the case of light passing from air through both glass and water. Notice that while both beams enter the denser material through the same angle of incidence with respect to the normal (60 degrees),

the refraction for glass

is almost 6 degrees

greater than that for

water due to the higher

refractive index of

glass.

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ProblemLight travels from a vacuum into water (cw =

2.25 x 108m/s). Determine the index of refraction of water.

n = c v / c w =

3x108m/s 2.25 x 108m/sn = 1.33

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Problem

A ray of light travels from air into water at an angle of 60.0 o with the surface.

A. Find the angle of refraction.n = sin i / sin fsin f = sin i/ n =sin30.0o/1.33 =sin f = 0.376f = sin-1 0.376 = 22.1 o

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B. Find the speed of light in water n = c v / c w

c w = c v / n

c w = 3 x 10 8 m/s

1.33c w = 2.26 x 10 8 m/s

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i c , critical angle - limiting angle of incidence that results in angle of refraction of 90 o (red)

For an angle greater than i c, total internal reflection occurs (dark blue)

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If a rod of glass is pulled to a very thin diameter,

and light is shone in at one end, it cannot

escape, and becomes "trapped" inside the glass

rod. Even if the rod is bent or curved, the light

continues to be totally internally reflected and

continues it's passage along the rod from one

end to the other with no loss to the outside.

Great use has been made of this property of

"light pipes" in recent years. A single glass fiber

can carry a stream of light pulses from one end

to another almost instantly, making for very rapid

very efficient telephone and data connections.

Also, if the fibers are bundled together correctly,

images can be transmitted, even round curves

and corners.

www.brooklyn.cuny.edu/.../

SBAM/SBAM.Prisms.html

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Diffraction- spreading of light around a barrier

www.ligo-wa.caltech.edu/teachers_corner/lesso...

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Single Slit Diffraction

n = s sin

n, dark band number

, wavelength (m)

s, slit width (m)

, angle defined by central band, slit, and dark band

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www.astrophys-assist.com/.../ses01p14.htm

Constructive

interference

yields bright

spots of light

Destructive

interference

yields no light,

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Double Slit Diffractionn = d sin

nbright band number (n = 0 for central)

, wavelength (m)d, distance between slits (m)n, angle defined by central band, slit, and band n

(order of magnitude, 0,1,2,… of bright bands)

This also works for diffraction gratings consisting of many, many slits that allow the light to pass through. Each slit acts as a separate light source.

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Problem

Find the angle of n=3 fringe (order of image) if

2 slits 0.4 mm apart are illuminated by yellow light, = 600 nm.

Sin = n/d = 3(600x10-9m)/4x10-4m = sin-1 (4.50x10-3) = sin-1 0.00450 = = 2.58x10-1 o

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Diffraction Grating ProblemA grating has 4000. lines per cm. At what

angles are maxima formed if it is illuminated with yellow light at 600.nm?

Slit spacing is: d = 1cm/4000lines = 2.5x10-4cm=

25x103nmsin=(n/d)= n(600nm)/2.5x103nm=n(0.24)n=1, =sin-1(1(0.24)=13.9o

n=2, =sin-1(2(0.24)=28.7o

n=3, =sin-1(3(0.24)=46.0o

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Polarization

www.edbergphoto.com/pages/Tip-polarizers.html

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Polarized Sunglasses

Polarized sunglasses work by filtering out certain frequencies and orientations of light, such as ultra-violet, which is harmful to human eyes. In order to polarize a material for light, etches of scratches must be microscopically put into the material, so that only the light waves that are lined up with the scratches can pass through. This is the basis behind polarized sunglasses.

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•REFRACTION CAUSES A CHANGE IN SPEED OF LIGHT AS IT MOVES FROM ONE MEDIUM TO ANOTHER.

•REFRACTION CAN CUAWE BENDING OF THE LIGHT AT THE INTERFACE BETWEEN MEDIA.

Light and Pigment

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Primary Colors of Light

Red Green Blue (These are the

secondary colors of pigment)

Primary Colors of Pigment

Yellow Cyan Magenta (These are the

secondary colors of light)

Mixing …

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Red+Blue = MagentaBlue+Green = CyanGreen+Red = Yellow

Magenta+Cyan = BlueCyan+Yellow = GreenYellow+Magenta = Red

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The three primary colors of light mixed together produce white light (all colors of light) - an additive process.

The three primary colors of pigment mixed together produce black (absorbing all or most light) - a subtractive process.

Light

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