light what is it?. light what is it: moving energy particle or wave?
TRANSCRIPT
Light
what is it?
Light
what is it: moving energy• particle or wave?
Light
what is it: moving energy• particle or wave? how do we decide?
Light
what is it: moving energy• particle or wave? how do we
decide?
• if a wave, what is waving?
(waving even in a vacuum?)
Light
what is it: moving energy• particle or wave? how do we
decide?
if a wave, what is waving:
(waving even in a vacuum)
Electric & Magnetic Fields
Properties of Light
• speed of light
• colors
• reflection
• refraction (bending)
• shadows
• energy theory
• absorption of light
• emission of light
Properties of Light
• speed of light Property #1• colors Property #2• reflection Property #3• refraction (bending) Property #4• shadows Property #5• energy theory consider in Part 4• absorption of light consider in Part 4• emission of light consider in Part 4
Property 1: Speed of Light
• particle (photon) prediction?
Property 1: Speed of Light
particle (photon) ? no prediction
• wave (E&M) prediction?
Property 1: Speed of Light
particle (photon): no prediction
wave (E&M):For a wave on a string, we can start from
Newton’s Second Law and get a wave equation that leads to the relation:
vphase = [T/] (speed of wave depends on parameters of the string the wave travels on - T is tension in the string and is the mass density of the string)
Property 1: Speed of Light
particle (photon): no prediction
wave (E&M): Maxwell’s Eqs.
In a similar way to the wave on a string, we can get a wave equation from Maxwell’s
Eqs for Electromagnetism. This predicts: vphase = [1/oo]
where the o and o are the electric and magnetic properties of vacuum.
Property 1: Speed of Light
particle (photon): no prediction
wave (E&M): Maxwell’s Eqs.in vacuum:
v = [1 / {o o}]1/2 where
o = 1/{4k} = 1 / {4 * 9x109 Nt-m2/Coul2}
o = 4 * 1x10-7 T-s /Coul
v = [4*9x109 / 4*1x10-7 ]1/2 = 3 x 108 m/s = cunits: [(Nt-m2/C2)*(C/[T-s])]1/2 = [({kg*m/s2}*m2/C2)*(C/[{Nt-s/C-m}*s])]1/2 = m/s
Property 1: Speed of Light
particle (photon): no prediction
wave (E&M): Maxwell’s Eqs.in material,
vphase = [1/]
= Ko , where K>1; and o ; sov < c
According to the wave theory, light should move slower in material than in vacuum.
Property 1: Speed of Light
particle (photon): no prediction
wave (E&M):
in vacuum, v = c; in material, v < c
we’ll come back to this when we look at refraction later in this part.
Property 2: Color
• experiment ?
• particle (photon) ?
• wave (E&M) ?
Property 2: Color
Experiment:– invisible as well as visible– total spectrum order:
• radio• microwave• IR• visible• UV• x-ray and gamma ray
Property 2: Color
Experiment:
– visible order:• red• orange• yellow (yellow)• green• blue• violet
Property 2: Color
particle (photon):
amount of energy per photon
determines “color”
Property 2: Color
particle (photon): amount of energy
• among different types:
x-ray - most energy; radio - least
• in visible portion:
violet - most energy; red - least
Property 2: Color
particle (photon): amount of energy
• wave (E&M) ?
Property 2: Color
particle (photon): amount of energy
wave (E&M): frequencyamong different types of “light”: low frequency is radio (AM is 500-1500 KHz) high frequency is x-ray & gamma rayin visible spectrum: red is lowest frequency (just above IR)
violet is highest frequency (just below UV)
Colors: frequencies & wavelengths (in vacuum)
AM radio 1 MHz 100’s of m
FM radio 100 MHz m’s
microwave 10 GHz cm - mm
Infrared (IR) 1012 - 4x1014Hz mm - 700 nm
visible 4x1014 - 7.5x1014 700nm -400nm
Ultraviolet (UV) 7.5x1014 - 1017 400 nm - 1 nm
x-ray & ray > 1017 Hz < 1 nm[This slide will be repeated after we see how we get these values.]
Property 3: Reflection
• particle (photon) ?
• wave (E&M) ?
Reflection
particle (photon): bounces “nicely”
wave (E&M): bounces “nicely”
experiment: bounces “nicely”
bounces nicely means:
angle incident = angle reflected
Reflection
Does a white paper reflect the light, or does a white paper emit from itself the light? - Obviously, the white paper reflects the light.
Does a mirror reflect light? Of course.
What is the difference between white paper and a mirror?
ReflectionA white paper is rough on a microscopic level, and
so a beam of light is reflected in all directions:
A mirror is smooth on a microscopic level, and so a beam of light is all reflected in just one direction.
rough paper smooth mirror
Red is incoming, blue is outgoing
Property 4: Refraction
• experiment ?
• particle (photon)?
• wave (E&M) ?
Property 4: Refraction
experiment: objects in water seem closer than they really are when viewed from air
air
water
real object
apparentlocation
eye
Property 4: Refraction
• particle (photon) ?
water
air
surface
refracted ray
incident ray
Property 4: Refraction
particle (photon):
water
air
surface
incident ray
refracted ray
vxa
vya
vxw
vyw
vxa = vxw
vya < vyw
therefore
va < vw
= a
= w
Refraction: particle theory
Since v1x = v2x, using the angles between the normal (the vertical) and the light rays, we have: vx1 = vx2, or v1 sin(1) = v2 sin(2) ,
v1 sin(1) = v2 sin() (faster speed means smaller angle)
Property 4: Refraction
• wave (E&M) ?
surface
air
water
incident wave
refracted wavenormal line
Property 4: Refraction
wave (E&M):
surface
air
water
incident wave
refracted wave
crest of wave
crest of preceding wave
x
a
w
normal linecrest of following wave
Property 4: Refraction
wave (E&M): + = 90o
+ = 90o
surface
air
water
incident wave
refracted wave
crest of wave
crest of preceding wave
x
a
w
normal line
sin() = a/x
sin() = w/x
Refraction: wave theorywave (E&M): Snell’s Law
sin(a) = a/x and sin(w) = w/x
eliminate x: a/sin(a) = w/sin(w)
and use: f = v (or = v/f) to get
f sin(a) / va = f sin(w) / vw or
(1/v1) sin() = (1/v2) sin()
(faster speed means bigger angle)
NOTE: since a > w, need va > vw which agrees with
wave prediction of Property 1 on speed!
Note: This is opposite to the prediction of the particle
theory: v1 sin() = v2 sin() with va < vw .
Property 4: Refraction
wave (E&M): Snell’s Lawnicer form for Snell’s Law:
f sin(a) / va = f sin(w) / vw
Multiply thru by c/f to get
(c/va) sin(a) = (c/vw) sin(w)
and use definition of index of refraction:
n = c/v to get
na sin(a) = nw sin(w) Snell’s Law
Properties 1, 2 & 4
Speed, Color and RefractionSpeed of light changes in different materials
Speed is related to frequency and wavelength: v = f
• If speed changes, does wavelength change, frequency change, or BOTH?
• Does color change with speed? (does color depend on frequency or wavelength?)
Properties 1, 2 & 4
Speed, Color and RefractionSpeed of light changes in different materialsSpeed is related to frequency and wavelength:
v = f• What changes with speed:
– Frequency remains constant regardless of speed
– Wavelength changes with speed– Color remains constant (so color depends on
frequency, not wavelength)
Property 4: Refraction
• particle (photon) theory: vw > va
• wave (E&M) theory: vw < va
• experiment ?
Property 4: Refraction
• particle (photon) theory: vw > va
• wave (E&M) theory: vw < va
• experiment: vw < va
particle theory fails!
wave theory works!
Property 4: Refraction
Snell’s Law:
na sin(a) = nw sin(w)
Note that angles are measured from the normal, not the surface.
Note that the index of refraction is bigger for slower speeds.
Property 4: Refraction
Snell’s Law: n1 sin(1) = n2 sin(2)• NOTE: If n1 > n2, THEN 1 < 2 .
• NOTE: All 1 values between 0 & 90 degrees work fine.
• NOTE: Not all values of 2 work!
Example: If n1 = 1.33, n2 = 1, and 1 = 75o, then
2 = inv sin [n1 sin(1) / n2] = inv sin [1.28] = ERROR
Property 4: Refraction
Snell’s Law: n1 sin(1) = n2 sin(2)
If n1 sin(1) / n2 > 1 THEN there is NO value of 2 that can satisfy Snell’s law (unless you count imaginary angles!).
The math is trying to tell us that there is NO transmitted ray. This is called
TOTAL INTERNAL REFLECTION.
Property 4: Refraction
The computer homework program entitled Snell’s Law (Vol. 5, #1) will give you practice in using Snell’s Law.
We will now temporarily halt our look at light’s different properties, and look at some important applications of Refraction.