Download What is light? For the purposes of this class, light will refer to visible

What is light?
For the purposes of this class, light will refer to visible
light - light that can be seen by the human eye.
Like all electromagnetic waves, light has a particlewave duality: it exhibits features of both a particle and
a wave.
Wave features
Light has the same properties of any other wave:
Absorption
Reflection
Interference
Diffraction
Refraction
We know that light behaves as a transverse wave,
moving at the speed of light in the vacuum of
space, and slowing down once it encounters a
medium.
Particle features
Light consists of little packets of
energy, called quanta, or photons
According to the particle view of light:
the brightness of the light is the number of
photons, the color of the light is the energy
contained in each photon.
Take the example of a beam of light striking a
metal plate, which causes the metal atoms to
release electrons.
If light is a wave:
the energy in the wave should depend only on
its amplitude (the intensity of the light).
Based on Max Planck's work, Einstein proposed:
light delivers its energy as photons;
each photon has an energy of Planck's constant
(6.626 x 10-34 J·s) times its frequency.
Higher-frequency photons have more energy
and make the electrons come flying out faster:
same light intensity but a higher frequency =
electrons released have maximum KE.
If the frequency is low enough, none of the
photons will have enough energy to knock an
electron out of an atom.
Very low-frequency light = NO electrons, no
matter how high the intensity.
In 1913-1914, Millikan's experiments involving the
photoelectric effect agreed exactly with Einstein's
predictions about photons, not with the wave theory.
Einstein actually won the Nobel Prize for his
work on the photoelectric effect, (not for the
theory of relativity).
Ultimately, light is seen as both a
particle and a wave, because it has
features of both!
Visible Light
Light ranges from wavelengths of 400 nm
(violet) to 700 nm (red).
Violet light has the shortest wavelength and highest
frequency of visible light; red has the longest
wavelength and the lowest frequency.
Humans see best in the yellow-green range of the
visible spectrum; this is why many fire companies
have switched to a yellow-green color for their
vehicles in recent years.
There are two basic types of light sources:
Incandescent and Luminescent
Incandescence involves the vibration of
entire atoms.
Incandescent light:
• produced when atoms are heated and release
some of their thermal vibration as
electromagnetic radiation.
• the most common type of light that you see
everyday:
Sunlight, light bulbs (not fluorescent ) and
fires also known as "black body radiation."
In luminescence, only the electrons vibrate.
Luminescent light:
• Occurs at lower temperatures
• Creates light when electrons rise to a higher
level, then fall back down, releasing energy
• Requires electrical current or chemical
reactions to boost the electrons to the higher
energy level to keep the cycle going:
Electrical current: fluorescent lights , neon light ,
mercury-vapor street lights, light emitting diodes,
television screens and computer monitors ;
Chemical reactions: light sticks and fire-flies
The Inverse Square Law
The total amount of light a source emits is called its luminosity.
The intensity of light observed from a source of constant luminosity
is reduced as the square of the distance from the object.
This is known as the inverse square law for light intensity.
The inverse-square law in action. A certain amount of light passes
through the hole at a distance of 1 foot from the light-bulb.
At distances of 2 feet, 3 feet, and 4 feet from the bulb, the same
amount of light spreads out to cover 4, 9, and 16 times the hole's
area, respectively.
What is a laser?
The acronym LASER stands for:
Light Amplification by Stimulated Emission of
Radiation
Stimulated Emission
An atom that is struck by a photon of light
becomes "excited". When another photon
strikes that atom, it releases a new photon that
is completely identical to the incoming photon;
same color, going in the same direction. The
atom is stimulated to emit the new photon.
Candelas and Intensity of Light
The candle (candela) is the unit used to
measure the intensity of light. The
measurement refers to the intensity of the light
emitted from a luminous source.
Originally the candle was defined in terms
of the intensity of a standard spermaceti
candle.
Spermaceti is a waxy substance produced in
the head of a sperm whale.
Illuminated
Objects reflect
light rays.
Luminous
Objects
emit light.
Luminous Flux, P
The rate at which
luminous energy is
being emitted,
transmitted, or
received from a
source
Unit: lumen, lm
Illuminance, E
Measures the illumination of a surface.
E = (Luminous flux, P)
(area around source)
may also be
shown in terms
E= P
of power
(4πd2)
(Watts)
d : distance from source
Unit = Lumens/m2 = lux or lx
Named for one of its discoverers (Willebrord Snel van Royen), Snell's law states
that the ratio of the sines of the angles of incidence and refraction is equal to the
ratio of velocities in the two media, or equivalently to the inverse ratio of the indices
of refraction:
Into a faster medium: the light bends away from normal
Into a slower medium:
light bends toward the normal
Normal: imaginary line perpendicular to the surface of the medium
(the boundary between mediums).
index of
refraction for the
original (incident)
medium
incident angle
index of refraction
for the second medium
angle of refraction
Use ray diagrams to illustrate the path of light and to find
the location and size of the image as it passes through
convex and concave lenses.
A converging lens (or convex lens) bends light so that the
light rays come together to a point. The human eye has a
single converging lens.
A diverging lens (or concave lens) bends light so it
spreads light apart instead of coming together. An object
viewed through a diverging lens appears smaller than it
would look without the lens.
A mirror reflects light and allows you to see the image
reflection. Flat mirrors show a true-size image.
Consider a ray of light coming from a light bulb and
striking a mirror. The incident ray is the light ray that
strikes the mirror. The reflected ray is the light ray that
bounces off the mirror
Narcissus, by Michelangelo Caravaggio, ca. 1598
The law of reflection
The law of reflection says the angle of incidence equals the
angle of reflection. Light rays reflect from a mirror at the
same angle at which they arrive. Angles are always
measured relative to the normal line.
Θi = Θr
The normal line
Between the incident and reflected rays, there is an
imaginary line called the normal line which is
perpendicular to the surface of the mirror.
The angle between the incident ray and the normal line is
called the angle of incidence.
The angle of reflection is the angle between the normal
line and the reflected ray.
A prism is another optical device. It is made of a solid
piece of glass with flat polished surfaces.
May also be made of acrylic.
Prisms can both refract and reflect light.
A clear cloud-less daytime sky is blue because molecules in the
air scatter blue light from the sun more than they scatter red
light.
Molecules of oxygen and nitrogen in the air scatter the
light.
In 1911, Einstein calculated the detailed formula for the
scattering of light from molecules; this was found to be in
agreement with experiments.
The molecules are able to scatter light because the
electromagnetic field of the light waves induces electric
dipole moments in the molecules.
What causes rainbows?
Rainbows: Refraction and Reflection
Refraction: each time light crosses a boundary from one
substance to another, the rays bend (change direction)
depending on the wavelength (color) of light.
In a rainbow white sunlight enters a raindrop and is
broken into different colors heading in slightly different
directions. The light is then reflected (and magnified) off
the back of the raindrop and passes back into the air
again, in the process being further refracted.