Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 4

Textbook Reading & Our Plans 2 Read Chapter #1 Reading is necessary to do well in solving HW and exam problems; Today – finish material of Chapter #1; Next class – Chapter #2

When does an electric field exert a force on a charge? A.Always B.Sometimes (changing field) C.Sometimes (depends on the charge) D.Sometimes (depends on many things) E.Something else

Speed, wavelength, frequency Speed = “c” in a vacuum Frequency, or oscillations per second (Hertz). f Wavelength (lambda), or how wide each oscillation is (meters) Units: m/s Either wavelength or frequency will tell you the color of light v = f λ

Clicker questions Which of the light waves has the longest wavelength? Which of the light waves is brightest? Which of the light waves has the highest speed in empty space? a)b) c) d)They all have the same speed A b) c) B C

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Let’s practice: Electromagnetic spectrum sheet v = f λ

Clicker Question 8 An FM radio station transmits at a frequency of: f = 100 MHz = 10 8 Hz then the wavelength is :  m B) 0.3 m C) 3 m D) 100 m E) None of these. c =  f f = c /  = c / f = [3 x 10 8 m/s] / [10 8 1/s] = 3 m

Clicker Question 9 Your microwave oven operates at a frequency of: f = 3 GHz = 3x10 9 Hz then the wavelength is :  cm B) 0.3 cm C) 3 cm D) 30 cm E) None of these. c =  f f = c /  = c / f = [3 x 10 8 m/s] / [3x10 9 /s] = 0.1 m Or 10cm

How does an ordinary incandescent light bulb work? Electrode leading to one side of the wall plug Electrode leading to the other side of the wall plug Filament with current of electrons which hitinto atoms causing light to be emitted Gas Atom Electron s Light emitted Atom

The hotter the source the more bluish the light. The cooler the source the more reddish the light These are curves of the intensity of each of the wavelengths present in an "off-white" light as the temperature of the filament in the bulb increases. These are curves of the intensity of each of the wavelengths present in an "off-white" light as the temperature of the filament in the bulb increases. We see only some of the emitted light:

Fluorescent lamp (based on fluorescent mineral coating) Fluorescent coating absorbs UV photons from mercury gas discharge and emits visible light photons with high efficiency. Calcium fluoride is fluorescent. 12 Lifetime: 8-15 times longer than incandescent Energy used: 3 – 5 times less Cost: 3 – 10 times more Demo: fluorescent minerals

13 Sources of light Flame (atomic emission) Incandescent Gas discharge (neon or mercury atomic emission) Fluorescent (mercury gas discharge + phosphor) Plasma TV (phosphors have color) Light emitting diode (LED) Laser All sources involve moving electrons !

Color temperature of "white" can be understood by mixing just three lights of different intensities IntensityIntensity IntensityIntensity Off-white in center is slightlyreddish. Low color tempOff-white in center is slightlyreddish. Low color temp Off-white in center is slightlybluish. High color tempOff-white in center is slightlybluish. High color temp

So now we can understand these different images as being due to different wavelengths and frequencies of light emitted and detected From small to large WAVELENGTH

18 How big is 500 nm? 500 nm = 0.5  m Human hair is 17 – 180  m in diameter. A 50  m hair is 100 wavelengths. There are 2000 wavelengths of green light per millimeter. A “light” microscope cannot see things smaller than about 0.5  m.

A prism spreads out the over- lapping wavelengths in white light into different spatial locations where they can be seen as colors.

We see color when waves of different wavelengths enter enter our eyes! Light with wavelength of 650 nm appears red when it enters a viewers eye Light with wavelength of 520 nm appears green when it enters a viewers eye Light with wavelength of 470 nm appears blue when it enters a viewers eye The speed of light in empty space is the same for all wavelengths

Clicker question What does Alex see when the wave at left with wavelength 650 nm goes by him? a)Red b)Blue c)Green d)White e)Nothing

What happens when two or more waves with different wavelengths reach your eye? Light with both wavelengths 650 nm and 520 nm appears yellow when it enters a viewers eye Light with only wavelength 580 nm ALSO appears yellow when it enters a viewers eye (A DEEPER YELLOW THAN FOR THE CASE ABOVE)

What is white light? Light which is a mixture of 650, 520 and 470 nm wavelengths (and possibly more wavelengths) appears WHITE when it reaches your eye No single wavelength (mono- chromatic) wave appears white when it reaches your eye!

26 Important to remember: How vision works Light from a source travels to the eye, OR Light from a source is reflected or scattered toward the eye. The sun is self-luminous. The moon shines by scattered light. Reflection in a mirror is a kind of scattering with a special direction (more later). Surface scattering: how we see pages of a book. Light leaves in all directions. Volume scattering: how we see fog, clouds, blue sky. Transparent: no scattering.

Different wavelengths scatter differently from really tiny particles Blue and green scatter more strongly – They have a small wavelength, so are better scattered by small particles smaller than the wavelength of light, so blue light comes at us from everywhere overhead. Sunset (“Raleigh scattering”)

So why does the sunset part look red? All the blue has been removed from scattering in the looong distance that the light has to travel from the horizon, leaving red behind

What can happen to incoming light Transmitted Through something opaque Reflected (including scattering) From something opaque

Opacity, transparency, and reflectivity depend on the light’s wavelength Infrared camera

31 Opacity of atmosphere

32 Transmission of glass Opacity is the opposite of transmission. 0% transmitting is 100% opaque. IR is blocked. UV is blocked. No sunburn indoors.

33 For THz radiation, skin is opaque and clothing is transmitting. For x-ray radiation, bones are opaque and skin is transmitting. Terahertz

34 Sunlight and the earth Sunlight is constant at 1000 W/m 2 (UV and x-ray output varies) Earth’s temperature is a balance between sunlight in and infrared radiation out Carbon dioxide blocks infrared going out and has a warming effect.

35 Properties of light (Ch. 1) 1. Light travels in vacuum. Sound travels in air (no sound in vacuum). 2. Light carries energy. (Sunlight warms, generates electricity.) 3. Light moves with a particular speed in vacuum, but moves less rapidly in other materials (water, glass). 4. Light travels in vacuum in straight lines (rays). Rays can be bent by materials. 5. Light has amplitude (intensity).

Review (Chapter 1) Light is an electromagnetic wave EM waves have both traveling electric and magnetic fields EM waves are created by accelerating charge. EM waves can make other charges move. EM waves travel outward like waves in a pond, with electric and magnetic vectors perpendicular to the direction of motion. EM waves carry energy 36

Review (Chapter 1) c = 300,000 km/s =3 x 10 8 m/s c = f  units are Hz for f and meters for wavelength of light spans 400 – 700 nm (blue to red) Atmosphere transmits light and radio, is opaque to UV, x-rays, some IR Amplitude of a wave is the half-height Wavelength is distance from crest to crest Period is the time for the wave to go up and down once at some location. 37