AlMughtaribeen University College of Medicine Physics

Course Descriptions & Course Outlines :This Course cover a range of basic physics concepts and are useful as a supplement to an introductory physics course with medical applications. The course present the concepts of physics simply and covers the most important applications of physics to help medical students to understand and apply these concepts.

Physics course include : Measurements , Measurements Errors Units , Units conversation , Significant digits (2 Lectures ) .Light ,Laser and its applications (4 Lectures) , Sound ,Electromagnetic waves ( 2 lectures ) ,Fluid Mechanics in medicine (2 lectures ), Temperature &heat ( 1Lecture ) , Electric charge & current (1 Lecture) , Nuclear physics & radio activity (2 Lectures) , concept of Nanotechnology (1 Lectures ) . Mr.Gazy Khatmi Email _ gazy@mu.edu.sd AlMughtaribeen University - College of Medicine

1May 1, 2023

Light

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Introduction : Why Light ??! These days light is frequently used for all sorts of therapies and

medical applications, from cancer diagnosis and treatment to monitoring of oxygen in the blood.

Biophotonics is a relatively new scientific discipline which is developing applications for light and lasers in the life sciences - for clinical diagnostics and therapy, and even semi-automated diagnostic systems for doctors and nurses.

Biophotonics also deals with the prevention of diseases; light

can be used for precision monitoring of our environment as well as assessing the quality of food. 

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The Nature of LightBefore the beginning of the nineteenth century, light was considered to be a stream of particles.The particles were either emitted by the object being viewed or emanated.Newton was the chief architect of the particle theory of light.Huygens argued that light might be some sort of a wave motion.Thomas Young (in 1801) provided the first clear demonstration of the wave nature of light.

He showed that light rays interfere with each other. Such behavior could not be explained by particles

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Confirmation of Wave NatureDuring the nineteenth century, other developments led to the general acceptance of the wave theory of light.Thomas Young provided evidence that light rays interfere with one another according to the principle of superposition.

This behavior could not be explained by a particle theory.Maxwell asserted that light was a form of high-frequency electromagnetic wave.Hertz confirmed Maxwell’s predictions.

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Particle NatureSome experiments could not be explained by the wave model of light.The photoelectric effect was a major phenomenon not explained by waves.

When light strikes a metal surface, electrons are sometimes ejected from the surface.

The kinetic energy of the ejected electron is independent of the frequency of the light.

Einstein (in 1905) proposed an explanation of the photoelectric effect that used the idea of quantization.

The quantization model assumes that the energy of a light wave is present in particles called photons.

E = hƒ

h is Planck’s Constant and = 6.63 x 10-34 J.s

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dual nature:wave vs.particle Wave: continuously changing force fields

energy travels as sine WAVE

macroscopic level

Particle: photon or quanta small packet of energy

acting as a PARTICLE microscopic level

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Dual Nature of EM Radiation

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Dual Nature of LightIn view of these developments, light must be regarded as having a dual nature.Light exhibits the characteristics of a wave in some situations and the characteristics of a particle in other situations.This chapter investigates the wave nature of light

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Ray ApproximationThe rays are straight lines perpendicular to the wave fronts.With the ray approximation, we assume that a wave moving through a medium travels in a straight line in the direction of its rays.

Section 35.3 9May 1, 2023

Ray Approximation, cont.If a wave meets a barrier, with λ<<d, the wave emerging from the opening continues to move in a straight line.

d is the diameter of the opening.

There may be some small edge effects.

This approximation is good for the study of mirrors, lenses, prisms, etc.Other effects occur for openings of other sizes.

See fig. 35.4 b and c

Section 35.3 10May 1, 2023

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Reflection of LightA ray of light, the incident ray, travels in a medium.When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium.

This means it is directed backward into the first medium.

For light waves traveling in three-dimensional space, the reflected light can be in directions different from the direction of the incident rays.

Section 35.4 12May 1, 2023

Specular ReflectionSpecular reflection is reflection from a smooth surface.The reflected rays are parallel to each other.All reflection in this text is assumed to be specular.

Section 35.4 13May 1, 2023

Diffuse ReflectionDiffuse reflection is reflection from a rough surface.The reflected rays travel in a variety of directions.A surface behaves as a smooth surface as long as the surface variations are much smaller than the wavelength of the light.

Section 35.4 14May 1, 2023

Law of ReflectionThe normal is a line perpendicular to the surface.

It is at the point where the incident ray strikes the surface.

The incident ray makes an angle of θ1 with the normal.The reflected ray makes an angle of θ1’ with the normal.

Section 35.4 15May 1, 2023

Law of Reflection, cont. The angle of reflection is equal to the angle of incidence. θ1’= θ1

This relationship is called the Law of Reflection.The incident ray, the reflected ray and the normal are all in the same plane.

Section 35.4 16May 1, 2023

Refraction of LightWhen a ray of light traveling through a transparent medium encounters a boundary leading into another transparent medium, part of the energy is reflected and part enters the second medium.The ray that enters the second medium changes its direction of propagation at the boundary.

This bending of the ray is called refraction.

Section 35.5 17May 1, 2023

Refraction, cont.The incident ray, the reflected ray, the refracted ray, and the normal all lie on the same plane. The angle of refraction depends upon the material and the angle of incidence.

v1 is the speed of the light in the first medium and v2 is its speed in the second.

2 2

1 1

sin sin

θ vθ v

Section 35.5 18May 1, 2023

Refraction of Light, finalThe path of the light through the refracting surface is reversible.

For example, a ray travels from A to B.

If the ray originated at B, it would follow the line AB to reach point A.

Section 35.5 19May 1, 2023

Refraction Details, 1Light may refract into a material where its speed is lower.The angle of refraction is less than the angle of incidence.

The ray bends toward the normal.

Section 35.5 20May 1, 2023

Refraction Details, 2Light may refract into a material where its speed is higher.The angle of refraction is greater than the angle of incidence.

The ray bends away from the normal.

Section 35.5 21May 1, 2023

Light in a MediumThe light enters from the left.The light may encounter an electron.The electron may absorb the light, oscillate, and reradiate the light.The absorption and radiation cause the average speed of the light moving through the material to decrease.

Section 35.5 22May 1, 2023

The Index of RefractionThe speed of light in any material is less than its speed in vacuum.The index of refraction, n, of a medium can be defined as

speed of light in a vacuum cnspeed of light in a medium v

Section 35.5May 1, 2023 23

Frequency Between MediaAs light travels from one medium to another, its frequency does not change.Both the wave speed and the wavelength do change.The wavefronts do not pile up, nor are they created or destroyed at the boundary, so ƒ must stay the same.

Section 35.5May 1, 2023 24

Index of Refraction ExtendedThe frequency stays the same as the wave travels from one medium to the other.

v = ƒλ ƒ1 = ƒ2 but v1 v2 so λ1 λ2

The ratio of the indices of refraction of the two media can be expressed as various ratios.The index of refraction is inversely proportional to the wave speed.

As the wave speed decreases, the index of refraction increases.The higher the index of refraction, the more it slows downs the light wave speed.

Section 35.5May 1, 2023 25

Fiber Optics

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Total Internal Reflection

A phenomenon called total internal reflection can occur when light is directed from a medium having a given index of refraction toward one having a lower index of refraction.

Section 35.8May 1, 2023 27

Critical AngleThere is a particular angle of incidence that will result in an angle of refraction of 90°.

This angle of incidence is called the critical angle, θC.

For angles of incidence greater than the critical angle, the beam is entirely reflected at the boundary.

This ray obeys the law of reflection at the boundary.

Total internal reflection occurs only when light is directed from a medium of a given index of refraction toward a medium of lower index of refraction.

21 2

1

sin (for )Cnθ n nn

Section 35.8May 1, 2023 28

Acceptance angle Having considered the propagation of light in an optical

fiber through total internal reflection at the core–cladding interface, it is useful to enlarge upon the geometric optics approach with reference to light rays entering the fiber. Since only rays with a sufficientlyshallow grazing angle (i.e. with an angle to the normal greater thanφc) at the core–cladding interface are transmitted by total internal reflection, it is clear that not all rays entering the fiber core will continue to be propagated down its length

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Numerical aperture

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Cont,

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Equation above, apart from relating the acceptance angle to the refractive indices, serves as the basis for the definition of the important optical fiber parameter, the numerical aperture (NA). Hence the NA is defined as

May 1, 2023

Example 1

A silica optical fiber with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and a cladding refractive index of 1.47.Determine: (a) the critical angle at the core–cladding interface; (b) the NA for the fiber; (c) the acceptance angle in air for the fiber.

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Attenuation

Attenuation is defined as the ratio of optical output power to the input power in the fiber of length L.

α= 10log10 Pi/Po [in db/km]

where, Pi= Input Power Po= Output Power, α is attenuation constant

The various losses in the cable are due to Absorption Scattering Dispersion Bending

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