Basics and history of geometrical optics

INTRODUCTION
• ELECTROMAGNETIC RADIATION :- it is
composed of oscillating electric and magnetic
fields emitted by charged particles
• It transport energy and travels at empty space
with a constant velocity C.
c equals the product of wavelength and
frequency.

• The study of light falls into two classes of
phenomena:
– physical optics
– geometrical optics

Physical optics
• Deals with theories of the nature of light and its
interaction with matter. The nature of light has been of
fundamental interest to physicists.
"What is light?“
• Isaac Newton's :- Capsular Theory of light
• corpuscular theory of light dominant but was
supplanted by wave theory originated by Christian
Huygens
• Thomas Young and Augustus Fresnel:- radiation is
emitted or absorbed in discrete packets or quanta.

Physical optics
• Einstein extended this idea to explain, where
the wave theory was unable to do so, the
photoelectric effect.
• He said that electromagnetic radiation existed
as quanta. A photon is a quantum of light.
• Contemporary optics treats photons as having
the properties of a particle and a wave.

Geometrical Optics
• concerned with how light is propagated,
reflected, and refracted, and the formation of
images. Light is assumed to consist of rays.
• Experimental facts have resulted in some basic
postulates of geometrical optics:

Postulates Of Geometrical Optics
1) Light Is Propagated In Straight Lines In A
Homogeneous Medium,
2) The Angle Of Reflection Equals The Angle Of
Incidence.
3) The Ratio Of The Sines Of The Angle Of Incidence
To The Angle Of Refraction Is A Constant That
Depends Only On The Media, And
4) Two Independent Beams Of Light That Intersect
Each Other Will In No Way Affect One Another.

THE ELECTROMAGNETIC SPECTRUM
• WAVE MOTION:- A pebble dropped into a
pond produces ripples that spread out in ever
widening circles. These ripples, or waves are
disturbances of particles of the water
medium. The particles move up and down.
• The properties of a train of waves are velocity,
frequency, wavelength, amplitude and phase.

• The number of waves that pass per unit time is
the frequency f.
• The peak-to-peak distance between waves is the
wavelength X
• Two such particles have the same displacement
and are moving in the same direction. They,
therefore, are in the same phase.
• The maximum height of the wave is its amplitude
a.

Wavelength (λ), amplitude (a) and phase. Points
ml and m2 are in phase since they occupy positions of
identical amplitude on the wave.

Basics and history of geometrical optics

• The velocity of any wave is given by: v = λ f.
• The visible spectrum is barely one octave
wide. It ranges from 400 nm violet light to 760
nm red light. Beyond red lies the infrared
region and below violet is the ultraviolet
region of the spectrum.

Measurement of the velocity of light
• The first evidence that light travels at a finite
speed was provided by the Danish astronomer
Romer about in 1675.
• A. Michelson, the first American physicist to
win the Nobel Prize, did so by measuring the
speed of light.

LIGHT SOURCES
• SELF-LUMINOUS:-
• SELF-LUMINOUS:- Thermally induced motions of the molecules of solids
produce incandescent light. Among incandescent sources are tungsten
lamps, candles, and the sun. Incandescent sources emit all visible
wavelengths in a continuous spectrum.
• Electric Arcs and Discharges:- Passing an electric current through gases
produces a disturbance of the atoms, which then emit light. The emitted
light is not a smooth continuous spectrum, but concentrated at
wavelengths that are characteristic of the gases. Sodium vapour produces
nearly monochromatic light at 589 and 589.6 nm.
• Fluorescent lamps:- The light produced by fluorescent lamps is emitted
by the fluorescent powders or phosphors that coat the inside of glass
tubes filled with a mercury vapour.
• Lasers. Lasers consist of solids, liquids or gases. They emit light because
either electrical or radiant energy stimulates their atoms. Laser stands for
light amplification by stimulated emission of radiation.

• LIGHT REFLECTING SOURCES :- Substances
that reflect light may serve as sources.
• The moon illuminates the earth by reflecting
sunlight

OPTICAL MEDIA
• Transparent substances, such as polished glass,
transmit about 96% of the light normally incident at
each surface. Transparent optical glass is
homogeneous and isotropic, i.e., has the same optical
properties in all directions. Glass and water are
isotropic substances.
• If a glass surface is roughened by sand blasting or acid
etching the transmitted light is diffused and the glass is
translucent.
• A substance is opaque if it only absorbs and reflects
light. Ordinary glass is opaque to short ultraviolet light;
black glass is opaque to visible light.

• light is transmitted, reflected and absorbed.
Transmittance (t) is the ratio of transmitted to
incident light; reflectance (r) is the ratio of
reflected to incident light; and absorptance is
the ratio of absorbed to incident light.

THE CORPUSCULAR AND WAVE
THEORY OF LIGHT
• THE CORPUSCULAR AND WAVE THEORY OF LIGHT:-
• Newton considered light to be streams of corpuscles
spreading out from a point source. The corpuscular
theory dominated scientific thinking for decades.
According to this theory, light sources emit particles
that travel in a straight line because Newton's first law
of motion requires this of bodies traveling in a uniform
medium, which are not acted on by any forces.
• The corpuscular theory was less successful in
explaining interference, diffraction and polarization.

Wave Theory of light.
• According to this theory, light is a transverse vibration that
travels outward from the source like ripples on the surface
of water. The theory could account for reflection and
refraction, and easily explained interference, diffraction,
and polarization
– Huygens postulated that a point source would produce a
wavelike disturbance that would spread out as an ever growing
sphere.
– At any instant all points on the surface of this sphere would
have the same state of excitation or phase and constitute a
wavefront.
– All points on the wavefront then become new sources or
centres from which secondary waves or wavelets spread out.
– These wavelets overlap and interfere with each other.

• The sharp shadows produced by point sources
are called umbras.
• the sharp boundary between an umbra and
the illuminated regions adjoining it will be
replaced by a region of partial shadow called a
penumbra.

REAL AND VIRTUAL OBJECTS AND
IMAGES
• One light ray reaching the eye will tell us the direction
of the point source from which it originated. However,
to locate a point source two rays are necessary. The
intersection of the two rays will locate the source or its
image if the rays travel through a homogeneous and
isotropic medium.
• Objects and images are real if the rays coming from the
object or going to the image actually intersect. If the
rays intersect when extended backwards or forward
the image and object, so formed, are virtual.

• STOPS:- FIELD STOP
• APERTURE STOP :- pupil of the eye.
• BAFFLES:- Baffles are diaphragms within an
optical system that block stray light from reaching
the image.

VERGENCE AND DIOPTERS
• If all object rays from a point meet in an image
point, the image is perfect or ideal, and the image
point is conjugate to the object point. Rays from a
real object diverge or have negative vergence.
• To form a real image the optical system must
produce positive vergence or converge the image
rays.
• Vergence is specified in diopters as the reciprocal
of the distance of the object or image in meters.
For example, light from a point source 0.5 meters
away has a vergence of -2.00 diopters.

VISUAL ANGLE
• Objects subtend visual angles at the eye. The
tangent of the visual angle is the apparent size
of the object.
• Given a tree with a height h, at a distance d
from the eye, the apparent size = tan h/d.

Basics and history of geometrical optics

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