Unit C: Light and Optical Systems

1. Unit C: Light and Optical Systems

2. Topic 1: What is Light? • Light: the form of energy that we can see • Natural light source: a non-human-made source of light, such as the Sun or fire • Light radiates(spreads out) from the Sun in all directions in a form of energy transfer called radiation (the transfer of radiant energy such as light). • A VERY small percentage of the Sun’s energy actually reaches the Earth

3. Our lives are totally dependant on the energy from the Sun. As sunlight is not always available, people have developed artificial light sources (ex. light bulb) • Chemicals can also produce light (ex. batteries, matches) • Nuclear fission also provides us with energy on a large scale

4. The First Basic Principle of Light • Light is a form of energy • When light is absorbed by a surface, it can be transformed into several different forms of energy Light-> Thermal Energy, Electrical Energy, Chemical Energy Ex. Trees, solar panels, a black sweater

5. The First Basic Principle of Light • The brightness or intensity of light indicates how much energy a surface will receive • A surface can absorb more energy if the brightness of the light intensifies

6. Sources of Light • Artificial sources of light have enabled us to be productive after the Sun is gone during the day • Torches, candles, lamps, light bulbs, etc. are artificial sources of light that help us to see at night • Observatories are located far from urban areas to avoid light pollution from large cities at night

7. Sources of Light • Communities are attempting to conserve the light energy they use • New street lights are more energy efficient and are being designed to direct light toward the ground to avoid illuminating the sky • In the headlights of cars, yellow sodium vapor lights are being used in newer cars as they are more energy efficient

8. Incandescent Sources • Incandescent source: an object that is heated to such a high temperature that it emits visible light • Incandescence: the emission of visible light by a hot object • Examples: candle flames, light bulbs • In a light bulb, the filament is heated a follows: Electrical Energy -> Thermal Energy -> Visible Light Energy

9. Incandescent Sources • 95% of the energy given off by incandescent light bulbs is released as heat • The filament of a light bulb is usually made of the chemical element tungsten (W)

10. Fluorescent Sources • Fluorescence: the process in which high-energy, invisible UV light is absorbed by the particles of an object, which then emits some of this energy as visible light, causing the object to glow UV Light Energy -> Energy Absorbed -> Visible Light by Particles Energy • Fluorescent source: a source that produces light when exposed to light of a particular wavelength

11. Fluorescent Sources • In fluorescent tubes, an electric current causes mercury vapors in the tube to give off UV radiation. A phosphor coating on the inside of the tube absorbs the UV energy and glows • Figure 3.6 page 181

12. Fluorescent Sources • Disadvantages of fluorescent tubes • Expensive to manufacture • Difficult to dispose of • Phosphor and mercury are toxic • Advantages of fluorescent tubes • Very little thermal energy is produced when lit • More energy efficient

13. Phosphorescent Sources • Similar to a fluorescent light source • Light energy is absorbed by particles that can store the light and later release it as visible light • Phosphorescence: the persistent emission of light following exposure to and removal of a source of radiation • Phosphorescent source: a substance that gives off visible light released after the light energy has been absorbed by particles and stored

14. Phosphorescent Sources • Examples include TV and computer screens, surfaces that glow in the dark (watches, dials, TV remote) Differences between Fluorescent and Phosphorescent Sources • Particles in a fluorescent source release their light energy immediately • Phosphorescent particles take longer to release their light energy and glow after the light source has been removed

15. Chemiluminescent Sources • Chemiluminescence: the emission of light resulting from a chemical reaction and not involving heat Chemical Energy -> Visible Light Energy • Chemiluminescent source: a chemical reaction that produces particles that give off visible light energy • Example: glow sticks

16. Bioluminescent Sources • Bioluminescence: the emission of light produced by chemical reactions inside the bodies of living creatures • Bioluminescent source: an organism that relies on chemical reactions inside its body to produce light

17. The Cost of Lighting • Electrical energy costs about 8¢ per kilowatt hour (kWh) • 8¢/kWh • A kilowatt hour (kWh) is approximately 1000 watts (W) of electrical power operating for one hour • We can calculate the cost of lighting by multiplying the power of the bulb (kW or W) by the time (h)

18. Example How much will it cost to leave a 60W light bulb on for 10 hours if the electrical energy cost is 8¢/kWh? • Convert W to kW by ÷ by 1000 • Calculate the number of kWh by multiplying the power (in kW) by the number of hours • Calculate the cost of leaving the light on for 10h by multiplying the number of kWh by the cost per kWh • Cost = 4.8¢

19. The Ray Model of Light • Ray Model: a model of light based on the observation that light travels in a straight line • Ray: straight line that represents the path of a beam of light • You can use this model to predict where shadows will form and how big they will be

20. The Ray Model of Light • When light travels through clear substances, the rays continue along in a straight path • Clear substances are transparent • When light travels through non-clear objects (ex. a piece of paper), the object scatters the light (bends the light rays) and you cannot see images from this light • These object are translucent • When light is completely blocked from a material, the material is called opaque • Figure 3.11A, 3.11B. 3.12 page 185

21. Topic 1 Review p. 187 #1-6

22. Topic 2: Reflection • Reflection: the process in which light strikes a surface and bounces back off that surface • Ex. Light reflecting off a mirror allowing you to see an image of yourself

23. Reflection • The difference between seeing your own image (mirror reflection) or seeing a solid object (words on a page) depends on the surface from which light reflects • A smooth surface is better at reflecting an image, while a rough surface tends to scatter the light in all directions.

24. Reflection • To analyze the process of reflection: • Imagine the surface is flat • The ray of light from a light source that strikes the surface is called the incident ray • The ray that bounces off that surface is called the reflected ray

25. To describe these rays: • Draw a line perpendicular to the reflecting surface (90°) at the point where the incident ray strikes the surface • This is called the normal line • The angle between the incident ray and normal line is the angle of incidence (i) • The angle between the normal line and the reflected ray is the angle of reflection (r) • Figure 3.15

26. Reflection • Inferring the Law of Reflection Lab, p. 190-191 • Plane mirror: a mirror with a flat surface • Shows the image exactly as it is Forming an Image • Angle of reflection should EQUAL the angle of incidence • This is the same on ALL types of surfaces, with NO exceptions!

27. Law of Reflection • The angle of reflection equals the angle of incidence • Ex. If the angle of incidence (i) is 60°, then the angle of reflection (r) will be 60° • The incident ray, the normal line, and the reflected ray also lie in the same plane (an imaginary flat surface)

28. Law of Reflection How do reflected rays form an image that you can see in a mirror? • Light shines on an object • Light reflects off the object in ALL directions • All of the object’srays that hit the mirror reflect off the mirror according to the law of reflection • The rays that reach your eyes appear to be coming from a point BEHIND the mirror • This happens for EVERY point on the object • Your brain interprets the pattern of light (travelling in straight lines) that reaches your eye as an image of the object behind the mirror

29. Law of Reflection • An image in the mirror is EXACTLY the same size as the object and appears to be the same distance from the mirror as the object • This is true only for plane (flat) mirrors

30. Curved Mirrors • Convex Mirrors: mirrors that bulge outward • Form images that appear much smaller and farther away than the object • Can reflect light from a large area • Ex. Security mirrors in stores, passenger mirrors in cars • Cannot focus an image on a screen, but you can see an image in the mirror (virtual image)

31. Curved Mirrors • Concave Mirrors: mirrors that are caved in • Can focus an image onto a screen • Magnifies the image in the mirror • Ex. Make-up mirrors

32. Rough Surfaces • When light strikes a smooth surface, the lines reflect according to the law of reflection • When light strikes a rough surface, the normallines go in many different directions. Each light ray that strikes the surface will reflect according to the law of reflection. However, since the normal lines point in different directions, the reflected rays will go in different directions. This results in the reflected rays appearing scattered.

33. Rough Surfaces • These seemingly scattered light rays reflect off the paper (for example) in all directions. • The light that reaches your eyes appears white because there is no pattern on the paper. • IF there is print on the paper, the ink absorbs light. Nothing that strikes the print is reflected, so your eyes see it as black.

34. Using Reflections • Reflectors on bicycles use the law of reflection to reflect light from other vehicles • Pool players use this law to improve their game • The pool ball will travel in a straight line until it hits something (bank shot)

35. Topic 2 Review • Page 199 #1-4

36. Topic 3: Refraction REFRACTION: the bending of light when it travels from one medium to another • Light bends because it changes speed when it moves between materials that have different densities

37. Refraction • Light travels slowly in a dense material • The bending of light makes the object’s image appear to be in a different position from where the object really is

38. Around a Bend with Light • When light moves from a less dense medium to a more dense medium (ex. air to water), the light will bend TOWARDthe normal • When light moves from a more dense medium to a less dense medium (ex. water to air), the light will BEND AWAY from the normal • The degree to which the light bends depends on the material the light travels through

39. Around a Bend with Light • Angle of Refraction (R): the new direction of the light following refraction • When the angle of incidence (i) increases, the angle of refraction (R) also increases • BUT doubling the angle of incidence DOES NOT double the angle of refraction

40. Around a Bend with Light • Refraction can also occur when light travels through air at different temperatures • WARM air is less dense than COLD air • Light bends as it travels through different densities of air • This can result in a mirage • The air close to the ground is hotter and less dense than the air higher up. As a result, light from the sky directed at the ground is bent upward as it enters the less dense air

41. Is that all there is to light?

42. Topic 4: Lenses and Vision Types of Lenses • Lens: a curved piece of transparent material such as glass or plastic • Light refracts as it passes through a lens, causing the light rays to bend

43. Types of Lenses • A double concave lens is thinner and flatter in the middle than around the edges • Light passing through the thicker, more curved areas of the lens will bend MORE than light passing through the flatter areas • This causes the rays of light to spread out (DIVERGE) after passing through the lens

44. Types of Lenses • A double convex lens is thicker in the middle than around the edges • This causes the refracting light rays to come together (CONVERGE)

45. Lenses and Images • An image forms where light rays from an object converge • The lens directs light from the left portion of the object to the right portion of the image • Light from the top of the object is directed to the bottom of the image • An image formed from a double convex lens is sometimes upside-down • Example: Film projectors are double convex lenses and the film must be placed upside-down to see the image properly on the screen

46. Eye Spy • The lens in a human eye is a convex lens • This lens takes light rays from objects and focuses them (brings them back to the point) through refraction • This allows us to see objects • In the eye, light refracts through the lens onto a light-sensitive area at the back of the eye called the retina • The image you see is formed on the retina

47. Disorders of the Eye • Near-sighted: people who have trouble seeing distant objects (eyes are too long) • Far-sighted: people who have trouble seeing objects close to them (eyes are too short) • Knowing how light behaves can help eye specialists correct vision problems • Figure 3.32B and 3.32C

48. Comparing the Eye and the Camera Eye • You can see objects when light from an object focuses on your retina Camera • In a camera, the lens refracts the light and the film senses the light • Figure 3.33A

49. Putting it in Focus • In a camera, if an object moves closer to the film, the lens must move farther from the film to keep the image in focus • In the eye, your ciliary muscles change the shape of the lens to focus an image • If an object moves closer to you, your eye muscles make the lens bulge in the middle, keeping the object in focus • This is called accommodation

50. Putting it in Focus • Near point: the shortest distance at which an object is in focus in the eye • Adult: 25cm • Babies: 7cm • Far point: the longest distance at which an object is in focus in the eye • Infinity (ex. stars)