Transcript of the light. - Hss-1.us

Phys Sci Week 13 - 14
Intro to Physics of Light
Next test: Week 15 Dec 19/21
Week 16 Dec 30 class at my home: 10 - 1 PM
Text: Module 15 pages 367 - 396
Reading Assignments
Module 15
Homework Assignment
Prep Questions 13:
Module 15 Study Guide Questions
(1) What is a light?
(2) What is the difference
between reflection, refraction
and the absorption of light?
Prep Questions 14:
(1) How does the human Eye
work?
(2) How do we perceive color?
See Introduction (p 284)
EM Waves
• Electromagnetic force is a fundamental force
– James Maxwell proved the electro and magnetic forces were really
one force (p 284 -285)
• Discovered and proved that electricity and magnetism were
caused/governed by the same force now called the electric force.
– Due to the movement interactions of the charges associated with
electrons.
– This force is also associated with the formation of electromagnetic
waves (forms of "light")
• ALL waves excerpt for one (electromagnetic waves)
requires a medium - something to move through.
• Two General Type of Waves (p 243)
– Transverse - wave that propagates perpendicular to its direction of
oscillation.
– Longitudinal - waves that propagates parallel to its direction of
oscillation.
• Compression - area of compression (higher pressure/greater density) like crest
• Rarefaction - pulled apart lower pressure/lower density - like trough.
Electromagnetic waves: Light (EM) Waves are very unusual in that
the are transverse waves that require no medium.
The Dual Nature of Light
• Photons and the Electromagnetic Force - The
Dual nature of Light
– Electromagnetic Waves has a dual nature - it can act
as wave and a particle. We don't understand this
because logically (deductive logic) a particle can't be a
wave and a wave can't be a particle. A wave should be
the movement of energy through some medium (bunch of
particles). But light violates this rule in that it sometimes
acts as a particle and sometimes act as wave.
• The Electromotive force can be viewed terms of
particle called photons.
– Particle theory of light : Photon - small packet of "light"
that acts like a particle.
• Wave theory of light: Light also acts as a wave.
Photons and EM Waves
Quantum Mechanical theory of light: - view of light as tiny packet
of waves
•
•
Speed of light 300,000,000 m/s
(186,000 m/s or 670,000,000
mph) in a vacuum- does not
depend upon temperature. Does
depend on medium.
Einstein's Special Theory tells
us that the speed of light is the
fastest anything with mass can
Substance
Speed of
Light
Substance
Speed of
Light
Air (25C)
300,000,000
m/s
Plastic
189,000,000
m/s
Alcohol
225,000,000
m/s
Crown Glass
185,000,000
m/s
Fresh Water
220,000,000
m/s
Flint Glass
175,000,000
m/s
Acrylic
220,000,000
m/s
Diamond
125,000,000
m/s
travel.
Wavelength and Frequency of Light
• Wavelength and frequency of sound determine its pitch.
• For light (EM waves) Wavelength and frequency
determine its energy and for visible light its color.
– The shorter the wavelength, the higher the frequency the greater
the energy.
– Review the EM spectrum to reinforce that we only see a small
part of the EM spectrum.
•
•
•
•
•
Radio - Microwaves
Infra red
Visible spectrum: Red - Yellow - Green - Violet
Ultra violet
X-rays, Gamma rays
Wavespeed
• Wavespeed = wavelength/frequency
– For light wave speed (c) = constant
– Because c is constant the frequency and
wavelength really tell us the same thing
because one wavelength can be associated
for a given frequency and vice versa.
• Sample problems: c = λ/f
Interaction of Light with Material Objects
(Absorption, Reflection, Refraction and Scattering)
• Absorption: Light is absorbed by the substance increasing the
energy of the material object. Gas, liquid and water
• Reflection: Light ray reflection is the turning back of the ray when it
encounters the edge of a medium.
• Angle of Incidence: Angle of incidence is a measure of deviation of
something from "straight on", for example in the approach of a ray to
a surface it the angle from the vertical.
• Angle of Reflection: The angle between the reflected ray and the
normal is known as the angle of reflection.
Law of Reflection: The angle of reflection equals the angle of incidence.
•
Refraction: Refraction is the change in direction of a wave due to a change
in its speed.
– This is most commonly observed when a wave passes from one medium to
another. Refraction of light is the most commonly observed example, but any
type of wave can refract when it interacts with a medium, for example when
sound waves pass from one medium into another or when water waves move
into water of a different depth.
– Refraction is described by Snell's law, which states that the angle of incidence is
related to the angle of refraction by
•
•
•
•
•
•
or
where
v1 and v2 are the wave velocities through the respective media.
θ1 and θ2 are the angles between the normal (to the interface)
plane and the incident waves respectively.
n1 and n2 are the refractive indices.
Two type of refractions:
1. Light slows down as it goes from one medium to
another
– Light bends up to line perpendicular to the surface.
Refraction of light at the interface between two media of different refractive
indices, with n2 > n1. Since the phase velocity is lower in the second medium (v2 <
v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray
in the higher-index medium(n2) is closer to the normal.
• 2. Light speeds up as it goes form one medium to another
- Light bends away from line perpendicular to the surface
Refraction of light at the interface between two media of different refractive indices,
with n2 < n1. Since the phase velocity is lower in the second medium (v2 <v1), the
angle of refraction θ2 is greater than the angle of incidence θ1; that is, the ray in the
lower-index medium (n2) is further away from the normal.
Fiber Optics: Total Internal
Reflection
Light Scattering
•
Scattering is a general physical process where radiation, such as light, sound, or moving particles, are
forced to deviate from a straight trajectory due to striking a particle
•
Wavelength much smaller than particle: Rayleigh scattering (named after the English physicist Lord
Rayleigh) is the uniform scattering of light or other electromagnetic radiation by particles much smaller than
the wavelength of the light. Rayleigh scattering of sunlight in clear atmosphere is the main reason why the
sky is blue: Rayleigh and cloud-mediated scattering contribute to diffuse light (direct light being sunrays).
•
Wavelength about same size as the particle: This is called Mie scattering and results in a directional
scattering of light cause the sky to appear white.
Lenses
•
A lens is an optical device with axial symmetry which transmits and refracts
light, converging or diverging the beam.
–
–
A simple lens is a lens consisting of a single optical element.
A compound lens is an array of simple lenses (elements) with a common axis;
• the use of multiple elements allows more optical aberrations to be corrected than is
possible with a single element.
• Manufactured lenses are typically made of glass or transparent plastic.
• Elements which refract electromagnetic radiation outside the visual spectrum are also
called lenses: for instance, a microwave lens can be made from paraffin wax.
Converging Lens:
•
If the lens is -convex, a parallel beam of light travelling parallel to the lens
axis and passing through the lens will be converged (or focused) to a spot
on the axis, at a certain distance behind the lens (known as the focal
length). In this case, the lens is called a positive or converging lens.
Diverging Lens:
•
If the lens concave, a parallel beam of light passing through the lens is diverged
(spread); the lens is thus called a negative or diverging lens. The beam after
passing through the lens appears to be emanating from a particular point on the
axis in front of the lens; the distance from this point to the lens is also known as
the focal length, although it is negative with respect to the focal length of a
converging lens
Other types of Lenses:
The Human Eye
• The eye is an organ which reacts to light for several purpose that allows
vision.
– Rods and cones in the retina allow conscious light perception and vision
including color differentiation and the perception of depth. The human eye can
distinguish about 10 million colors.
• Photoreceptors - Rods, Cones and Photosensitive Ganglion:
– A photoreceptorcell, is a specialized type of neuron (nerve cell) found in the eye's
retina that is capable of phototransduction (process by which light is converted
into electrical signals sent on to the brain ).
– Rods and cones in the retina allow conscious light perception and vision
including color differentiation and the perception of depth. The human eye can
distinguish about 10 million colors.
– Cones are adapted to detect colors, and function well in bright light;
• In humans there are three different types of cone - responding respectively to short
(blue), medium (green) and long (yellow-red) light
– Rods are more sensitive, but do not detect color well, being adapted for low light.
• The human retina contains about 120 million rod cells and 6 million cone cells
– Photosensitive Ganglion Cells in the retina receive light signals which:
• Sets the body clock,
• Adjustment of the size of the pupil and
• Regulation and suppression of the hormone melatonin (helps us sleep)
The Human Eye
Perceiving Color
•
Color is based upon the frequency (or wavelength) of the light.
– We perceive color when the different wavelengths composing white light are
selectively interfered with by matter (absorbed, reflected, refracted, scattered, or
diffracted) on their way to our eyes, or when a non-white distribution of light has
been emitted.
– We can detect the range of light spectrum from about 400 nanometers (violet) to
about 700 nanometers (red). We perceive this range of light wavelengths as a
smoothly varying rainbow of colors, otherwise known as the visual spectrum.
100 nm
1000 nm
Light Interaction With Itself Interference
• Interference: In physics, interference is
the addition (superposition) of two or more
waves that results in a new wave pattern. .
• Constructive and Destructive
interference
– Constructive Interference pattern reinforce
waves and produced bright bands
– Destructive interference pattern cancel out
waves and produce dark bands (no light).
Constructive and Destructive
interference
• Interference pattern produced with a Michelson interferometer.
Bright bands are the result of constructive interference while the
dark bands are the result of destructive interference.
• When two sinusoidal waves superimpose, the resulting waveform depends
on the frequency (or wavelength) amplitude and relative phase of the two
waves.
– If the two waves have the same amplitude (A) and wavelength (λ) the resultant
waveform will have an amplitude between 0 and 2A depending on whether the
two waves are in phase or out of phase.
– In phase: Consider two waves that are in phase, with amplitudes A1 and A2.
Their troughs and peaks line up and the resultant wave will have amplitude A =
A1 + A2. This is known as constructive interference.
– Out of phase: If the two waves are π radians, or 180°, out of phase, then one
wave's crests will coincide with another wave's troughs and so will tend to
cancel out. The resultant amplitude is A = |A1 − A2|. If A1 = A2, the resultant
amplitude will be zero. This is known as destructive interference.
Electromagnetic Interference
(EMI)
• Electromagnetic interference (or EMI, also called radio frequency
interference or RFI) is a disturbance that interfers an electrical circuit
due to either electromagnetic conduction or electromagnetic
radiation emitted from an external source.
– The disturbance may interrupt, obstruct, or otherwise degrade or limit
the effective performance of the circuit.
– The source may be any object, artificial or natural, that carries rapidly
changing electrical currents, such as an electrical circuit, lightning, the
Sun activity or the Northern Lights.
• EMI can be intentionally used for radio jamming, as in some forms of
electronic warfare, or can occur unintentionally, as a result of
spurious emissions of static.
– It frequently affects the reception of AM radio in urban areas.
– It can also affect cell phone, FM radio and television reception, although
to a lesser extent.
Adding and Subtraction Colors:
• Colors are perceived by either emitted or reflective light
– Emitted light causes additive colors - they add together to create color
Visible light color: http://www.youtube.com/watch?v=5J8iEIQBlxc
– Reflective light causes subtracted color - everything but the light we see
is absorbed or subtracted by object
– The human eye can distinguish about 10 million different colors
• Primary Colors: Other colors can be made from either adding
or subtraction these three colors
• Additive Primary Colors: Red, Green, Blue - used for
generated light (example color TV)
– Mixing all additive primary colors = white
• Subtractive Primary Colors: Yellow, Magenta, Cyan - used
for reflected colors (paints)
– Mixing all subtractive primary colors = black
Labs
• Lab 15.1 p 370 Wavelengths of Light
– Other Light experiments
• Lab 15.2. p 374 Law of reflection
– Fiber optic experiment Tyndall Bucket
• Lab 15.3 p 378Refraction
• Lab 15.4 p 380 Magical Quarter
• Lab 15.5. p 388 How the eye detects
color