2. Chapter Twenty-Five: Light  25.1 Properties of Light  25.2 Color and Vision  25.3 Optics

3. 25.1 Properties of light  Light travels fast over long distances and carries energy and information.  Light travels in straight lines, but can be bent by lenses or reflected by mirrors heat and warmth.  Light has color and can be bright or dim.

4. 25.1 The electromagnetic spectrum  Light, like sound and heat, is a form of energy.  The visible light we see is part of the electromagnetic spectrum.

5. 25.1 Properties of light  You see book pages because light in the room reflects from the page to your eyes.  Your eyes and brain use the information carried by the light to make a mental picture.

6. 25.1 Light is produced by atoms  Most light is produced by atoms.  When you put some energy into the atom, it excites the atom’s electrons.  Light is produced when the electron releases this energy.

7. 25.1 Incandescent light  Making light with heat is called incandescence.  Atoms in the filament convert electrical energy to heat and then to light.  Incandescent bulbs are inefficient, but their waste heat can be useful.

8. 25.1 Fluorescent light  To make light, fluorescent bulbs use high-voltage electricity to energize atoms of gas in the bulb.  These atoms release the electrical energy directly as light (not heat), in a process called fluorescence.

9. 25.1 Color and energy  When all the colors of the rainbow are combined, we see light without any color.  We call the combination of all colors white light.

10. 25.1 Color and energy  Compare the hot, blue flame from a gas stove to the orange flame of a match.  The light from a gas flame is blue (high energy) and the light from a match is red-orange (low energy).

12. 25.1 The speed of light  The speed at which light travels through air is about 300 million meters per second.  The speed of light is so important in physics that it is given its own symbol, a lower case “c”.

13. 25.1 Speed of light  The speed at which electromagnetic waves travel through air is about 300 million meters per second.  The speed of light is so fast that when lightning strikes a few miles away, we hear the thunder after we see the lightning.

14. 25.1 Wavelength and Frequency of Light  Because the wavelength of light is so small, scientists measure it in nanometers.  One nanometer (nm) is one billionth of a meter (0.000000001 m).

15. 25.1 Wavelength and Frequency of Light

16. 25.1 What kind of wave is light?  A sound wave is a oscillation of air.  A water wave is an oscillation of the surface of water.  An oscillation of electricity or magnetism creates electromagnetic waves.

17. 25.1 Electromagnetic waves  If you could shake the magnet up and down 450 trillion times per second, you would make waves of red light with a frequency of about 450 THz.

18. 25.1 Electromagnetic spectrum  The entire range of electromagnetic waves, including all possible frequencies, is called the electromagnetic spectrum.  This spectrum includes visible light and invisible waves:  radio wave  microwaves  infrared light  ultraviolet light  X-rays  gamma rays

20. 25.2 The human eye  The eye is the sensory organ used for vision.  The retina contains light-sensitive cells called photoreceptors.  Photoreceptors convert light into nerve impulses that travel through the optic nerve to the visual cortex of the brain.

21. 25.2 Photoreceptors  The human eye has two types of photoreceptors— cones and rods.  Cones respond to color and rods respond to the intensity of light.  Rod cells “see” black, white, and shades of gray.

22. 25.2 How we see color  Our eyes work according to an additive color process — 3 photoreceptors (red, green, and blue) in the eye operate together so that we see millions of different colors.

24. Wavelength and Frequency of Visible Light

25. 25.2 Making an RGB color image  A television makes different colors by lighting red, green, and blue pixels in different proportions.  Color images in TVs and computers are based on the RGB color model.

27. 25.2 Making an RGB color image  Like the rods and cones in your retina, a video camcorder has tiny light sensors on a small chip called a CCD.  There are three sensors for each pixel of the recorded image: red, green, and blue.

28. 25.2 How objects appear to be different colors  Your eye creates a sense of color by responding to red, green, and blue light.  You don’t see objects in their own light, you see them in reflected light!

29. 25.2 Subtractive color process  A blue shirt looks blue because it reflects blue light into your eyes.  Chemicals known as pigments in the dyes and paints absorb some colors and reflect other colors.

31. 25.2 The CMYK color process  The subtractive color process is often called CMYK for the four pigments it uses.  CMYK stands for cyan, magenta, yellow, and black.

34. 25.2 Why plants are green  Plants absorb energy from light and convert it to chemical energy in process called photosynthesis.  Chlorophyll is the main pigment of plants absorbs red and blue light and reflects green light.

35. 25.2 Why plants are green Plants must reflect some light to avoid absorbing too much energy.  A plant will die if placed under only green light!

36. 25.3 Basic optical devices Three useful optical devices are: 1. lenses 2. mirrors 3. prisms

37. 25.3 Basic optical devices  A magnifying glass is a converging lens that can be used in survival situations to make a hot spot.  Mirrors can attract the attention of rescue teams from great distances.

38. 25.3 Four ways light is affected by matter

39.  All four interactions almost always happen together.  Green colored paper absorbs some light, reflects some light, and is partly translucent. Can you tell which colors are reflected and which are absorbed?

40. 25.3 Four ways light is affected by matter  A glass window is mostly transparent, but also absorbs, scatters, and reflects some light.  See if you can identify where certain colors are absorbed and reflected in this picture.

41. 25.3 Light rays  Reflection occurs when light bounces off a surface and when light bends while crossing through materials.

42. 25.3 Reflection  There are two types of reflection; but not all reflections form images.  Rays light that strikes a shiny surface (like a mirror) create single reflected rays.  This type of reflection is called specular reflection.

43. 25.3 Reflection  A surface that is dull or uneven creates diffuse reflection.  When you look at a diffuse reflecting surface you see the surface itself.

45. 25.3 Law of reflection  A ray diagram is an accurately drawn sketch showing how light rays interact with mirrors, lenses, and other optical devices.

46. 25.3 Refraction  Materials with a higher index of refraction bend light by a large angle.  The index of refraction for air is about 1.00.  Water has an index of refraction of 1.33.

48. 25.3 Refraction  Vegetable oil and glass have almost the same index of refraction.  If you put a glass rod into a glass cup containing vegetable oil, the rod disappears because light is NOT refracted!

49. 25.3 Lenses  An ordinary lens is a polished, transparent disc, usually made of glass.  The shape of a converging lens is described as being “convex” because the surfaces curve outward.

50. 25.3 Lenses  The distance from the center of the lens to the focal point is the focal length.  Light can go through a lens in either direction so there are always two focal points, one on either side of the lens.

51. 25.3 Lenses  For a converging lens, the first surface (air to glass) bends light rays toward the normal.  At the second surface (glass to air), the rays bend away from the normal line.