Light and Color

1. Light and Color Electromagnetic Radiation

2. Properties and Sources of Light Key Question: What are some useful properties of light?

3. Properties and Sources of Light • Light travels almost unimaginably fast and far. • Light carries energy and information. • Light travels in straight lines. • Light bounces and bends when it comes in contact with objects. • Light has color. • Light has different intensities, it can be bright or dim.

4. Light carries energy and power • Light is a form of energy that travels. • The intensity of light is the power per square meter falling on a surface. • Most light sources distribute their light equally in all directions, making a spherical pattern. • Because light spreads out in a sphere, the intensity decreases the farther you get from the source.

5. Light intensity • The intensity of light from a small source follows an inverse square law because its intensity diminishes as the square of the distance.

6. Light carries information • The fiber-optic networks you read about are pipelines for information carried by light.

7. Light carries information • In some cities, a fiber-optic cable comes directly into homes and apartments carrying telephone, television, and Internet signals.

8. The speed of light • The speed of light is so important in physics that it is given its own symbol, a lower case c. • The best accepted experimental measurement for the speed of light in air is 299,792,500 m/sec. • For most purposes, we do not need to be this accurate and may use a value for c of 3 × 108 m/sec.

9. The Speed of Light Albert Michelson • The most famous experiment measuring the speed of light was performed by the American physicist Albert Michelson in 1880. • Light was directed by a lens to an octagonal mirror. • A beam of light was reflected to a stationary mirror on a mountain 35 km away and then reflected back. • The distance was known, so Michelson had to find only the time it took to make a round trip.

10. The Speed of Light • When the mirror was spun, short bursts of light reached the stationary mirror and were reflected back to the spinning octagonal mirror. • If the rotating mirror made one-eighth rotation while the light made the trip, the mirror reflected light to the observer. • If the mirror was rotated too slowly or too quickly, it would not be in a position to reflect light.

11. The Speed of Light • Light is reflected back to the eyepiece when the mirror is at rest.

12. The Speed of Light • Light is reflected back to the eyepiece when the mirror is at rest. • Reflected light fails to enter the eyepiece when the mirror spins too slowly . . .

13. The Speed of Light • Light is reflected back to the eyepiece when the mirror is at rest. • Reflected light fails to enter the eyepiece when the mirror spins too slowly . . . • . . . or too fast.

14. The Speed of Light • Light is reflected back to the eyepiece when the mirror is at rest. • Reflected light fails to enter the eyepiece when the mirror spins too slowly . . . • . . . or too fast. • When the mirror rotates at the correct speed, light reaches the eyepiece.

15. The Speed of Electromagnetic Waves Michelson timed a light beam as it traveled from one mountain to another and back again. His experiment measured the speed of light more accurately than it had been measured before. Mt. San Antonio Mirror 35.4 km Telescope Light Source Octagonal Rotating Mirror Mt. Wilson

16. Early Concepts of Light Light has been studied for thousands of years. Some ancient Greek philosophers thought that light consists of tiny particles, which enter the eye to create the sensation of vision. Others thought that vision resulted from streamers or filaments emitted by the eye making contact with an object.

17. Wave or Particle? Evidence for the Wave Model A beam of light passes first through a single slit and then through a double slit. • Where light from the two slits reaches a darkened screen, there are alternating bright and dark bands. • The bands are evidence that the light produces an interference pattern. • Interference occurs only when two or more waves overlap.

18. Wave or Particle? When light passes through a single slit and then a double slit, it produces an interference pattern. Interference pattern appears on screen. Card with two slits Dark bands show destructive interference. Card with one slit Bright bands show constructive interference. Light source Light from single slit produces coherent light at second card.

19. Wave or Particle? Evidence for the Particle Model When dim blue light hits the surface of a metal such as cesium, an electron is emitted. A brighter blue light causes even more electrons to be emitted. Red light, no matter how bright it is, does not cause the emission of any electrons from this particular metal.

20. Wave or Particle? • Red light or infrared rays, no matter how bright, does not cause electrons to be emitted from this metal surface. • When blue light or ultraviolet rays strike the metal surface, electrons are emitted, even if the light is dim. Electrons are emitted. No electrons are emitted. Dim blue light or ultraviolet rays Bright red light or infrared rays Metal plate Metal plate

21. Wave or Particle? The emission of electrons from a metal caused by light striking the metal is called the photoelectric effect. In 1905, Albert Einstein (1879–1955) proposed that light, and all electromagnetic radiation, consists of packets of energy. These packets of electromagnetic energy are now called photons.

22. Photons and Atoms • Key Question: How does light fit into the atomic theory of matter?

23. Photons and atoms • Just like matter is made of tiny particles called atoms, light energy comes in tiny bundles called photons. • White light is a mixture of photons with a wide range of colors (energies).

24. Photons and intensity • Intensity measures power per unit area. • There are two ways to make light of high intensity. • One way is to have high- energy photons. • A second way is to have a lot of photons even if they are low-energy. The number and energy of photons determine the intensity of the light.

25. Intensity Intensity is the rate at which a wave’s energy flows through a given unit of area. A wave model also explains how intensity decreases. • As waves travel away from the source, they pass through a larger and larger area. • The total energy does not change, so the wave’s intensity decreases.

26. Intensity The closer you are to a surface when you spray paint it, the smaller the area the paint covers, and the more intense the paint color looks.

27. What Are Electromagnetic Waves? Electromagnetic waves are transverse waves consisting of changing electric fields and changing magnetic fields. • Like mechanical waves, electromagnetic waves carry energy from place to place. • Electromagnetic waves differ from mechanical waves in how they are produced and how they travel.

28. What Are Electromagnetic Waves? How They Are Produced Electromagnetic waves are produced by constantly changing electric fields and magnetic fields. • An electric field in a region of space exerts electric forces on charged particles. Electric fields are produced by electrically charged particles and by changing magnetic fields. • A magnetic field in a region of space produces magnetic forces. Magnetic fields are produced by magnets, by changing electric fields, and by vibrating charges.

29. What Are Electromagnetic Waves? Electromagnetic waves are transverse waves because the fields are at right angles to the direction in which the wave travels.

30. What Are Electromagnetic Waves? How They Travel Changing electric fields produce changing magnetic fields, and changing magnetic fields produce changing electric fields, so the fields regenerate each other. • Electromagnetic waves do not need a medium. • The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation.

31. The Waves of the Spectrum The full range of frequencies of electromagnetic radiation is called the electromagnetic spectrum. • Visible light is the only part of the electromagnetic spectrum that you can see, but it is just a small part. • Each kind of wave is characterized by a range of wavelengths and frequencies. All of these waves have many useful applications.

32. The Waves of the Spectrum The electromagnetic spectrum consists of radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

33. Waves of the electromagnetic spectrum • Visible light is a small part of the energy range of electromagnetic waves. • The whole range is called the electromagnetic spectrum and visible light is in the middle of it.

34. Color and Vision Key Question: How do we see color?

35. Color and Vision • When all the colors of the rainbow are combined, we do not see any particular color. • We see light without any color. • We call this combination of all the colors of light "white light".

36. Color and Vision • We can think of different colors of light like balls with different kinetic energies. • Blue light has a higher energy than green light, like the balls that make it into the top window. • Red light has the lowest energy, like the balls that can only make it to the lowest window.

37. How the human eye sees color • The retina in the back of the eye contains photoreceptors. • These receptors release chemical signals. • Chemical signals travel to the brain along the optic nerve. optic nerve

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

39. How we see colors • 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.

40. How we see other colors • The three color receptors in the eye allow us to see millions of different colors. • The additive primary colors arered, green, and blue. • We don’t see everything white because the strength of the signal matters. • All the different shades of color we can see are made by changing the proportions of red, green, and blue.

41. How we see the color of things When we see an object, the light that reaches our eyes can come from two different processes: • The light can be emitted directly from the object, like a light bulb or glow stick. These are called luminous objects • The light can come from somewhere else, like the sun, and we see the objects by reflected light. These objects are illuminated.

42. How we see the color of things • Colored fabrics and paints get color from a subtractive process. • Chemicals, known as pigments, in the dyes and paints absorb some colors and allow the color you actually see to be reflected. • Magenta, yellow, and cyan are the three subtractive primary colors.

44. Why are plants green? • Plants absorb energy from light and convert it to chemical energy in the form of sugar (food for the plant). • Chlorophyll is an important molecule that absorbs blue and red light.

45. How does a color TV work? • Televisions give off light. • To make color with a TV, you can use red, green, and blue (RGB) directly. • The screen is made of tiny red, green, and blue dots. • The dots are called pixels and each pixel gives off its own light. • TV sets can mix the three colors to get millions of different colors.

46. Reflection and refraction • When light moves through a material it travels in straight lines. • When light rays travel from one material to another, the rays may reflect. • The light that appears to bounce off the surface of an object is shown by a reflected ray.

47. Reflection and refraction • Objects that are in front of a mirror appear as if they are behind the mirror. • This is because light rays are reflected by the mirror. • Your brain perceives the light as if it always traveled in a straight line.

48. Reflection and refraction • The light that bends as it crosses a surface into a material refracts and is shown as a refracted ray.

49. Reflection and refraction • Another example of refraction of light is the twinkling of a star in the night sky • As starlight travels from space into the Earth’s atmosphere, the rays are refracted. • Since the atmosphere is constantly changing, the amount of refraction also changes.