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# Light

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### Light

1. 1. HOW DO WE SEE? WHAT IS LIGHT? HOW DOES IT CAUSE US TO SEE THINGS?
2. 2. FACTS ABOUT LIGHT• It is an electromagnetic wave that transmits energy• It is a part of the Electromagnetic Spectrum and the only part we can really see
3. 3. FACTS ABOUT LIGHT The speed of light, c, is constant in a vacuum. Light can be: •REFLECTED •ABSORBED •REFRACTEDLight is an electromagnetic wave in that it has wave like properties which can be influenced byelectric and magnetic fields.
4. 4. CHRISTIAAN HUYGENS(1629-1695)• Used a wave model to explain diffraction• Huygens’ principle--light waves spreading out from a point source are made up of tinier wave fronts.
5. 5. CHARACTERISTICS OF LIGHT• What we recognize as “white” light is actually light that can be separated into elementary colors of the visible spectrum: ROY G. BIV Red, orange, yellow, green, blue, indigo, violet
6. 6. LIGHT HAS BOTH PARTICLE AND WAVECHARACTERISTICS• A particle: • Definition: a small discrete quantity of matter that has an interface with the surrounding environment • Travels in a straight line, like a bullet • Bounces off objects, like a ball off of a wall • Its speed varies in different types of matter, like a ball bearing in molasses• A wave: • It travels in a straight line • It bounces off of object (we can hear our own echo) • Its speed varies in different types of medium
7. 7. WAVE-PARTICLE DUALITY THEORY• Light is a more complex phenomenon than just a simple wave or a simple beam of particles! Phenomenon Explained in terms of Explained in terms of waves? particles? Reflection ✔ ✔ Refraction ✔ ✔ Interference ✔ ✗ Diffraction ✔ ✗ Polarization ✔ ✗ Photoelectric Effect ✗ ✔
8. 8. LIGHTvocabulary activities• Absorption 1. Laser lights• Reflection 2. Light around an obstacle• Refraction 3. Pig mirage• Dispersion 4. Polarizing filters and mica• Diffraction 5. Write and see squares• Polarization 6. Concave mirror• Concave mirrors 7. Triple slits & prisms 8. Observing a pin through water
9. 9. DOUBLE-SLIT EXPERIMENTTwo slits made in a screen. Light was shined through to see what image would display on theother side.• If light was a particle, the experiment would show similar results to the ones below:
10. 10. THOMAS YOUNG--FAMOUS EXPERIMENT IN 1801• This experiment was a MAJOR reason for the discrediting of the particle theory of light• Monochromatic light was interfering constructively and destructively when passed through narrow slits onto a screen• Bright and darkfringes formedon the screen.
11. 11. THE PHOTOELECTRIC EFFECT• Showed that if photons are shined at an object, an electric current could be produced in the object. • Found that by increasing intensity (amplitude), more electrons will be emitted • supports WAVE THEORY: more intensity, more electrons knocked out of orbit! • BUT, it also found that if you increase the frequency (shift toward blue end of spectrum), the energy of each electron does increase • Supports PARTICLE THEORY: according to wave theory, frequency should not influence the amount of energy – only intensity should• Einstein: light comes in discrete units called photons that have a specific energy associated with in depending on its frequency
12. 12. SEEING COLOUR• The colour an object appears depends on the colours of light it reflects.For example, a red book only reflects red light: White Only red light is light reflected
13. 13. A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue): Purple light A white hat would reflect all seven colours: White light
14. 14. • We see things because they reflect light into our eyes: Homework
15. 15. PART 2 - REFLECTION• Reflection from a mirror: NormalIncident ray Reflected ray Angle of incidence Angle of reflection Mirror
16. 16. • The Law of Reflection Angle of incidence = Angle of reflectionIn other words, light gets reflected from a surface at ____ _____ angle ithits it. The same !!!
17. 17. CLEAR VS. DIFFUSE REFLECTION• Smooth, shiny surfaces have a clear reflection:Rough, dull surfaces have a diffuse reflection.Diffuse reflection is when light is scattered indifferent directions
18. 18. USING MIRRORS • Two examples: 2) A car headlight 1) A periscope
19. 19. REFLECTION• Sun rays are hit by spherical water droplets and are reflected off the back surface.• Red is bent the least (since it has the lowest wavelength) so it reaches the observer’s eye from higher in the sky!
20. 20. REFRACTIONRefraction is based on the idea that LIGHT is passing through one MEDIUM into another. The question is, WHAT HAPPENS? Suppose you are running on the beach with a certain velocity when you suddenly need to run into the water. What happens to your velocity? IT CHANGES! Refraction Fact #1: As light goes from one medium to another, the velocity CHANGES!
21. 21. REFRACTIONSuppose light comes from air, which in this case will be considered to be a vacuum, strikes a boundary at some angle of incidence measured from a normal line ,and goes into water. The ratio of the two speeds can be compared. The denominator in this case will ALWAYS be smaller and produce a unitless value greater or equal to 1. This value is called the new medium’s INDEX OF REFRACTION, n.All substances have an index of refraction and can be used to identify the material.
22. 22. REFRACTIONSuppose you decide to go spear fishing, but unfortunately you aren’t having much luck catching any fish. The cause of this is due to the fact that light BENDS when it reaches a new medium. The object is NOT directly in a straight line path, but rather it’s image appears that way. The actual object is on either side of the image you are viewing. Refraction Fact #2: As light goes from one medium to another, the path CHANGES!
23. 23. REFRACTIONWhat EXACTLY is light doing when it reaches a new medium? Not ALL of the light refracts. Some of the light REFLECTS off the boundary and some of the light REFRACTS through the boundary. Angle of incidence = Angle of Reflection Angle of Incidence > or < the Angle of refraction depending on the direction of the light
24. 24. REFRACTION – GOING FROM AIR TO WATER The index of refraction, n, for air (vacumm) is equal to 1. The index of refraction for water is 1.33. If you are going from a LOW “n” to a HIGH “n”, your speed DECREASES and the angle BENDS TOWARDS the normal
25. 25. REFRACTION – GOING FROM WATERINTO AIR The index of refraction, n, for air (vacumm) is equal to 1. The index of refraction for water is 1.33. If you are going from a HIGH “n” to a LOW “n”, your speed INCREASES and the angle BENDS AWAY the normal Note: If the angles are EQUAL, then the “n” must be equal for each. The ray will pass straight through.
26. 26. REFRACTION – SNELL’S LAWA scientist by the name of Snell discovered that the ratios of the index ’s and the ratio of the sine of the angles are the same value! n1 sin  2  n2 sin  1 Snell s Law n1 sin 1  n2 Sin 2
27. 27. EXAMPLEThe refractive index of the gemstone, Aquamarine, is 1.577. Suppose a ray of light strikes a horizontal boundary of the gemstone with an angle of incidence of 23 degrees from air. Calculate the SPEED of light in Aquamarine 8 c c 3x10 n  vm   vm n 1.577 vm  1.90 x 108 m/s Calculate the angle of refraction within Aquamarine n1 sin 1  n2 sin  2 (1)(sin 23)  1.577 sin  2 sin 23   sin ( 1 ) 14.34 degrees 1.577
28. 28. COLOUR• White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.• The act of splitting light into its colors using a prism is called dispersion • Each color has a different wavelength, so they bend differently when going into a new medium since index of refraction depends on the wavelength for that media. • It is greater for shorter wavelength We can demonstrate this by splitting white light with a prism:
29. 29. LIGHT & USES: DIFFRACTION• Diffraction – Bending of waves around the edge of a barrier. New waves are formed from the original. breaks images into bands of light & dark and colors.• Refraction – Bending of waves due to a change in speed through an object.
30. 30. LIGHT & USES: DIFFRACTION © 2000 Microsoft Encarta• A diffraction grating. Each space between the ruled grooves acts as a slit. The light bends around the edges and gets refracted.
31. 31. CONVERGING (CONCAVE) MIRROR Since the mirror is spherical it technically has a CENTER OF CURVATURE, C. The focal point happens to be HALF this distance. C f  2 C 2f We also draw a line through the center of the mirror and call it the PRINCIPAL AXIS.
32. 32. RAY DIAGRAMA ray diagram is a pictorial representation of how the light travels to form an image and can tell you the characteristics of the image. object C f Principal axis Rule One: Draw a ray, starting from the top of the object, parallel to the principal axis and then through “f” after reflection.
33. 33. RAY DIAGRAMS object C f Principal axisRule Two: Draw a ray, starting from the top of the object, through the focal point, then parallelto the principal axis after reflection.
34. 34. RAY DIAGRAMS object C f Principal axisRule Three: Draw a ray, starting from the top of the object, through C, then back upon itself. What do you notice about the three lines? THEY INTERSECT The intersection is the location of the image.
35. 35. RAY DIAGRAM – IMAGE CHARACTERISTICS object C f Principal axisAfter getting the intersection, draw an arrow down from the principal axis to the point of intersection. Then ask yourself these questions:1) Is the image on the SAME or OPPOSITE side of the mirror as the object?Same, therefore it is a REAL IMAGE.2) Is the image ENLARGED or REDUCED?3) Is the image INVERTED or RIGHT SIDE UP?
36. 36. POLARIZED WAVES• Definition: a wave that is polarized so that oscillations only occur in a certain plane.