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ISNS 3371 - Phenomena of Nature
The Doppler Effect - Wavelength Shift Due to Motion.
Each circle represents the crests of sound waves going in all directions from
the train whistle. The circles represent wave crests coming from the train at
different times, say, 1/10 second apart. If the train is moving, each set of
waves comes from a different location. Thus, the waves appear bunched up
in the direction of motion and stretched out in the opposite direction.
ISNS 3371 - Phenomena of Nature
Hearing the Doppler Effect Animation
ISNS 3371 - Phenomena of Nature
Doppler Shift vs Velocity Animation
ISNS 3371 - Phenomena of Nature
Sonic Boom
As the airplane moves, it pushes
air molecules out of its way,
continuously creating waves of
compressed and uncompressed
air. These air pressure waves move
away from the airplane in all
directions at the speed of sound.
Next, break the sound barrier by
increasing the airplane's speed to
supersonic - the air pressure waves
(called shock waves) cannot precede
the plane, and so accumulate in a
cone behind the plane. The shock
waves will move out and back from
the plane, towards the ground. There
is a sudden change in pressure when
the shock wave hits your eardrum.
You hear this as a loud sonic boom.
ISNS 3371 - Phenomena of Nature
There are actually two shocks - the first shock forms at the nose of the
aircraft and the second near the tail - so you actually hear two sonic
booms. They start out very close together (one body length apart, which
means they are separated by much less than 0.1 sec), but, as they
propagate the long distance to the Earth's surface, they spread apart, and
the time between them gets as large as 1 sec. That's why you can actually
hear two closely-spaced booms. The longer the aircraft's fuselage and the
higher it is flying, the easier it is to distinguish the two shocks.
To increase the intensity of a sonic boom, increase the size of the
airplane. The larger the aircraft, the more air it displaces and the stronger
the shock waves become.
ISNS 3371 - Phenomena of Nature
F/A-18 Hornet photographed just as it broke the sound barrier.
A leading theory is that a drop in air pressure at the plane described by the
Prandtl-Glauert Singularity occurs so that moist air condenses there to form
water droplets.
ISNS 3371 - Phenomena of Nature
Light
ISNS 3371 - Phenomena of Nature
Light
What is light? - A vibration in an electromagnetic field through which energy
is transported. Since it is an electromagnetic field, it does not require a
medium -
In space everyone can see you scream! (they just can’t hear you)
The dual nature of light or wave-particle duality - light can be treated as a
wave or as a particle:
Light as a wave
f=c
Light as a particle
E = hf
photon
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Dual Nature of Light
Light may show properties of a wave or of a particle, called a
photon.
Newton (1680) explained light as a particle of energy. In
reflection and refraction, light behaved as a particle.
Young, (circa. 1800) showed that light interfered with itself.
Therefore, it must be a wave. Reflection and refraction could
be explained by light being a wave.
Maxwell (1850) showed that light was a form of high frequency
electromagnetic wave.
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Dual Nature of Light
Einstein (1905) showed that in the photoelectric effect (light
causing electrons to be emitted from a metal surface) light
must act as a particle.
Planck (1900) developed a model that explained light as a
quantization of energy. Energy of a light wave is present in
bundles of energy called photons; the energy is said to be
quantized into the photons.
Therefore, light must be regarded as having a dual nature;
in some cases light acts as a wave; in others it acts like a
particle.
ISNS 3371 - Phenomena of Nature
The Photoelectric Effect
First observed by Heinrich Hertz in 1887 - light
shining on a metal plate causes electrons to be
knocked loose (ejected) from the metal plate.
Several aspects of the phenomena could not be
explained in terms of an electromagnetic wave:
Increasing the brightness of the light did not eject faster
electrons - think of light as a wave - brighter light (bigger
amplitude wave) should eject more energetic (faster) electrons.
Energy and number of ejected electrons depends on color of
light - for some metals, red light would not eject any electrons at
all - blue lights ejects very fast electrons even if very dim.
The electrons were emitted immediately - no time lag - if light is
dim, expect a delay while the waves wiggle the electrons and
break them loose.
ISNS 3371 - Phenomena of Nature
The Photoelectric Effect
In 1905 Einstein gave a very simple interpretation of the results - the
incoming radiation should be thought of as quanta of energy hf - a
photon - with f the frequency.
- one of the crucial landmarks in the development of modern
physics.
- introduced in one of his three papers, published in 1905
(called the Remarkable Year of Physics)
- helped to initiate the fundamental revolution in science that
we now call Quantum Physics.
- it was for this work that he was awarded a Nobel Prize in
1921 - not relativity!
Idea used in a number of technologies today including photovoltaic cells
- solar cells.
ISNS 3371 - Phenomena of Nature
Light as a Particle (Photon)
• Light propagates as quanta of energy called photons
• Photons
•move with speed of light
•have no mass
•are electrically neutral
•
Energy of a photon or electromagnetic wave:
E = hf = h c/ l
where
h = Planck’s constant
f = frequency of a light wave - number of
crests passing a fixed point in 1 second
c = velocity of light
l = wavelength of a light wave distance between successive crests
ISNS 3371 - Phenomena of Nature
Light as a Wave
Remember: Light is a vibration in an electromagnetic field through
which energy is transported.
So electrons can be manipulated by light. Electrons wiggle up and
down as light passes by. It is a transverse wave - the vibration of
particles is perpendicular to the propagation of the wave.
ISNS 3371 - Phenomena of Nature
ELECTROMAGNETIC WAVES (LIGHT WAVES)
Velocity
186,000 miles/second
300,000 kilometers/second
3 x 106 m/second
• It takes 1 1/3 second for light to travel from the earth
to the moon.
• It takes 8 1/3 minutes for light to travel from the sun
to the earth.
ISNS 3371 - Phenomena of Nature
Light Travel Time
From Earth to the Moon
From Earth to the Sun
From the Sun to Jupiter
From the Sun to Saturn
our natural satellite
1.25 seconds
the centre of our Solar System
8.3 min
the largest planet
41 min
the furthest naked eye planet
85 min
the furthest of the Sun's
From the Sun to Pluto
5.5 hr
planets
From the Sun to Alpha Centauri
the nearest star to us
4.3 yr
From the Sun to Sirius
the brightest star in our sky
8.6 yr
Distance where the Sun would no longer be visible to naked eye
60 yr
From the Sun to Polaris
the north pole star
650 yr
From the Sun to the Galactic
the centre of our Galaxy
31,000 yr
centre
Galactic diameter
the diameter of our Galaxy
81,500 yr
To the Andromeda Galaxy
the nearest large galaxy
2,200,000 yr
Extinction of the dinosaurs
65,000,000 yr
To Q0134+329
typical quasar
4,500,000,000 yr
Formation of the Earth and Sun
4,700,000,000 yr
To remotest quasars
discovered in 1998
14,000,000,000 yr
Edge of Universe
limit of observable Universe
15,000,000,000 yr
ISNS 3371 - Phenomena of Nature
Light as a Wave
•
For a wave, its speed: s = f ( is wavelength)
•
But the speed of light is a constant, c.
•
For light: f  = c
•
The higher f is, the smaller  is, and vice versa.
•
Our eyes recognize f (or ) as color!
ISNS 3371 - Phenomena of Nature
Visible light ranges through 7 major colors from long wavelengths (low
frequency - red) to short wavelengths (high frequency - violet) - Red,
orange, yellow, green, blue, indigo, violet (Roy G Biv)
Visible Light Waves Animation
ISNS 3371 - Phenomena of Nature
The Electromagnetic Spectrum
Most wavelengths of light can not be seen by the human eye.
The visible part of the electromagnetic spectrum lies between ultraviolet
and infrared light (between about 400 and 700 nm). The higher the
frequency (shorter the wavelength), the higher the photon energy. Radio
waves are at the long wavelength end of the spectrum and gamma rays
are at the short wavelength end of the spectrum.
ISNS 3371 - Phenomena of Nature
Color
Additive primary colors - adding light of additive primary colors
produces complementary colors - all colors produces white
Red
Green
Blue
Red + Green  Yellow
Red + Blue  Magenta
Green + Blue  Cyan
Red
Magenta
Yellow
Blue
Green
Cyan
White
ISNS 3371 - Phenomena of Nature
Color
Subtractive primary colors - mixing pigments (that absorb light) of
various subtractive primary colors produce complementary colors added all together, produce black.
Yellow
Magenta
Yellow + Magenta  Red
Yellow + Cyan  Green
Magenta + Cyan  Blue
Cyan
ISNS 3371 - Phenomena of Nature
Only four colors are used to print color illustrations and photos: (a)
magenta, (b) yellow, (c) cyan, (e) black. (d) is with all but black, (f) is with
all.
ISNS 3371 - Phenomena of Nature
Four Ways in Which Light can Interact with Matter
1.
emission – matter releases energy as light
2.
absorption – matter takes energy from light
3.
transmission – matter allows light to pass through it
4.
reflection – matter repels light in another direction
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Properties of Light
Law of Reflection -
Angle of Incidence = Angle of reflection
Law of Refraction - Light beam is bent towards the normal
when passing into a medium of higher Index of Refraction.
Light beam is bent away from the normal
when passing into a medium of lower Index of Refraction.
Index of Refraction -
n
Speed of light in vacuum
Speed of light in a medium
Inverse square law - Light intensity diminishes with square of
distance from source.
ISNS 3371 - Phenomena of Nature
Mirror reflects light at
angle equal to
incoming angle -
Most materials reflect
light randomly scattering. Movie
screen scatters light
into array of beams
that reaches every
member of the
audience
ISNS 3371 - Phenomena of Nature
Materials that transmit light are transparent
Materials that absorb light are opaque
In general - some combination of reflection, absorption, and transmission
Red glass transmits red light - absorbs all other colors
Green grass reflects green light - absorbs all other colors
ISNS 3371 - Phenomena of Nature
Atmospheric Light Scattering
Atmospheric gases are largely transparent to visible light
Most photons penetrate to the ground, warming it as the light is
absorbed
Small portion of light is scattered
- why our sky is bright
- light is not scattered on Moon, Mercury
- their skies are dark - stars are visible during day
- shadows extremely dark
Gas molecules scatter blue light more effectively than red light
ISNS 3371 - Phenomena of Nature
Why is the Sky Blue?
Sunlight is scattered off the molecules
of the atmosphere - called Rayleigh
scattering - more effective at short
wavelengths (the blue end of the visible
spectrum). Little of the red, orange and
yellow light is affected. Whichever
direction you look, some of this
scattered blue light reaches you. Since
you see the blue light from everywhere
overhead, the sky looks blue. As you
look closer to the horizon, the sky
appears much paler in color. To reach
you, the scattered blue light must pass
through more air. Some of it gets
scattered away again in other
directions. Less blue light reaches your
eyes. The color of the sky near the
horizon appears paler or white.
ISNS 3371 - Phenomena of Nature
Why are Sunsets Red or Orange?
The blue light has been scattered out
and away from the line of sight.
As the sun begins to set, the light must
travel farther through the atmosphere
before it gets to you. More of the light
is reflected and scattered. As less
reaches you directly, the sun appears
less bright. The color of the sun itself
appears to change, first to orange and
then to red. This is because even more
of the short wavelength blues and
greens are now scattered. Only the
longer wavelengths are left in the
direct beam that reaches your eyes.
Dust particles can scatter red and
orange light in all directions leading to
spectacular sunsets - why sunsets in
LA are so beautiful - pollution.
ISNS 3371 - Phenomena of Nature
Moonrise
Earthrise
On the moon, there is no atmosphere to scatter light and the sky is black
Atmosphere on Mars too thin to scatter light effectively - sky is reddish from
presence in the atmosphere of reddish dust from surface. On Venus, almost
all blue light scattered away - atmosphere dimly lit and appears reddish
orange.