Ch 2 OpticsContents

1- The Nature of Light

2 Reflection and Refraction

3 The Law of Refraction

4 Total Internal Reflection, Binocular, Optical Fiber

5 Dispersion and Prisms, Color Matching

6 Eye vision

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Objectives

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• 1- Understand the nature, origin and sources of light

• 2-Use the laws of Reflection and Refraction to understand some optical devices

• 3- Discuss the theory of operation of some medical instruments.

• 4- Understand light dispersion and the theory of Color Matching

•

Nature of Light Theories

• Light was though to be a stream of particles (Corpuscular theory)

• James Clerk Maxwell (1831-1879) developed the

electromagnetic theory and pronounced that light is a form of high frequency electromagnetic wave

• Max Planck (1858-1947) put forward the quantum theory of light (light is emitted in the form of photons)

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What is the light?

Waves

Particles

Is it?

Or

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Properties of light - two models

Light ray model• Particle-like view• Photons travel in

straight lines• Applications

– Mirrors– Prisms– Lenses

Wave model• Traces motions of wave

fronts• Best explains

– Interference– Diffraction– Polarization

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Light is an electromagnetic wave.

• The electric (E) and magnetic (B) fields are in phase.

• The electric field, the magnetic field, and the propagation direction are all perpendicular.

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Waves can interfere.

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Light is not only a wave, but also a particle.

• Photographs taken in dimmer light look grainier.

• When we detect very weak light, we find that it’s made up of particles. We call them photons. April 19, 2023 8W3

Where does light come from?

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Sources of light

Accelerating charges emit light

B

1- Linearly accelerating charge

2-Synchrotron radiation—light emitted by charged particles deflected by a magnetic field

3-Bremsstrahlung (Braking radiation)— light emitted when charged particles collide with other charged particles

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4-But the vast majority of light in the universe comes from molecular vibrations emitting light.

•Electrons vibrate in their motion around nuclei High frequency: ~1014 - 1017 cycles per second.

•Nuclei in molecules vibrate with respect to each other Intermediate frequency: ~1011 - 1013 cycles per second.

•Nuclei in molecules rotate Low frequency: ~109 - 1010 cycles per second.April 19, 2023 11W3

Polarized and unpolarized media

Unpolarized medium Polarized medium

On the right, the displacements of the charges are correlated, so it is polarized at any given time (and its polarization is oscillating).

Note that matter’s polarization is analogous to the polarization of light. Indeed, it will cause the emission of light with the same polarization direction.

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Sources of light • Matter constantly emits

and absorbs radiation• Emission mechanism

– Accelerated, oscillating charges produce electromagnetic waves

• Absorption mechanism– Oscillating

electromagnetic waves accelerate charges within matter

• Different accelerations lead to different frequencies

• Luminous – Producing light– The Sun versus the

nonluminous Moon

• Incandescent – Glowing with visible light

from high temperatures – Examples: flames,

incandescent light bulbs

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The Interaction of Light and Matter:

•The interaction of light and matter is what makes life interesting.

•Everything we see is the result of this interaction.

•Why is light absorbed or transmitted by a particular medium?

•Light causes matter to vibrate. Matter in turn emits light, which interferes with the original light. •Traces motions of wave fronts•Best explains

– Interference– Diffraction– PolarizationApril 19, 2023 14W3

Light interacts with matter

• Interaction begins at surface and depends on– Smoothness of surface– Nature of the material– Angle of incidence

• Possible interactions– Absorption and

transmission– Reflection– Refraction

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Geometrical Optics: Study of reflection and refraction of light from

surfaces using the ray approximation.

1-The ray approximation states that:light travels in straight lines until it is reflected or refracted and then travels in straight lines again.

2-The wavelength of light must be small compared to the size of the objects or else diffractive effects occur.

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Reflection details

• Angles measured with respect to the “surface normal”– Line perpendicular to

the surface

• Law of reflection

i = r

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Light refraction

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Experiment shows that the path of a light ray through a refracting surface isreversible. For example, the ray in Figure a travels from point A to point B. If the ray originated at B, it would follow the same path to reach point A, but the reflected ray would be in the glass.

Refraction

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The Fundamental Law

2211 sinsin nn

1

2

n1

n2

Snell’s Law

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Sin 0=0Sin 90= 1

n2

n1θC

P

θ1θ1

θ1 θ1

θ2

θ2

Critical Angle

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Total internal reflection

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212

1

21

0

21

,sin

sin

1sin90

sinsin

nnforn

n

nn

at

nn

c

i

rr

ri

Refraction, cont. • Critical angle

– Light refracted parallel to surface

– No light passes through surface - “total internal reflection”

– Applications - fiber optics, gemstone brilliance

Substance Index of refraction

Light speed

Air Approx. 1 ~c

Water 1.333 0.75c

Glass 1.5 0.67c

Diamond 2.4 0.42c

BE condensate

18,000,000 38 mph!

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Optical Instruments Binoculars

Many optical instruments, such as binoculars, periscopes, and telescopes, use glass prisms and total internal reflection to turn a beam of light through 90° or 180°.

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AssignmentWrite a short report on Binoculars taking into consideration the following points1- Optical phenomena2- Structure

Fiber Optics

Light can travel with little loss in a curved optical fiber because the light is totally reflected whenever it strikes the core-cladding interface and because the absorption of light by the core itself is small.

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Optical Fiber Structure

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nc

ncore

ncore > nc

Utilizations

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Physicians often use fiber-optic cables to aid in the diagnosis and correction of certain medical problems without the intrusion of major surgery. For example, a fiber-optic cable can be threaded through the esophagus and into the stomach to look for ulcers. In this application, the cable consists of two fiber-optic lines: one to transmit a beam of light into the stomach for illumination and the other to allow the light to be transmitted out of the stomach.

Example-EndoscopyIn the field of medicine, optical fiber cables have had extraordinary impact. In the practice of endoscopy, for instance, a device called an endoscope is used to peer inside the body.

A colonoscope reveals a polyp (red)

attached to the wall of the colon. A bronchoscope is being used to look

for signs of pulmonary disease.

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Arthroscopic SurgeryOptical fibers have made arthroscopic surgery possible, such as the repair of a damaged knee shown in this photograph:

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Answer the following question

•  

• Are there applications in dentistry

•  

• Justify your answer

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Light Refraction

• Here h and h”

• are heights of

• the body and

• its imageApril 19, 2023 W3 31

Thin LensesThin Lenses

Thin Lenses

• The magnification of a thin lens is

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• Combine definition of focal length with lensmaker’s equation

• If f > 0, we have a converging lens

• If f < 0, we have a diverging lens

• What if f = infinity?

Power of a Lens

• The power of a lens in diopters is the inverse of its focal length

P = 1/f

diopters 5.2 m 40.0

diopters 0.5 m 20.0

Pf

Pf

Exercises

• Calculate image positions, si

• Calculate image heights, hi, for (a) and (b)

• Calculate image length for (c)

• Are images real or virtual, upright or inverted, and reduced or enlarged

Fig. 17-24, p. 538

Image Reconstruction (Lens)1. Rays parallel to the optical axis, go through the focal point.

2. Rays through the focal point, emerge parallel to the optical axis.

3. Rays through the center of a lens or through the center of curvature of a mirror are undeviated.

Optical Axis

The eye

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Vision

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The near point is the closest distance for which the lens can accommodate to focus light on the retina.

Typically, the near point of the eye is at age 10 about 18 cm. at age 20 about 25 cm,at age 40, 50 cmat age 60. 500 cm or greater

The far point of the eye represents the farthest distance for which the lens of the relaxed eye can focus light on the retinaA person with normal vision is able to see very distant objects, such as the Moon, and so has a far point at infinity.

Farsightedness (or hyperopia)

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Nearsightedness (or myopia)

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Assignment

• Write a report, not more than 2 bages about eye vision including the following points

• 1-lens system of the eye

• 2-Accomodation

• 3-Resolving power of the eye

• 4-Retina structure and function

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Dispersion and colors• White light

– Mixture of colors in sunlight– Separated with a prism

• Dispersion– Index of refraction varies with

wavelength– Different wavelengths refract at

different angles– Violet refracted most (blue sky)– Red refracted least (red sunsets)– Example: rainbows

• Wavelength/frequency related

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Dispersion of the refractive index allows prisms to separate white light into its components and to measure the wavelength of light.

Dispersion is the tendency of optical properties to depend on wavelength.

Dispersion can be good or bad, depending on what you’d like to do.

Dispersive element

White light

Dispersed beam

n()

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Color Matching•  To understand color you must appreciate the

three dimensional nature of color.

• The dimensions of color are:

• HUE(color) is commonly referred to as color, E.g. Blue, Orange etc. It is associated with the wavelength of the light received.

• VALUE (brightness) is how we tell a light hue from a dark one.

• CHROMA (saturation) is the intensity or saturation of a hue.

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Hue -colors

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VALUE (brightness)

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Saturation

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Additive primary colors theory

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1 Red + 1 Blue = Magenta

1 Blue + 1 Green = Cyan

1 Green + 1 Red = Yellow

1 Red + 1 Blue + 1 Green = White

2 Red + 1 Green = Orange

2 Green + 1 Red = Lime

1 Blue + 1 Green + 4 Red = Brown

Important

•A good site to visit

• http://www.dentalxp.com/

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Assignment

• Solve the following problems

• 1-4-7-10-13-15-18

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