Transcript What_Is_Light

Fiat Lux
Let there be Light
Kingshuk Majumdar
Berea College, Berea, KY
Overview
Part I:
 Light and some of its properties.
 Sources of light.
 Different phenomena: Reflection, Refraction, Dispersion,
Polarization, Interference
 Blackbody Radiation
Part II:
 Strange properties of light
 Wave-particle duality
Part III:
 Food for thought
What is light? What are its properties?
 Light is an electromagnetic wave.
First predicted by James Clark Maxwell (1831 – 1879).
Experimentally detected by Heinrich Hertz (1857 – 1894).
 Light wave is transverse (wave propagation is
perpendicular to electric and magnetic field oscillations).
 Light does not need a medium to propagate (different
from sound wave for example).
Experimentally verified by Albert A. Michelson and E. W.
Morley (1880).
First American to receive a
Nobel prize in physics in
1907
E. W. Morley
 Speed of light c is maximum in vacuum (= 3 x 108 m/s)
First precise measurement made by Albert Michelson.
 No information can be sent at a speed more than the
speed of light - one of the fundamental postulate of
Einstein’s special theory of relativity in 1905.
 c is a fundamental constant of nature.
Examples of other fundamental constants are charge of an
electron, mass of an electron, Planck’s constant, etc.
Light can only be studied indirectly, that is, in terms of how
it behaves.
Sources of Light:
• Luminous:
 When something produces light it is said to be luminous.
Examples: Sun, Stars, light bulbs, burning materials.
• Incandescent:
 When light is given off as a result of high temperatures an
object is said to be incandescent.
Examples: Sun, a flame from a burning source, light bulbs
• Black Body Radiation:
The radiation given off by an object at any temperature.
A luminous, incandescent source
Betelgeuse, the brightest star in the constellation Orion. (Produced
with ESA's Faint Object Camera (FOC), Hubble Space Telescope.)
Light interacts with Matter:
A ray of light travels in a straight line from a source until it
encounters some object or particles of matter.
Light that interacts with
matter can be reflected,
absorbed or transmitted
through transparent
materials.
Reflection
(A) Rays reflected from a
perfectly smooth
surface are parallel to
each other (specular
reflection).
(B) Diffuse reflection
from a rough surface
causes rays to travel in
many random
directions.
Law of Reflection
The law of reflection states that the angle of incidence I is equal
to the angle of reflection R. Both angles are measured from the
normal, a reference line drawn perpendicular to the surface at the
point of reflection.
Application of Reflection – Image Formation
Light rays from the block are reflected according to the
law of reflection, and reach your eyes. You see a
virtual image of the block, because light rays do not
actually come from the image.
Refraction
When a light ray
moves from one
transparent
material to another,
such as from air to
water, the ray
undergoes a change
in the direction of
travel at the
boundary.
This change in
direction is called
refraction.
Law of Refraction
Refractive index (n) of a medium
n = c/v
c = speed of light in vacuum (= 3 x 108 m/s)
v = speed of light in the medium (Note: v < c)
Law of Refraction:
n1 sin I = n2 sin R
Two cases possible:
(A) A light ray moving to a
denser material is refracted
toward the normal
( I > R).
(B) A light ray moving to a
less dense material from a
denser material is refracted
away from normal
( I < R ).
An application of refraction
Total Internal Reflection
The angle of incidence for a material that results in an angle of
refraction of 90o is called the critical angle. When the incident ray
is at this critical angle or greater, the ray is reflected internally.
Then it is called total internal reflection. The critical angle or water
is about 49o, and for a diamond it is about 25o.
Application of Total Internal Reflection - Mirage
Mirages are caused by hot air near the ground refracting,
or bending light rays upward into the eyes of a distant
observer. The observer believes he is seeing an upside
down image reflected from water on the highway.
Dispersion and Color
 Different colors of light have different frequencies and
wavelengths.
 The speed of light (c) is related to the wavelength () and
frequency (f) by
c=f
 Visible light is the part of the electromagnetic spectrum
visible to humans.
 Ordinary light (white light) is composed of 7 colors:
Red, Orange, Yellow, Green, Blue, Indigo, Violet.
Dispersion of light with Prism
The color of an object depends on the wavelengths of light
the object reflect. Each of these flowers absorbs the colors
of white light and reflects the color that you see.
The flowers appear to be red because it reflects light in
the 7.9 x 10-7 m to 6.2 x 10-7 m range of wavelengths.
Electromagnetic spectrum
Polarization
For unpolarized light, the electric
field vectors oscillate in all
transverse directions.
Light can be polarized – electric
field vector oscillates only in one
plane (plane polarized)
Polaroid
Filter
Picket Fence
Analogy
Circularly and Elliptically polarized light
Optical Rotation and Polarization
1. Plane of a polarization of a plane polarized light rotates
as it passes through a optically active (chiral) solution.
2. Rotation angle depends on (a) path length of the medium
and (b) concentration of the chiral substance.
Interference
Question: What happens when we superpose two waves?
Case I: A wave of maximum intensity.
(Constructive interference)
Case II: A wave of minimum intensity.
(Destructive interference)
Young’s (Thomas Young, 1773 – 1829) double slit
experiment:
Animation
Blackbody Radiation
Blackbody:
a body that would absorb all the radiation falling on it.
Blackbody radiation: Radiation emitted by a blackbody.
Spectrum of light emitted by a body depends on it’s
temperature.
Animation
Question: A red star and a blue star – which one is
hotter?
Part II: Strange Properties of Light
Photoelectric effect (A. Einstein got Nobel prize on this):
Animation
Compton effect (discovered by A. H. Compton in 1923):
Animation
Question: What can we learn?
Wave - particle duality
Answer:
Light behaves as a particle (photon) and also as a wave
but not both at the same time.
This is known as wave-particle duality (essence of
quantum physics)
Question: Is this wave-particle duality just special for
light?
Answer: NO!!
Wave-particle duality continued….
In 1923 de Broglie proposed (de Broglie’s hypothesis) :
Matter (for example electrons) also have wave like
properties.
Wavelength = Planck’s constant / Momentum of the particle
Or,
= h / p
This hypothesis was confirmed several years later.
Thought experiments with electrons
Experiment 1:
Watching the electrons!!
Experiment 2:
Part III: Food for thought !!
Question 1:
Light travels in a straight line. But, in presence of
strong gravity light bends (Bending of light was
first proposed by A. Einstein in his theory of
gravity).
So is there a contradiction?
Hint: How do you define a straight line? A line that appears curved
can be a straight line!!!
Physics of current interest: Gravitational lensing in Astronomy
Question 2:
Holograms are three dimensional images. When we see an object,
we actually see the hologram (three dimensional image) of that
object embedded in two dimensions (in our retina)!!
Note: The correspondence between the image in our retina and our
brain’s interpretation of the image is not one to one
So is it that we perceive a holographic universe?
Are there extra spatial dimensions (more than 3) that our brain fails
to interpret?
Physics of current interest: Holographic Universe, Extra dimensions
Question 3:
Fundamental constants of nature:
Speed of light c, Gravitational constant G, mass of an
electron me, charge of an electron e, Planck’s constant
h, mass of up and down quarks mu and md, QCD Mass
LQCD, etc.
A fundamental theory of our universe should explain
1. Are these constants fundamental?
2. If they are fundamental, why there are so many or so
few?
Thank you all !!