I Love Lucy Airs for First Time (1951) READING: reread chapter 7 HOMEWORK – DUE TUESDAY 10/20/15

HW-BW 7.1 (Bookwork) CH 7 #’s 5, 7-12 all, 14, 15, 20, 21, 24, 28-31 all, 34 HW-WS 12 (Worksheet) (from course website)

HOMEWORK – DUE THURSDAY 10/22/15 HW-BW 7.2 (Bookwork) CH 7 #’s 39, 42, 48-52 all, 55-60 all, 64, 69, 71, 72,

78, 90 HW-WS 13 (Worksheet) (from course website)

Lab Next Monday/Tuesday – EXP 9

Prelab

Next Wednesday/Thursday – EXP 10

The Nature of Light: Its Wave NatureLight is a form of electromagnetic radiation

made of perpendicular waves, one for the electric field and one for the magnetic field

Light is a form of electromagnetic radiationmade of perpendicular waves, one for the electric field

and one for the magnetic fieldAll electromagnetic waves move through space at

the same, constant speed2.998 x 108 m/s in a vacuum = the speed of light, c

The Nature of Light: Its Wave Nature

Characterizing WavesThe amplitude is the height of the wave

the distance from node to crest or node to trough

Characterizing Waves

Node

Characterizing WavesThe amplitude is the height of the wave

the distance from node to crest or node to troughthe amplitude is a measure of how intense the light is –

the larger the amplitude, the brighter the light

Characterizing Waves

Characterizing WavesThe wavelength (l) is a measure of the distance

covered by the wavethe distance from one crest to the next (or the

distance from one trough to the next, or the distance between alternate nodes)

Characterizing Waves

Node

Characterizing WavesThe wavelength (l) is a measure of the distance

covered by the wavethe distance from one crest to the next (or the

distance from one trough to the next, or the distance between alternate nodes)

For visible light, the wavelength is related to the color of light

Characterizing Waves

The frequency (n) is the number of waves that pass a point in a given period of timethe number of waves = number of cyclesunits are hertz (Hz) or cycles/second = s−1

1 Hz = 1 s−1

Characterizing Waves

LIGHT!!!wavelength and frequency are INVERSLY proportional

wavelength frequency

LIGHT!!!wavelength and energy are INVERSLY proportional

wavelength energy

LIGHT!!!energy and frequency are DIRECTLY proportional

energy frequency

Wavelength and FrequencyWavelength and frequency of electromagnetic

waves are inversely proportional because the speed of light is constant, if we know

wavelength we can find the frequency, and vice versa

c

Calculate the wavelength of red light (nm) with a frequency of 4.62 x 1014 s−1

649 nm

8

14 92.998 10 1 1

1 4.62 10 1 10

m n s m

s m

Calculate the wavelength (m) of a radio signal with a frequency of 106.5 MHz

2.815 m

8 12.998 101 ms

Hz11 s

1 MHz61 10 Hz

1106.5

MHz

Color The color of light is determined by its

wavelength or frequency White light is a mixture of all the

colors of visible light a spectrum RedOrangeYellowGreenBlueViolet

When an object absorbs some of the wavelengths of white light and reflects others, it appears colored the observed color is predominantly

the colors reflected

Types of Electromagnetic Radiation

low frequency and energy

high frequency and energy

• Electromagnetic waves are classified by their wavelengthRadio waves = > 0.01 m

4 2Microwaves = 1 10 m < < 1 10 m Infrared (IR)

5 4far IR = 1 10 m < < 1 10 m 6 5middle IR = 1 10 m < < 1 10 m

7 6near IR = 1 10 m < < 1 10 m 7 7Visible light = 4 10 m < < 8 10 m

.YO GR BIV

Ultraviolet (UV)7 7near UV = 2 10 m < < 4 10 m

8 7far UV = 1 10 m < < 2 10 m 10 8X-rays = 1 10 m < < 1 10 m

10Gamma rays = < 1 10 m

Electromagnetic Spectrum

InterferenceThe interaction between waves is called

interferenceWhen waves interact so that they add to make a

larger wave it is called constructive interferencewaves are in-phase

InterferenceThe interaction between waves is called

interferenceWhen waves interact so they cancel each other it is

called destructive interferencewaves are out-of-phase

Diffraction When traveling waves encounter an obstacle or opening in

a barrier that is about the same size as the wavelength, they bend around it – this is called diffraction traveling particles do not diffract

Diffraction When traveling waves encounter an obstacle or opening in a

barrier that is about the same size as the wavelength, they bend around it – this is called diffraction traveling particles do not diffract

The diffraction of light through two slits separated by a distance comparable to the wavelength results in an interference pattern of the diffracted waves An interference pattern is a characteristic of all light waves

2-Slit Interference

https://www.youtube.com/watch?v=hRFQd_fkzws

The Photoelectric Effect Many metals emit electrons when a light shines on them.

called the photoelectric effect

The Photoelectric Effect

The Photoelectric Effect Many metals emit electrons when a light shines on them.

called the photoelectric effect Classic wave theory said this effect was due to the light

energy being transferred to the electron. The energy of a wave is directly proportional to its amplitude and

its frequency If the wavelength of light is made shorter, more electrons should be

ejectedLight waves’ intensity made brighter, more electrons should be ejected

Predicts that if a dim light were used there would be a lag time before electrons were emitted to give the electrons time to absorb enough energy

Experiments showed that a minimum frequency was needed before electrons would be emittedcalled the threshold frequencyno dependence on intensity

It was observed that high-frequency light from a dim source caused electron emission without any lag time

The Photoelectric Effect: The Problem

chE h

Einstein’s ExplanationEinstein proposed that the light energy was

delivered to the atoms in packets, called quanta or photonsThe energy of a photon of light is directly proportional to

its frequency and inversely to wavelengththe proportionality constant is called Planck’s Constant,

(h) and has the value

cE h

h

346.626 10 J sphoton

h cE h

3 92

34 8

1 1 10 1 1 103.83 3.37 10

6.626 10 1 3.00 10 1

nmnm

J

J

photonmJ

m

m

J m

s

s

Calculate the number of photons in a laser pulse with wavelength 337 nm and total energy 3.83 mJ

6.49x1015 photons

What is the frequency of radiation required to supply 1.0 x 102 J of energy from 8.5 x 1027 photons?

2

34 27

1 1.0 10 1

6.626 10 1 8.5 10

photo n

ph

J

s nJ oto

1.8x107 s-1 or 1.8x107 Hz or 18 MHz

Ejected ElectronsOne photon at the threshold frequency gives the

electron just enough energy for it to escape the atombinding energy, f

When irradiated with a shorter wavelength photon, the electron absorbs more energy than is necessary to escape

This excess energy becomes kinetic energy of the ejected electron

Kinetic Energy = Ephoton – Ebinding

KE = hn − f