Optics looks at the properties and behaviour of light!

Transcription

1 Optics looks at the properties and behaviour of light! Chapter 4: Wave Model of Light Past Theories Pythagoras believed that light consisted of beams made up of tiny particles that carried information about an object to the eye so we could see it Galileo believed to be the first person to try to determine the speed of light. He and an assistant stood on two hilltops about 1 km apart with lanterns. Galileo uncovered his lantern first and his assistant was suppose to uncover his lantern when he saw Galileo s light. This did not work well and Galileo could not calculate the speed of light! Michelson He is the first person to accurately carry out experiments to measure the speed of light. He used a strong light source, an 8 sided rotating mirror and another large mirror about 35 km away. Using the distance the light travelled and the speed at which his mirrored wheel was spinning he was able to calculate the speed of light. Scientific knowledge of light has led to the development of early technologies such as: Microscope created by the Janssen s who experimented with lenses and tubes. By moving the tubes in and out they made small objects appear larger Telescope created by Galileo who made his own lenses to magnify objects in space 1

2 Light Specifics Light a form of energy that can be detected by the human eye Speed of light The speed of light is m/s or 3 x 10 8 m/s * Compare speed of light with speed of sound: - Speed of sound at sea level is about 330 m/s (1200 km/h) compared to the speed of light at m/s ( km/h) - light travels extremely fast so fast that we cannot notice the time required for light to travel normal distances around us - The light from a distant lightening strike reaches us almost instantly but the sound from the strike (the thunder) takes longer to reach us! The longer it takes for you to hear the thunder after seeing the lightening, the further away the lightening is! Properties of light waves Light waves have the same features as ocean waves: Relationship between frequency and wavelength: High frequency waves have short wavelengths while low frequency waves have long wavelengths The red has the longest wavelength but the least refraction The violent has the shortest wavelength but the most refraction 2

3 The light that we see is called visible light. It is only one small part of the larger electromagnetic spectrum. The physical make up of our eyes allow us only to see visible light in the form of the colors below! Visible Light Spectrum Visible light a form of energy that can be detected with our eyes. Roy G Biv to remember colors! Red has the smallest refraction, orange refracts a little more, yellow a little more and so on... violet has the largest refraction! A prism refracts light and disperses it, separating its colors. Different colors of light are carried by light waves that have different wavelengths. An object appears blue in sunlight because only the blue color is reflected. The other colors are absorbed because of the properties of their wavelengths. Other examples of light dispersion to separate colors occur in sun catchers, rainbows in the sky or when you use a sprinkler on the lawn. 3

4 Electromagnetic radiation movement of electric and magnetic energy through space Electromagnetic radiation is always present but we do not realize it because its wavelengths are too short or too long for our eyes to see! We can only see the visible light portion of the electromagnetic spectrum! The electromagnetic spectrum has 7 types of electromagnetic radiation which can be categorized in order by the size of their wavelengths, their frequencies and their energy: Everyday Uses: 1. Radio waves used in telecommunications (phone, radios, radar, satellite communication) 2. Microwaves cooking food, Wireless LAN, Bluetooth devices 3. Infrared motion sensors, night vision devices, infrared cameras detect heat loss in homes 4. Visible light Everything we see, microscopes, CD players, fax machines, photocopiers 5. Ultraviolet sun tanning, black lights, glow in the dark objects, fluorescent lamps 6. X-rays x-rays, radiation treatment for cancer, airport security scanners 7. Gamma rays Gamma radiation sterilizes hospital equipment, used to kill some cancer cells 4

5 Positive and Negative effects of Electromagnetic Radiation * Higher energy radiation, such as X-rays and Gamma rays, is more harmful and dangerous. Electromagnetic Radiation Positive Effects Negative Effects Radio Waves Improved telecommunication Uncertain of long term exposure effects Microwaves Quick cooking of food May decrease nutritional value of foods when used in heating Infrared Improved night vision Long term exposure can have irreversible effects on eyesight Ultraviolet Used to treat jaundice in Skin cancer babies X-rays Medical detection Over exposure can lead to cancer Gamma rays Radiation therapy for cancer May kill other exposed cells To remember the electromagnetic spectrum visit the electromagnetic song at: 5

6 Chapter 5: Laws of Reflection Properties of visible light 1. Rectilinear propagation light travels in a straight line, like when we make shadows 2. Reflection specular reflection using mirrors and diffuse reflection using dust 3. Refraction bending or changing direction of wave as it passes from one material to another such as a popsicle stick appearing bent in a glass of water 4. Dispersion formation of a rainbow as light separates into its different colors 5. Travels through a vacuum does not require a medium such as the light from stars that reaches earth by travelling through space 6. Travels through transparent, translucent and opaque materials (in varying amounts) Transparent can see through it (glass, air, water) Translucent cannot see through (frosted or stained glass) Opaque light cannot pass through and so we cannot see through it (doors, wood) 6

7 Laws of Reflection in Mirrors Ray diagram uses straight lines to show the path of light rays Incident light ray the incoming light ray Reflected light ray the ray that bounces off the surface of the barrier (surface, mirror etc) Normal the imaginary line that is perpendicular to the barrier Angle of incidence the angle formed by the incident ray and the normal (i) Angle of reflection the angle formed by the reflected ray and the normal (r) Two types of reflection: Specular reflection reflection from a mirror like surface which produces an image of the Surroundings Diffuse reflection reflection from a rough surface that does not produce a clear image but instead allows you to see what is on the surface 7

8 Examples of specular versus diffuse reflection: Matte versus Glossy Paint: Matte paints have a higher proportion of diffuse reflection resulting in lower luster. Gloss paints have a greater proportion of specular reflection resulting in a shinier appearance. Unglazed versus Glazed ceramics: Unglazed has higher proportion of diffuse reflection; glazed has greater proportion of specular reflection Matte versus Glossy photographs: same effect as matte versus glossy paint! Types of Mirrors Plane mirror flat, smooth mirrors like bathroom mirrors Concave mirror has a reflecting surface that curve inwards like the inside of a metal spoon Convex mirror has a reflecting surface that curves outward like the safety mirror on the front of a school bus 8

9 Ray Diagrams for Mirrors Key Backgroud Knowledge: Law of Reflection observations on all types of surfaces show that the angle of incidence is alway equal to the angle of reflection Object the initial object facing the mirror (if you look at a mirror, you are the object) Image the appearance of the object that was facing the mirror (the image of yourself in the mirror is not you, just a likeness of you) Real image happens when reflected or refracted rays meet and the image appears to be in front of the mirror. It is often distorted and you need a screen to see it clearly Virtual image the reflected rays do not meet, but their extended rays meet at the object. The image appears to be behind the mirror 9

10 Object size size of the original object Image size size of the reflected/refracted image Object distance distance between the object and the mirror Image distance distance between the image and the mirror Upright same drection as the original object Inverted upside down from original object Prinipal axis a straight line that is perpindicular to the centre of a mirror or lens Vertex the point where the principal axis meets the mirror Focal point the point where converging light ray meet or diverging light rays diverge (converging means come together and diverging mean to spread out as per the diagram below) Focal length distance from the lens (vertex) to the focal point 10

11 Drawing Ray Diagrams for Plane Mirrors SPOT Characteristic Plane Mirror S = size (sizes of object and image) Image size = Object size P = position (object distance or image distance) Image distance = Object distance O = Orientation (upright or inverted) Upright, flipped in plane mirrors T = Type (real or virtual) Virtual Drawing Ray Diagrams for Concave Mirrors Concave mirrors are more complicated because the characteristics depend on location of object: SPOT Characteristic Object between focal point and mirror S = size (sizes of object and image) P = position (object distance and image distance) O = Orientation (upright or inverted) T = Type (real or virtual) Object between focal point and 2x focal point Image is larger than Object beyond the 2x focal point Image is larger than object object Image is smaller than object Image distance is Image distance is Image distance is larger than object larger than object smaller than object distance distance distance Upright Inverted Inverted Virtual Real Real In curved mirrors: - incident rays travelling parallel to the principal axis are reflected through the focal point - incident rays going throug the focal point are reflected parallel to the principal axis 11

12 Object between mirror and focal point * Any ray that is drawn beyond the mirror is an extended ray and should be a dotted line, not a solid line. 12

13 Object between the focal point and 2x the focal point 13

14 Object beyond the 2F point 14

15 Drawing Ray Diagrams for Convex Mirrors SPOT Characteristic Convex Mirror S = size (sizes of object and image) Image is smaller than object P = position (object distance or image distance) Image distance is smaller than object distance O = Orientation (upright or inverted) Upright T = Type (real or virtual) Virtual * The focal point for convex ray diagrams is behind the mirror! Any lines drawn beyond the mirror are dotted, not solid. * rays travelling parallel to the principal axis will reflect so that there extended rays go through the focal poing * rays travelling through the focal point will cause the reflected ray to be parallel to the principal axis 15

16 Chapter 6: Lenses Refract Light to Form Images (The first part of this section is found in chapter 5 but fits better here with chapter 6) Refraction Refraction the bending of light rays when they travel from one medium to another (ie: from air to water) The human brain does not recognize that light rays become bent or refracted as they travel from water to air and so the apparent position of an object is different from its actual position. As the light rays travel from one medium to another their speed changes. - speed will decrease as it travels from one medium to another that has a greater density (slows down as it goes from air to water). This will result in the ray bending toward the normal - speed will increase if it travels from one medium to another with a lesser density. This will result in the ray bending away from the normal. 16

17 Lenses Lens a curved piece of transparent material such as glass or plastic that refracts light in a predicatable way (like camera lenses or contact lenses). There are 2 types: concave and convex: 17

18 Converging vs Diverging Uses of Concave and Convex Lenses: Concave Eye glasses (fix near-sightedness) Convex Magnifying glasses Eye Glasses (fix far-sightedness) How do lenses fix your vision? The type of lens you need depends on your vision problem does your eye lens converge light rays to a point in front of your retina or behind it?? If your eye refracts light too much then you are nearsighted and you will need to use concave lenses in your glasses. If your eye does not refract enough light then you are far-sighted and you will need to use a convex lens in your glasses. Usually, convex lenses in glasses make someone s eyes look lager where concave lenses make someone s eyes and face look smaller! 18