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Light & Waves

Light & Waves. L2 NCEA Achievement Standard 2.3 Text Book reference: Chapters 12,13 &14. Why Waves?. A wave is a method of transferring energy from one place to another without having to move any matter. Examples of everyday waves include: water, light, sound, seismic waves.

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Light & Waves

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  1. Light & Waves • L2 NCEA • Achievement Standard 2.3 • Text Book reference: Chapters 12,13 &14

  2. Why Waves? • A wave is a method of transferring energy from one place to another without having to move any matter. • Examples of everyday waves include: water, light, sound, seismic waves. • They come in two forms: Transverse and Longitudinal

  3. Direction of Wave Propagation Particle Motion Transverse Waves • The particles that make up the wave vibrate at right angles to the direction of wave propagation. • Example: Light

  4. Direction Of Wave Propagation Particle Motion Longitudinal Waves • The particles that make up the wave vibrate back and forth in the same direction as the direction of wave propagation. • Example: Sound

  5. Amplitude Wave Terms • Amplitude - The distance from the undisturbed position of the particle to it’s maximum displacement • Symbol: A • Measured in metres

  6. Wavelength Wave Terms • Wavelength - The distance from one point on a wave to where it begins to repeat itself. • Symbol: l (Greek letter “lam-da”) • Measured in metres

  7. Period - the time it takes one wavelength to pass a given point Symbol: T Measured in seconds Wave Terms • Wave speed - the speed of wave propagation • Symbol: v • Measured in ms-1

  8. Frequency - the number of waves that pass a given point per second Symbol : f Measured in Hertz Hz (or cycles per second s-1) Note: Frequency and Period are inverses of each other ie. f =1/T or T = 1/f Wave Terms

  9. Area of Compression- part of a longitudinal wave where the particles are squashed up Area of Rarefaction- part of a longitudinal wave where the particles are spread out Compression Rarefaction Wave Terms

  10. Crest Trough Wave Terms • The top or peak of a transverse wave is called a crest • The bottom or dip of a transverse wave is called a trough

  11. Wavefronts Ray S Wave Terms • Waves generated from a point source travel outwards in concentric circles called wavefronts. • A line in the direction of propagation is called a ray.

  12. This is the equation that relates wave speed, frequency and wavelength. “c” is sometimes substituted for “v” when the wave is light. Wave Equation

  13. Light to Heavy String Heavy to Light String Reflection & Transmission of Pulses • When a pulse moves from one medium into another, some of the pulse is reflected and some is transmitted. Pg 224 Questions 14A 1-6

  14. Normal Reflection • Waves will bounce (reflect) off a flat surface at the same angle at which they hit it • A line at right angles to the surface is called the normal

  15. Curved Reflectors • Convex Reflectors – make the waves diverge (spread out)

  16. Curved Reflectors • Concave reflectors – make the wave converge (meet at a point)

  17. Refraction • The bending of a wave as it goes from one medium into another. • When a wave travels from one medium into another it’s speed alters. • If the wave hits the boundary at an angle, one side will change speed before the other, skewing the wave around and changing it’s direction of propagation.

  18. Fast Medium Slow Medium Refraction • Because the frequency of the wave is determined by the source, if the wave slows down, it’s wavelength must decrease. (And vice versa)

  19. Angle of Incidence Angle of Refraction Angles in refraction • The angle between the incident (incoming) ray and the normal is called the angle of incidence • The angle between the refracted ray and the normal is called the angle of refraction.

  20. Refractive Index • How much a wave is bent depends on the refractive indices of the two media. • Relative refractive index(2n1)is a ratio of the speeds of the waves in the two media • Absolute refractive index (n1or n2)is a measure of how much the speed is slowed when entering a medium from air ( or vacuum)

  21. Snell’s Law • Where: n= refractive index q= angle of incidence/refraction v= wave speed l= wavelength Medium 1 is the one the wave is leaving. Medium 2 is the one it is entering.

  22. Diffraction • The bending of waves as they travel through gaps….. • The smaller the gap, the more the diffraction

  23. Diffraction • ….or around edges. Pg 229 Questions 14B 1-6

  24. Interference • When two waves meet at one point they interfere. • Constructive interference is where a crest meets a crest, or a trough meets a trough. • This creates a really big crest or a really deep trough.

  25. Interference • Destructive interference is where a crest meets a trough. The result is that they cancel each other out leaving no wave.

  26. Superposition • The ability of waves to superimpose (add their displacements and energy) as they move through each other. • They carry on after as if the other wave was not present • Eg, if several people in a room talk all at once, the different sounds move from place to place with no effect on each other

  27. Superposition

  28. Superposition

  29. A A N N N Standing Waves • These are produced when a wave is reflected back on itself • The original wave and it’s reflection interfere to form a standing wave. • They have constant positions of no motion (called a node) and maximum motion (called an antinode)

  30. 2 Source Interference • Having 2 sources of concentric waves will produce a pattern like this • There appear to be lines radiating out from between the sources

  31. 2 Source Interference • Anti-nodal lines are lines of constructive interference. ie the water is choppy • Nodal lines are lines of destructive interference. ie the water is flat

  32. n=1 n=0 n=1 2 Source Interference • The n value is called the path difference • It tells you how many wavelengths further one wave has traveled compared to the other

  33. B A 2 Source Interference • If the waves were sound, a person walking from A to B would hear a series of loud and soft noises as they moved across the antinodal and nodal lines

  34. Low Energy Low Frequency Long Wavelength Radio Waves Microwaves Infra-red Visible Light Ultra Violet (UV) X-rays High Energy High Frequency Short Wavelength Gamma Rays Light • Visible Light is part of the electromagnetic spectrum.

  35. Colour

  36. Light Source Light • In general, light travels in straight lines • Light spreads out in all directions from it’s source. • The further from the source the less the illumination

  37. qi qr Reflection • A light ray can be bounced off a flat surface. This is called reflection. • Law of Reflection: The angle of incidence = the angle of reflection. (Remember: angles are measured from the normal)

  38. Plane Mirrors • To form images, light rays have to meet or focus. • The image is laterally inverted by a plane mirror (ie. You wave left hand, image waves right) • The image is virtual. It is formed behind the mirror, in a place where no light actually went. (a real image is formed when light rays meet at a point)

  39. Eye sees image back here Light from object reflects into eye Plane Mirrors Do Page 187 Questions 12A

  40. Curved mirrors • The centre of the mirror is called the pole. • A line at right angles to this is called the principal axis. • The focal length of a mirror is half the radius of curvature • The radius of curvature is the radius of the ball that the mirror would have been cut from

  41. c pa P F C f Curved Mirrors • C = centre of curvature • c = radius of curvature • F = Focal point or focus f = focal length • pa = principal axis P = pole

  42. Concave Mirrors • Concave (or converging) mirrors focus light at the focal point.

  43. Convex Mirrors • Convex mirrors have a focal point behind the mirror. • Convex (or diverging) mirrors spread the light rays apart so that they appear to have come from the focal point

  44. Ray Diagrams • Used to find the size, nature and position of images. • The nature of an image formed by a mirror or lens can be described according to 3 characteristics: Is it • a) upright or inverted • b) magnified, diminished or the same size • c) Real or virtual

  45. Ray Diagrams • Rule One: An incident ray parallel to the pa is reflected back through the focal point.

  46. Ray Diagrams • Rule Two: An incident ray headed towards the pole reflects back at an equal angle

  47. Ray Diagrams • Rule Three: An incident ray that passes through the focal point on the way to the mirror is reflected back parallel to the pa.

  48. Ray Diagrams • All three combined allow you to find the image. • In this example the image is inverted, diminished and real.

  49. Ray Diagrams • The same can be applied to convex mirrors with a few small changes… • All convex mirror images are virtual.

  50. Mirror Formulae • Descartes’ Formula: • Or: • m=magnification factor • h=height of image or object • d=distance from mirror to image or object • Distances behind the mirror are negative

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