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Light Rays
Dr. Entesar Ganash
1. The Nature of Light
“Is light a wave or a particle?”
The particle model of light (Newton’s Particle Model):
- Light was considered to be a stream of particles.
- Newton held that particles were emitted from a light source &they stimulated the
sight sense upon entering the eye.
- Reflection and refraction can be explained. However, light Interference phenomenon
could not be explained.
 The wave model of light:
This model was proposed that light might be some type of wave motion.
To Huygens, light was a type of vibratory motion, spreading out & generating a light
sensation when impinging on the eye.
in 1678, Huygens explained reflection & refraction on the basis of this theory.
In 1801, Young showed light Interference phenomenon
In 1873, Maxwell stated that light was a type of high-frequency electromagnetic wave.
In 1887, Hertz confirmed Maxwell’s theory.
1. The Nature of Light
“Is light a wave or a particle?”
In 1905, Einstein explained the photoelectric effect using quantization model, which
assumes the energy of a light wave is existed in particles called photons. According to
Einstein’s theory, the photon energy (E) is proportional to the frequency (f) of the
electromagnetic wave (E= h f ), where h is Planck’s constant.
Hence, “Is light a wave or a particle?” is an inappropriate question
Light must be regarded as having a dual nature.
Sometimes light acts like a wave, and other times it acts like a particle.
2. The Ray Approximation in Ray Optics
 The field of ray optics (geometric optics) involves the study of the light propagation.
Geometric optics is an approximate treatment of light in which light are represented
as straight- line rays.
The direction of light ray propagation
is changed by refraction & reflection.
knowledge of geometrical optics is useful for the important design & manufacture
of magnifiers, microscopes & telescopes.
We can understand the formation of an image with the fundamentals of ray optics.
2. The Ray Approximation in Ray Optics
A plane wave of wavelength λ is incident on a barrier in which there is an opening of diameter d.
A barrier
Diffraction
The ray approximation is very useful for the study of mirrors, lenses, prisms, &
optical instruments
3. Reflection
When a beam of light strikes such an interface, some light is scattered backward. This
phenomenon is known as reflection.
smooth surface
Specular reflection,
Reflection of light from a smooth
surface . The reflected rays are all
parallel to one another.
any rough surface
Diffuse reflection,
Reflection from any rough
surface. The reflected rays travel
in random directions.
Lows of Reflection
 At a planar mirror
Angle of incidence = Angle of reflection
(the first part of Law of reflection)
Be carful the angles are always measured from the normal to the surface.
 The incident and reflected beams lie within a plane together with the surface normal.
(the second part of Law of reflection)
4. Refraction
 At a planar dielectric surface such as glass plate, reflection & transmission occur at
the same time.
 Refraction is a phenomenon that describes the propagation of light through
interface, which separates 2 materials. The refraction changes the light’s direction of
travel.
The transmitted part of the incident beam is refracted.
Its change of direction can be explained by the refraction
index (n) that is defined by
n=
c
v
8
where c is speed of the light in a vacuum ( c = 2.998 ×10 m / s ) & v is the velocity of
light in a medium.
4. Refraction
Optically Dense medium has a higher n than thinner medium.
E.g. Water ( n ≅ 1.333 ) is optically more dense than air ( n ≅ 1.00029 ).
Low of Refraction
A refracted ray lies in the plane of incidence and has a refracted angle (θ t ) which is
related to incident angle( θ i ) by Snell’s law or Law of refraction that is given as
ni sin θ i = nt sin θ t
Low of Refraction
we can have three basic results:
a) nt = ni ⇒ θ t = θ i
i.e. the refraction does not bend the light beam, which continues
moves in the same direction.
b) nt > ni ⇒ θ t < θ i
i.e. the refraction bends the light beam toward the normal
c) nt < ni ⇒ θ t > θ i i.e. the refraction bends the light beam a way the normal.
Here the subscript
1 ≡i &2 ≡t
5. Total Internal Reflection
Rays of light from point source (S) in glass incident on glass-air interface
ray a: part of ray light reflects at interface & the rest travels without change in
direction .
Rays b, c and d: there are both reflection and refraction at interface.
5. Total Internal Reflection
Rays of light from point source (S) in glass incident on glass-air interface
Ray e : as the incident angle increases the refracted angle increases. When the
refracted ray reaches the interface that means the refracted angle is equal to 90 (relative
to a normal), then the incident angle is called the critical angle ( θ C ). This
Snell's law.
Rays f and g: here the incident angles larger than θ C & there is no refracted ray &
all the light is reflected. This situation is known as total internal reflection.
The total internal reflection can
occur when the incident light in
medium has a high (n).
that means the light travels from
a dense medium to a thinner one
( here from glass to air).
Total Internal Reflection
Optical Fibers
Losing of light intensity is very little
A cladding is a material that has a lower n than
the core.
Physicians use it to
1. inspect internal organs of the body
2. do surgery without making large incisions.
6. Huygens’s Principle
All points on a given wave front are taken as point sources for the production
of spherical secondary waves, called wavelets, that propagate outward
through a medium with speeds characteristic of waves in that medium. After
some time interval has passed, the new position of the wave front is the surface
tangent to the wavelets.
The laws of reflection & refraction can be derived by using Huygens’s principle.
A plane wave
A spherical wave
Radius of each circular arc is c∆t
Wavelets
8. Dispersion
The refraction index depends on the wavelength of the light i.e. n( λ).
A light beam consists of rays of different wavelengths which will refracted at different
angles.
Dispersion is the spreading of light according to its wavelengths
λ
References
•“ Physics for Scientists & Engineers with Modern Physics ” by Serway & Jewett 2014,ch 35.
•“ Fundamental of Physics ” by Halliday, Resnick & Walker 2008 John Wiley & Sons, 8 ed.
•http://sst2011-s208sci.blogspot.com/2011/08/applications-of-total-internal.html
•http://www.physicsclassroom.com/class/refrn/Lesson-1/The-Direction-of-Bending
•http://cnx.org/content/m12895/latest/
•http://thewaythetruthandthelife.net/index/2_background/2-1_cosmological/2-1-12_childoverview/sir_isaac_newton.htm
•http://turningmirrors.com/22-halo/halo-plot-dispersion_ice