Ch 16

Section 1 Key Terms 1) Luminous Source – an object that emits light 2) Illuminated source – an object that is visible as light reflects off it. 3) Opaque – media that don’t transmit light but may reflect it. (ex: bricks…) 4) Transparent – media that transmits light. 5) Translucent – media that transmits light, but the object can not be seen clearly.

Ch 14 Characteristics of Light

The Electromagnetic spectrum (E-M)- the types of Energy from a star

Electromagnetic waves vary depending on f and λ (study chart page 521)  Radio waves λ > 30 cm (tv, am, fm)  Micro waves 30cm > λ > 1 mm (radar…)  Infrared 1 mm> λ > 700 nm ( photo….)

Electromagnetic waves vary depending on f and λ (study chart page 521)  Visible 700nm (red)> λ > 400nm (violet)  UV light 400 nm > λ > 60 nm (sterilize)  X- rays 60 nm > λ > nm (medical)  Gamma 0.1 nm > λ > nm ( cancer treatment, etc…

Telescopes- used to observe the different spectrum of stars  1a) VISIBLE SPECTRUM – the light that we see

Telescopes- used to observe the different spectrum of stars  1b) INFRARED SPECTRUM-  i ) Cool stars are RED  Ii) Hot stars are BLUE

1c) Other Spectrums i) X- RAY ii) UltraViolet (UV)

Important E-M Concepts:  ALL E-M waves move at the speed of Light (you need this for calculations!!)  Movement of E-M wave:  The Electric field oscillates perpendicular to E-M wave motion  The Magnetic field oscillates perpendicular to E-M wave & perpendicular to Electric field  Diagram page 521

λ = v/f Longest λShortest λ Shortest f Longest f

COLOR-wavelengths of visible light measured in nanometers (nm) *** 1 nm = m  Violet λ = 380 – 430 nm (shortest λ)  Blue λ = 450 – 500 nm  Green λ = 520 – 565 nm  Yellow λ = 565 – 590 nm  Orange λ = 590 – 625 nm  Red λ = 625 – 740 nm (Longest λ)

The speed of light: c = f λ

Measuring the speed of light 1)Roemer determined # of eclipses of Io during 6 th month period (=103) 2) Each eclipse was 13sec which yields 1.3 x 10 3 sec for 103 eclipses. 3) Earth’s orbital diameter = 2.9 x m 4) Use v = d/t to determine how fast light crosses Earth orbit  2.2 x 10 8 m/sec 5) Pretty good calculation, but in reality…

**Speed of light (c ) = 3.00 x 10 8 m/s (in a vaccuum)  Most famous : Michelson’s experiment…  Used rotating mirrors to measure the time it took for light to make a round trip between 2 mountains in CA.  Found c = x 10 8 m/s  Accepted value c = x 10 8 m/s  (so light travels 9.46 x km in a year = 1 light-year)

SPEED OF LIGHT KEY CONCEPTS 1. c = 3.00 x 10 8 m/s (in a vacuum) 2. c = f( λ) 3. c is CONSTANT. Same value throughout universe! 1. (What if it’s not?)

Practice problem 1.  Find the range of frequencies of blue light (wavelengths nm)  USE c = fλ  c = 3.00 x 10 8 m/s  All e-m waves travel at speed of light.

Practice problem 2.  Find the wavelength for the FM radio band 90.9 MHz. ( 1MHz = 10 6 Hz. All SI prefixes are in the back cover of the book)

3 models of light

Model 1: Ray model of light- (via Isaac Newton) 1) Light waves travel in a straight path 2) Path is only changed by contact with another object. 3) Straight lines are used to represent lights path.

BUT…(model 2: Wave model)  Light also behaves like a wave.  It has characteristics of a wave  Wavelength, frequency, speed, etc…

AND…(model 3: Particle model)  Light also behaves like a particle called photons (light particles)  Some characteristics :  Any single photon has a fixed, discrete energy level.  Each color has unique energy level.  not possible to Change the energy of that single photon without changing its wavelength  The intensity of visible light changed only by changing the number of photons present.

Our Presumptions of Light waves for our concepts we study  It is a point source  Can the sun be considered a point source. Why / why not?  It is far enough away from source  It moves out spherically from source.  It strikes surface perpendicular

It strikes surface perpendicular

illuminance- “related to how bright an object is” E = P 4πr 2 Find Illuminance at r = 2m E= 1750/ (4π(2) 2 ) E = lx Find Illuminance at r = 3m E= 1750/ (4π(3) 2 ) E = lx

What has occurred to the illuminance as distance from source is increased?  This is called the INVERSE SQUARE LAW- The illuminance is proportional to 1/r 2 The amount of light spreads out in ALL directions; therefore, the number of light rays available to illuminate a unit area decreases as square of distance.

Fewer direct rays at tilting away (hence SEASONS!!)

So, how do you increase illuminance (brightness?)  Brighter luminous source  Closer distance

The candle displays more light on the screen. For the bulb to have same illuminance (at given distance), it will need to be 4 x as bright. THEREFORE—have 4x the luminous intensity of the candle

Homework:  P 523 # 1-6  P 525 #1-4  Remember!! ALL electromagnetic waves move at the speed of light through space!

The wave nature of light  Dutch scientist Grimaldi noticed  Edge of shadows weren’t sharp  Shadow wider than it should be  Some colored bands on edge of shadows Known as Diffraction- the bending of light around a barrier

More terms… Luminous flux – the rate at which energy is emitted from a luminous source (how much light comes out in a certain time) SYMBOL (P) UNITS (lumen – lm) Illuminance – the rate at which light strikes a surface. SYMBOL (E)UNITS (lux - lx)

Equation for Illuminance of object from a point source (small source of light) E = P Illuminance at object = luminous flux 4πr 2 of light source, divided by the surface area of the sphere with radius equal to distance object is from source. E = illuminance (lux or lumens / square meter) P = luminous flux (lumens) r = distance of object from light source

The Illuminance equation is valid ONLY if Light source  is a POINT source (small enough) OR  Is far enough from source  Is spreading out spherically  And light is assumed to be striking object perpendicular to its surface  CAN the Sun be considered a POINT SOURCE? WHY? Or WHY NOT?

Example: Illuminance E = P 4πr 2 Find Illuminance P = 1750 lm, r = 1m E= 1750/ (4π(1) 2 ) E = lux OR E = lm/m 2

How to increase the illuminance of an object?? LUMINOUS INTENSITY…  Brighter luminous source  Closer distance  The amount of luminous flux (P) that falls on 1m2 of a 1m radius sphere  OR… P/ 4π  UNITS candela (cd)

DIFFRACTION-via Christiaan Huygens (p439) 1) The points of light become sources of new, smaller waves 2) They expand in all directions, but even with each other 3) The new wave front is parallel 4) Leads to alternating bands light & dark, OR different wavelengths of light (color)

16-2: The wave nature of Light Diffraction

Page 440  Primary Colors- Red, green, & blue. When 3 correct intensities of light blend to make WHITE  Secondary Colors- the combination of 2 primary colors produces yellow, cyan, and magenta  Complementary Colors- 2 colors of light that combine to form white (like yellow & blue)

Polarization of light- production of light in one plane of oscillation (Amplitude drops = lower intensity= dimmer with certain angles) Light passes through Parallel to polarizingperpendicular to Axis. Polarizing axis

Polarize by…  Filtering- as light passes through a filter (sunglasses) the molecules of filter cause less light to pass through.  Reflection- Partial polarization occurs due to angle of reflection.

Polarization (p444)

To determine Intensity of light coming through filter  Malus’ Law – explains reduction of light through a filter. I 2 = I 1 cos 2 θ I 1 Intensity of light coming to filter I 2 Intensity of light coming out of filter Θ Polarizing angle.

Doppler effect and light f obs = f ( 1 + v/c) use to determine frequency of light observed. 1 + v/c if moving toward 1 – v/c if moving away

Doppler effect and wavelength- to determine observed wavelength change Δλ =( λ obs - λ )= + v λ c Applications (of both equations) – Star spectra, sub-atomic particles

P446 # 14-17, 22, 23, p 452 #30, 36, 39, 47, 48  Other concept questions:  P452 # 40 – 52 (exclude 43, 44, 49-51)  Extra credit problems: (any- but due Friday)