LIGHT AND
OPTICS
The Nature of Light and the
Principles of Ray Optics
OUTLINE
Historical Background
Geometric Optics
The Law of reflection
Refraction
Dispersion
HISTORICAL BACKGROUND
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“Light is comprised of colored particles.”
- technology on lenses, prisms, mirrors,
telescopes, microscopes and optical
(mirror/lens) polishing
“New Theory on Light and Color”
- light is a mixture of various colors having
different refractivity
“Light is a wave.”
- Discovered Saturn’s satellite Titan, the Saturn’s
ring, and the Great Nebula of Orion
- light reflection and refraction phenomenon
- became the mainstream scientific concept.
HISTORICAL BACKGROUND
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“Proof of the wave theory of light”
- showed that when light coming from a point light
source is shined onto two pinholes, interference
fringes can be observed on a screen an
appropriate distance away (Young’s Experiment)
and advocated his theory that light behaves like a
wave
“Predicted the existence of electromagnetic waves”
- electromagnetic waves propagate at the same speed as
light, and as horizontal waves
- light is a form of a high-frequency electromagnetic
wave.
HISTORICAL BACKGROUND
“Light is a photon”
- greatest physicist of the 20th century
- photoelectric effect theory
- theory of Brownian motion
- theory of special relativity
- photon has many mysterious physical
properties - the dual properties of a wave and a
particle
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“GEOMETRIC OPTICS”
-
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study of the propagation of light
light travels in a straight line in a fixed direction
“WAVEFRONT”
-
locus of all adjacent points at which the phase of vibration of a
physical quantity associated with the wave is the same
“RAYS”
- direction in which light propagates
- the path of the particles
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“TWO TYPES OF REFLECTION”
1. specular reflection – reflection on a smooth surface
2. diffuse reflection – reflection of light from a rough surface.
THE LAW OF REFLECTION
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• The principle when the light rays fall on the smooth
surface, the angle of reflection is equal to the angle of
incidence.
• The incident ray, the reflected ray, and the normal ray all
lie to the surface in the same plane.
Normal
• Retroreflection occurs when the reflected beam returns to
its source parallel to its original path.
CHECK YOUR UNDERSTANDING
1. Consider the diagram at the left.
• Which one of the angles (A, B, C, or D) is the angle of
incidence?
• Which one of the angles is the angle of reflection?
2. A ray of light is incident towards a plane mirror at an
angle of 30-degrees with the mirror surface. What will be
the angle of reflection?
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REFRACTION
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- the bending of light (it also happens with sound, water
and other waves) as it passes from one transparent
substance into another.
A straw is placed with a diagonal orientation within a half-filled beaker of water
REFRACTION
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- EM waves travels faster in a medium that has a lower index of
refraction.
- As the index of refraction of the medium increases, the velocity of
the wave decreases.
- Light travels fastest in vacuum
REFRACTION
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If a wave moves faster in a medium, it bends away from the
normal. If it travels slower, it bends towards the normal. (FAST)
The velocity of a wave is given by the equation
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PROBLEM
A light ray of wavelength 589 nm travelling through air is
incident on a smooth, flat slab of crown glass with
n1=1, n2 = 1.52 at an angle of 30° with the normal.
1. (A) Find the angle of refraction.
(B) What is the wavelength of this light in glass?
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SOLUTION
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Measuring n
using a Prism
For a prism with an apex angle Φ and minimum
angle of deviation δmin, the index of refraction is
determined using the formula;
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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 equal to the speed
of the waves in that medium
Huygen’s
Principle
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The index of refraction of a given material varies
depending on the wavelength of light passing
through the material. This behavior is called
dispersion.
Dispersion
A wave having shorter wavelength is more refracted
than a wave with longer wavelength.
When white light is passed through a prism,
dispersion occurs. The band of colors that emerge
is called the visible spectrum.
Total Internal
Reflection
Polarization
- happens when the angle of incidence is greater than the
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critical angle. The critical angle
required for total internal reflection is;
- determined by the direction of the electric field vector
of the wave.
- The intensity of an electromagnetic wave is
proportional to the square of the amplitude of the
wave.
Malus’s Law
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The light coming from the sun is absorbed and is
reradiated in a variety of direction. This process is
called scattering.
Polarization
Brewster’s law states that the reflected light is
completely polarized perpendicular to the plane of
incidence (parallel to the interface) if the angle of
incidence equals the polarizing angle, θp
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PROBLEM 2.
Find the critical angle for an air-water boundary.
The index of refraction of water is 1.33, while for air it
is approximately equal to 1.
Find the critical angle for an air-water boundary. 21
The index of refraction of water is 1.33, while for air it
is approximately equal to 1.
SOLUTION
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CONCEPT
CHECK 1
The transmission of light in optical fibers used in
modern communication systems is due to?
a. refraction
b. reflection
c. dispersion
d. reflection and refraction
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CONCEPT
CHECK 2
Total internal reflection occurs when the angle of
incidence is ______________ the critical angle.
a. equal
b. less than
c. greater than
d. at least equal to
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CONCEPT
CHECK 3
What phenomenon is responsible for the creation
of rainbow?
a. refraction
b. reflection
c. dispersion
d. refraction and dispersion
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CONCEPT
CHECK 4
The dependence of the index or refraction on the
wavelength of light is called?
A. aberration
B. dispersion
C. deviation
D. refraction
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CONCEPT
CHECK 5
Violet is the lowermost color in a rainbow. Why?
A. because it has the highest wavelength
B. because it has the highest frequency
C. because it has the lowest index of refraction
D. because it has the shortest wavelength
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CONCEPT
CHECK 6
In which of the following interface will total
internal reflection of light occur?
A. air to water
B. water to glass
C. glass to water
D. none of these
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PROBLEM 3.
A narrow beam of sodium yellow light, with
wavelength 589 nm in vacuum, is incident from air
onto a smooth water surface at an angle of incidence of
35.0°. Determine the angle of refraction and the
wavelength of the light in water.
A narrow beam of sodium yellow light, with wavelength 589 nm in vacuum,
29 of
is incident from air onto a smooth water surface at an angle of incidence
35.0°. Determine the angle of refraction and the wavelength of the light in
water.
SOLUTION