Physics 1230 Light and Color

Transcription

1 Physics 1230 Light and Color phys1230_sm15/ Dr. Ka'e Hinko Office: JILA A502

2 Ques'ons (5 min) Finish Module 7 Agenda, Day 18: Ac'vity 17: Spectra (25 min) Scien'fic Nota'on (20 min) Start Module 8 (30 min) R&E #5 is due now Homework #8 is due Thursday Quiz 3 is on Friday Modules 6 and 7

3 10 Class Modules Each equal to about one week in regular term 1. How does light travel? 2. How do we see things that don t emit light? 3. Does light always travel in a straight line? 4. How do lenses make things look different than they are? 5. How does our eye see light? 6. How does our brain process light? 7. How is yellow between red and blue? 8. Why is there no pink light? 9. What use is light we can t see? 10. How does a laser work? Or, is light really a wave?

4 WHAT IS COLOR?

5 What is color? Color is our brain s interpretation of light of different wavelengths/frequencies entering our eyes Which of these has the highest frequency? A. Red Light with wavelength of 650 nm appears red when it enters a viewers eye Light with wavelength B. Green of 520 nm appears green when it enters a viewers eye C. Blue Light with wavelength of 470 nm appears blue when it enters a viewers eye

6 So this is how yellow is between yellow and blue Yellow has a larger wavelength than blue, smaller than red. Red = 700 nanometers (nm) Yellow = 600 nm Violet = 400 nm ROYGBIV

7 This is why dispersion (refrac'on of different colors) results in this ordering of the colors Red, orange, yellow, green, blue, violet have different wavelengths. These wavefronts bend at the interface. (Remember our car analogy). The short wavelength of blue bends more.

8 Diffrac'on can separate colors, because the amount that light is diffracted depends on its wavelength If monochroma'c (one color) we see dark and light bands instead. How many colors do you see through the gra'ng?

9 Ac'vity 17: Spectra

10 Incandescent light bulbs Filament with current of electrons which hit into atoms causing light to be emitted! Atom! Electrons! Atom! Atom! Light emitted at many different resonance frequencies of atoms appears as white light!

11 Incandescent light bulbs A cononuous light source Almost 90% of its emission is invisible to the human eye, producing heat and was'ng energy

12 Fluorescent light bulbs Fluorescent bulbs have a lower current and power usage for the same light output in the visible range. How do they do this?

13 Fluorescent light bulbs The atoms inside a fluorescent bulb only wiggle in the ultraviolet range (invisible, high energy light). Atom! Electrons! Atom! Atom! Phosphors! Invisible ultra-violet light! white light! This makes phosphors on the coating of the bulb glow

14 Fluorescent light bulbs Because the phosphors emit at very specific resonant frequencies, the spectrum is not continuous

15 Incandescent vs fluorescent light bulbs

16 Fluorescent light bulbs: Neon lights Produced the same way, but with a different set of atoms in the tube to produce the different colors

17 Frequency and wavelength are inversely related As frequency goes up, wavelength goes down Big wavelength Small frequency Low energy Small wavelength Big frequency High energy There is a HUGE spectrum of possible wavelengths/ frequencies of electromagne'c waves!

18 The ElectromagneKc Spectrum Small wavelength Big frequency High energy Big wavelength Small frequency Low energy

19 Which has the highest frequency? (A) Visible light (B) Ultraviolet (C) Gamma Rays (D) Radio waves (E) Infrared

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21 The electromagne'c spectrum

22 How big are these waves? Visible wavelengths are nanometers. A nanometer is a billionth of a meter, or 1 x 10-9 meters

23 Things on the nanometer scale include: Virus (30-50 nm), DNA (2.5 nm), buckyballs (~1 nm in diameter), carbon nanotubes (~1 nm in diameter). Atoms are smaller than a nanometer. One atom measures ~ nm, depending on the element. One inch equals 25.4 million nanometers. A sheet of paper is about 100,000 nanometers thick. A human hair measures roughly 50,000 to 100,000 nanometers in diameter. Your fingernails grow one nanometer every second.

24 Size and Scale 10 minute walk A child Your pinky Width of dime Width of hair Virus, DNA Atoms See size and scale sheet

25 Scien'fic nota'on We also write 1 nanometer as 1x10-9 meters That s because m is really unwieldy to write For example, 300 m Or 0.03 m X10 2 means mul'ply by 10*10, or 100 X10 3 means mul'ply by 10*10*10 or 1000 Etc. X 10-2 means divide by 10*10 or 100 X10-3 means divide by 10*10*10 or 1000 Etc. Add that number of zeroes to the end Move the decimal point to the leq that many 'mes For example, 6 x 10 7 m 5 x 10-5 m

26 Scien'fic Nota'on Prac'ce Write these two numbers out in regular (long) form. Which one is LARGER? (A) 5 x 10 2 (B) 5 x Write these two numbers out in regular (long) form. Which one is LARGER? (a) 5 x 10-2 (b) 5 x 10-12

27 Speed, wavelength, frequency Speed of light, v = c in a vacuum Wavelength (lambda), or how wide each oscilla'on is (meters) c = f λ Units: m/s = waves/sec * meters/wave Either wavelength or frequency will tell you the color of light Frequency, or oscilla'ons per second (Hertz). ν or f

28 c = f λ f = c/ λ λ = c/f So knowing the frequency, we can calculate the wavelength. OR knowing the wavelength we can calculate the frequency When light is traveling in a vacuum or in air, v = c. Remember: c = 300,000 km/s = 3x10 8 m/s

29 Which wave has the highest frequency? Oscilla'ons per second A c = f λ 600 nm wave, f =? b) B c) C

30 So this is why increasing frequency decreases the wavelength c = f λ

31 Ac'vity 18: Scien'fic Nota'on & Wavelength/Frequency Calcula'ons PRACTICE x 10-6 m = m x 10 5 m = 182,400 m 4,375,000 m = x 10 7 m m = 7.25 x 10-7 m

32 Conversions between units

33 How to convert To convert: 20 cm = how many meters? 20 cm x (how many meters per cm) = 20 cm x (0.01 m)/cm = 20 x 0.01 m = 0.2 m 10 cm = 0.2 meters

34 Ac'vity Scien'fic Nota'on How many pennies in $3? $3 x 100 pennies / $1 = 300 pennies How many nanometers in 20 m? 20 m x (1 nm / m) = 20 m x (1nm /1 x 10-9 m) = 20 x 10 9 nm = 2 x nm

35 Frequency and wavelength are inversely related Small wavelength Big frequency High energy Big wavelength Small frequency Low energy c = f λ When light is traveling in a vacuum or in air, v = c. Remember: c = 300,000 km/s = 3x10 8 m/s

36 Clicker Ques'on #1 An FM radio sta'on transmits at a frequency of: f = 100 MHz = 10 8 Hz then the wavelength is : Α) 1 m B) 0.3 m C) 3 m D) 100 m E) None of these. c = λ f f = c / λ λ = c / f c = 3x10 8 m/s λ = [3 x 10 8 m/s] / [10 8 /s] = 3 m 36

37 Clicker Ques'on #2 An FM radio sta'on transmits at a frequency of: f = 100 MHz = 10 8 Hz. We already found that the wavelength is 3m. If the frequency doubles, what is the new wavelength? (No calculators!) Α) 6 m B) 0.3 m C) 1.5 m D) 100 m c = λ f f = c / λ λ = c / f c = 3x10 8 m/s E) It does not change c = λ f c = (½ λ)(2f) 37

38 Clicker Ques'on #3 Your microwave oven operates at a frequency of: f = 3 GHz = 3x10 9 Hz then the wavelength is : Α) 1 cm B) 0.3 cm C) 3 cm D) 30 cm E) None of these. c = λ f f = c / λ λ = c / f c = 3x10 8 m/s λ = [3 x 10 8 m/s] / [3x10 9 /s] λ = 0.1 m = 0.1 m x 100 cm/m = 10 cm 38