Phys Sci Week 13 - 14 Intro to Physics of Light

Next test: Week 15 Dec 19/21

Week 16 Dec 30 class at my home: 10 - 1 PMText: Module 15 pages 367 - 396Reading Assignments Module 15 Homework AssignmentModule 15 Study Guide Questions

Prep Questions 13:(1) What is a light?(2)  What is the difference between reflection, refraction and the absorption of light?

Prep Questions 14:(1) How does the human Eye work?(2)  How do we perceive color?

See Introduction (p 284)EM Waves

• Electromagnetic force is a fundamental force – James Maxwell proved the electro and magnetic forces were really

one force (p 284 -285)• Discovered and proved that electricity and magnetism were

caused/governed by the same force now called the electric force. – Due to the movement interactions of the charges associated with

electrons. – This force is also associated with the formation of electromagnetic

waves (forms of "light") • ALL waves excerpt for one (electromagnetic waves)

requires a medium - something to move through. • Two General Type of Waves (p 243)

– Transverse - wave that propagates perpendicular to its direction of oscillation.

– Longitudinal - waves that propagates parallel to its direction of oscillation.

• Compression - area of compression (higher pressure/greater density) - like crest

• Rarefaction - pulled apart lower pressure/lower density - like trough.

Electromagnetic waves: Light (EM) Waves are very unusual in that

the are transverse waves that require no medium.

The Dual Nature of Light• Photons and the Electromagnetic Force - The

Dual nature of Light– Electromagnetic Waves has a dual nature - it can act

as wave and a particle. We don't understand this because logically (deductive logic) a particle can't be a wave and a wave can't be a particle. A wave should be the movement of energy through some medium (bunch of particles). But light violates this rule in that it sometimes acts as a particle and sometimes act as wave.

• The Electromotive force can be viewed terms of particle called photons.– Particle theory of light : Photon - small packet of "light"

that acts like a particle.

• Wave theory of light: Light also acts as a wave.

Photons and EM Waves

Quantum Mechanical theory of light: - view of light as tiny packet of waves

• Speed of light 300,000,000 m/s (186,000 m/s or 670,000,000 mph) in a vacuum- does not depend upon temperature. Does depend on medium.

• Einstein's Special Theory tells us that the speed of light is the fastest anything with mass can

travel.

Substance Speed of Light

Substance Speed of Light

Air (25C) 300,000,000 m/s

Plastic 189,000,000 m/s

Alcohol 225,000,000 m/s

Crown Glass 185,000,000 m/s

Fresh Water 220,000,000 m/s

Flint Glass 175,000,000 m/s

Acrylic 220,000,000 m/s

Diamond 125,000,000 m/s

Wavelength and Frequency of Light• Wavelength and frequency of sound determine its pitch.• For light (EM waves) Wavelength and frequency

determine its energy and for visible light its color. – The shorter the wavelength, the higher the frequency the greater

the energy.– Review the EM spectrum to reinforce that we only see a small

part of the EM spectrum.• Radio - Microwaves • Infra red• Visible spectrum: Red - Yellow - Green - Violet• Ultra violet• X-rays, Gamma rays

WavespeedWavespeed

• Wavespeed = wavelength/frequency– For light wave speed (c) = constant– Because c is constant the frequency and

wavelength really tell us the same thing because one wavelength can be associated for a given frequency and vice versa.

• Sample problems: c = λ/f

Interaction of Light with Material Objects (Absorption, Reflection, Refraction and Scattering)

• Absorption: Light is absorbed by the substance increasing the energy of the material object. Gas, liquid and water

• Reflection: Light ray reflection is the turning back of the ray when it encounters the edge of a medium.

• Angle of Incidence: Angle of incidence is a measure of deviation of something from "straight on", for example in the approach of a ray to a surface it the angle from the vertical.

• Angle of Reflection: The angle between the reflected ray and the normal is known as the angle of reflection.

Law of Reflection: The angle of reflection equals the angle of incidence.

• Refraction: Refraction is the change in direction of a wave due to a change in its speed.

– This is most commonly observed when a wave passes from one medium to another. Refraction of light is the most commonly observed example, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth.

– Refraction is described by Snell's law, which states that the angle of incidence is related to the angle of refraction by

• or• • where• v1 and v2 are the wave velocities through the respective media. • θ1 and θ2 are the angles between the normal (to the interface)

plane and the incident waves respectively. • n1 and n2 are the refractive indices.

Two type of refractions:

1. Light slows down as it goes from one medium to another– Light bends up to line perpendicular to the surface.

Refraction of light at the interface between two media of different refractive indices, with n2 > n1. Since the phase velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in the higher-index medium(n2) is closer to the normal.

• 2. Light speeds up as it goes form one medium to another

- Light bends away from line perpendicular to the surface

Refraction of light at the interface between two media of different refractive indices, with n2 < n1. Since the phase velocity is lower in the second medium (v2 <v1), the angle of refraction θ2 is greater than the angle of incidence θ1; that is, the ray in the lower-index medium (n2) is further away from the normal.

Fiber Optics: Total Internal Fiber Optics: Total Internal ReflectionReflection

Light ScatteringLight Scattering• Scattering is a general physical process where radiation, such as light, sound, or moving particles, are

forced to deviate from a straight trajectory due to striking a particle

• Wavelength much smaller than particle: Rayleigh scattering (named after the English physicist Lord Rayleigh) is the uniform scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. Rayleigh scattering of sunlight in clear atmosphere is the main reason why the sky is blue: Rayleigh and cloud-mediated scattering contribute to diffuse light (direct light being sunrays).

• Wavelength about same size as the particle: This is called Mie scattering and results in a directional scattering of light cause the sky to appear white.

Lenses• A lens is an optical device with axial symmetry which transmits and refracts

light, converging or diverging the beam.– A simple lens is a lens consisting of a single optical element.– A compound lens is an array of simple lenses (elements) with a common axis;

• the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element.

• Manufactured lenses are typically made of glass or transparent plastic.• Elements which refract electromagnetic radiation outside the visual spectrum are also

called lenses: for instance, a microwave lens can be made from paraffin wax.

Converging Lens: • If the lens is -convex, a parallel beam of light travelling parallel to the lens

axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (known as the focal length). In this case, the lens is called a positive or converging lens.

Diverging Lens: • If the lens concave, a parallel beam of light passing through the lens is diverged

(spread); the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens

Other types of Lenses:

The Human Eye• The eye is an organ which reacts to light for several purpose that allows

vision.– Rods and cones in the retina allow conscious light perception and vision

including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.

• Photoreceptors - Rods, Cones and Photosensitive Ganglion: – A photoreceptorcell, is a specialized type of neuron (nerve cell) found in the eye's

retina that is capable of phototransduction (process by which light is converted into electrical signals sent on to the brain ).

– Rods and cones in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.

– Cones are adapted to detect colors, and function well in bright light; • In humans there are three different types of cone - responding respectively to short

(blue), medium (green) and long (yellow-red) light– Rods are more sensitive, but do not detect color well, being adapted for low light.

• The human retina contains about 120 million rod cells and 6 million cone cells– Photosensitive Ganglion Cells in the retina receive light signals which:

• Sets the body clock, • Adjustment of the size of the pupil and • Regulation and suppression of the hormone melatonin (helps us sleep)

The Human Eye

Perceiving Color• Color is based upon the frequency (or wavelength) of the light.

– We perceive color when the different wavelengths composing white light are selectively interfered with by matter (absorbed, reflected, refracted, scattered, or diffracted) on their way to our eyes, or when a non-white distribution of light has been emitted.

– We can detect the range of light spectrum from about 400 nanometers (violet) to about 700 nanometers (red). We perceive this range of light wavelengths as a smoothly varying rainbow of colors, otherwise known as the visual spectrum.

1000 nm100 nm

Light Interaction With Itself - Interference

• Interference: In physics, interference is the addition (superposition) of two or more waves that results in a new wave pattern. .

• Constructive and Destructive interference– Constructive Interference pattern reinforce

waves and produced bright bands – Destructive interference pattern cancel out

waves and produce dark bands (no light).

Constructive and Destructive interference

• Interference pattern produced with a Michelson interferometer. Bright bands are the result of constructive interference while the dark bands are the result of destructive interference.

• When two sinusoidal waves superimpose, the resulting waveform depends on the frequency (or wavelength) amplitude and relative phase of the two waves. – If the two waves have the same amplitude (A) and wavelength (λ) the resultant

waveform will have an amplitude between 0 and 2A depending on whether the two waves are in phase or out of phase.

– In phase: Consider two waves that are in phase, with amplitudes A1 and A2. Their troughs and peaks line up and the resultant wave will have amplitude A = A1 + A2. This is known as constructive interference.

– Out of phase: If the two waves are π radians, or 180°, out of phase, then one wave's crests will coincide with another wave's troughs and so will tend to cancel out. The resultant amplitude is A = |A1 − A2|. If A1 = A2, the resultant amplitude will be zero. This is known as destructive interference.

Electromagnetic Interference(EMI)

• Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a disturbance that interfers an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. – The disturbance may interrupt, obstruct, or otherwise degrade or limit

the effective performance of the circuit. – The source may be any object, artificial or natural, that carries rapidly

changing electrical currents, such as an electrical circuit, lightning, the Sun activity or the Northern Lights.

• EMI can be intentionally used for radio jamming, as in some forms of electronic warfare, or can occur unintentionally, as a result of spurious emissions of static. – It frequently affects the reception of AM radio in urban areas. – It can also affect cell phone, FM radio and television reception, although

to a lesser extent.

Adding and Subtraction Colors:

• Colors are perceived by either emitted or reflective light – Emitted light causes additive colors - they add together to create color

Visible light color: http://www.youtube.com/watch?v=5J8iEIQBlxc– Reflective light causes subtracted color - everything but the light we see

is absorbed or subtracted by object– The human eye can distinguish about 10 million different colors

• Primary Colors: Other colors can be made from either adding or subtraction these three colors

• Additive Primary Colors: Red, Green, Blue - used for generated light (example color TV)– Mixing all additive primary colors = white

• Subtractive Primary Colors: Yellow, Magenta, Cyan - used for reflected colors (paints) – Mixing all subtractive primary colors = black

Labs

• Lab 15.1 p 370 Wavelengths of Light– Other Light experiments

• Lab 15.2. p 374 Law of reflection– Fiber optic experiment Tyndall Bucket

• Lab 15.3 p 378Refraction

• Lab 15.4 p 380 Magical Quarter

• Lab 15.5. p 388 How the eye detects color