Light and Color Chapters 16 & 19

1. Light and Color Chapters 16 & 19

2. Light and the Electromagnetic Spectrum • Visible Light is an electromagnetic wave that stimulates the retina of the eye. Its wavelengths are between 400nm (violet) and 700nm (red). • The Electromagnetic Spectrum is the full range of wavelengths and frequencies at which all electromagnetic radiation exists and the corresponding names that we give to certain “zones”.

5. Primary and Secondary Pigments Primary and Secondary Light

6. Absorbing and Emitting Light • White paper absorbs and then emits photons of all colors of light. • Black ink absorbs the photons of all colors and emits none.

7. Relative Speed of Light and the Doppler Effect with Light • Doppler Effect with light determined based on relative speed • Relative speed – magnitude of the difference between the velocities of the light source and the observer of the light

8. Blue-Shifted or Red-Shifted? • Blue-shifted - When light source approaches observer, there is an increase in measured frequency, so wavelength of light shortens. Called blue-shifted because the increase is towards the higher frequency (or blue) end of the color spectrum. (Ex: side of a star turning towards us as it spins) • Red, green and blue channels represent the red-shifted and blue-shifted motions of the ionised material in the halo. The positions of the two galaxies C11 and C15 are marked.

9. Red-Shifted? • Red-shifted - When a light source recedes (goes away) from observer, there is a decrease in measured frequency, so wavelength lengthens. Called red-shifted because the decrease is towards the lower (or red) end of the color spectrum (Ex: the side of a star spinning away from us; distant galaxies show a red shift as they move away)

10. Light speed – History of Discovery • In a vacuum, light has a speed of 3.00 x 108m/s (symbol c) • Before 17th century, people believed light traveled instantaneously or that speed of light was too fast to be measured. • Roemer used the orbit of Jupiter’s moon, Io, and the orbit of Earth around the sun to first measure the speed of light in 1674. • Michelson, using a specially designed experiment sending light between two mountains, more closely estimated the speed of light in 1926. His measurement is almost the exact same that we use today!

11. Light… • has a dual personality- it can behave as both a particle (advanced physics) and a wave. • can travel fastest in a vacuum! • The speed of light through the other types of mediums (solid, liquid, and gas) is dependent on the density of that medium. More dense substances slow down light more than less dense. (Opposite of sound!)

12. Speed of Light wavelength (m) c = f l speed of light 3 x 108 m/sec frequency (Hz)

13. Speed of Sound vs. Speed of Light • Light travels almost a million times faster • Light travels around the earth 7.5 times per second

14. The speed of light • Speed of light in a vacuum is equal to a constant c, which equals 3.00 x 108 m/s • Rearrange the speed of light formula to find wavelength of a light wave by dividing c by frequency of that light. • We can also rearrange the formula to find frequency, if we know the wavelength and speed. • Try p. 447, #14 and #8 p. 455 (Note that a nanometer is 10-9 m)

15. Answers to practice problems • P. 447 #14. 5.85 x 1014 Hz • P. 455 #8. 7.43 x 108 Hz

16. Chapter Review Questions • Answer the following 19 questions: • p. 438 #7 and 12 (hint: v = d/t - rearrange) • P. 447 : #18 – 22 (4 questions) • 13 chapter review questions on page 452-454: #25, 33, 35, 36, 37, 38, 39, 43, 48, 51, 54 (hint: remember v = d/t ), 62, and 68 (just explain in general terms, no need to solve) • DUE FRIDAY

17. Colors of Light • White light is a combination of the spectrum of colors, each having different wavelengths. • When combined, the three primary light colors of red, blue, and green will produce white light. • In other combinations, they will produce other colors.

18. Primary and Secondary Pigments Primary and Secondary Light

19. Absorbing and Emitting Light • White paper absorbs and then emits photons of all colors of light. • Black ink absorbs the photons of all colors and emits none.

20. Why do we see colors? • Pigments reflect color of light that we see. So… a shirt that has been dyed to appear red is only red because it reflects mostly red light and absorbs most of the other two primary light colors of blue and green. So that shirt must contain cyan pigment. • When mixed, the primary pigments create the secondary pigments (red, blue, and green).

21. Examples • White light is incident on the three shapes below. Explain why they appear to be the color they are by using reflected and absorbed colors as well as identifying which pigments must be present in the object. Yellow color is seen. Red and Green light are reflected, Blue light is absorbed. Blue pigment must be present. Blue color is seen. Blue light is reflected, Red and Green light are absorbed. Yellow pigment must be present. Magenta color is seen. Red and Blue light are reflected, Green light is absorbed. Green pigment must be present.

22. How the human eye sees light • Photoreceptors release chemical signals. • Chemical signals travel to the brain along the optic nerve. optic nerve

23. Cones respond to three colors: red, green and blue. Rods detect intensity of light: black, white, shades of gray. Photoreceptors in the eye

24. Which chemical signal gets sent depends on how much energy the light has. If the brain gets a signal from ONLY green cones, we see green. How we see colors

28. Normal vision Weak green color vision No red color vision Color Blindness

29. What’s the difference between light and pigments? • Unlike light, pigments are referred to as subtractive colors. For instance, when you mix the primary pigments of yellow and cyan, the color of green is what you will see. That is because green pigment absorbs the light with colors of blue and red and reflects back only green.

30. Thin Films • Colors in soap and oil films are caused by the interference of specific wavelengths of light reflected from the front and back surfaces of the thin films. • The colors actually show up because an antinode (constructive interference) is formed.

31. Categories of Materials based on absorption or reflection of light • Transparent - allow most light to pass through them. • Translucent - allow some light to pass through, but some is absorbed and some is reflected. • Opaque - do not allow any light to pass through, but instead absorb or reflect all light.

32. Spectrum Diffraction Patterns on a CD • The metallic coatings on CDs are less than 100nm thick • Each coating partially reflects and partially transmits incident light. • Light rays reflected from different coating boundaries interfere with each other to produce the colorful patterns

33. Diffraction • Bending of light around a barrier • Diffraction pattern – a pattern of bright and dark bands produced by constructive and destructive interference; white light shows all the colors of spectrum • Diffraction gratings (ex: goggles used in 1st lab) are devices made up of many single slits that bend light and form diffraction patterns • See fig. 19-14 p. 528 red light versus white light

34. Polarized light • Normally, light (just like all electromagnetic radiation) vibrates in two dimensions as it travels. • Polarized light consists of waves vibrating in a particular plane. Polarization is done by using VERY small filters (slits) that block vibrations from other planes. • Application of Polarization – sunglasses polarized to reduce glare reflected off water or off the road

35. Polarized Light

36. Wavelength and Frequency of Visible Light

37. Thin slits and diffraction

41. How do scientists analyze light? • A Spectroscope (containing a diffraction grating) is used to measure light from glowing elements and separate that light into its various frequencies. It is able to measure Doppler shift of the wavelengths of the light from elements.

42. What does a spectroscope do? • Helps chemists determine the elemental composition of heated gases or materials using the spectra they emit. • Astronomers can use spectra seen to determine what elements make up the stars and galaxies and to see if they are moving towards (blue- shifted) or away (red-shifted) from us.