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Light & Reflection Mirrors

Light & Reflection Mirrors. Notes: Ref p446> HRW. EM Spectrum – Basic Properties. Radio Microwave Infrared Visible Ultraviolet X-rays Gamma rays. Light Properties. Light is e/m radiation

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Light & Reflection Mirrors

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  1. Light & ReflectionMirrors Notes: Ref p446> HRW

  2. EM Spectrum – Basic Properties • Radio • Microwave • Infrared • Visible • Ultraviolet • X-rays • Gamma rays

  3. Light Properties • Light is e/m radiation • Light rays travels in a straight line in a medium, according to the wave equation (“c” is speed of light , 3 x 108 m/s) • c = λ*f (wavelength * frequency) • Question • How long (t) does it take for reflected light to reach the earth from the moon, if the moon is 3.84 x 108 m (d) away? • Solve d = v*t for t • 3.84e8 = 3e8 * t • t = 3.84/3 = 1.3 seconds

  4. Practice EM radiation • Microwave ovens emit waves of ~2450 MHz. What is the wavelength (λ) of this radiation? • Solve c = λ*f for λ • 3 x 108 = λ * 2450 x 106 • λ = 3 x 108/2450 x 106 • λ = 0.122 m

  5. Focus on Visible frequencies • Visible spectrum • Red – violet (ROYGBIV) • 700 nm – 400 nm (nm = nanometer 10-9) 700 x 10-9 m 400 x 10-9 m

  6. Light Characteristics • When light encounters an object it can be: • Reflected • Refracted • Diffracted • Diffused • Polarized

  7. Reflection • Fundamental Law of Reflection • Angle of incidence = angle of reflection Light ray Normal • The observer is on the reflected side!

  8. Practice • In a house of mirrors a man stands so that light shining on his face is incident on one mirror at an angle of 50°, as shown. At what angle will this light reflect from the second mirror? • Solution • 40°

  9. Reflection • Consider reflection in 3 different types of mirrors • Plane (flat) • Converging (concave) • Diverging (convex)

  10. Flat (plane) mirror • Image is behind the mirror (virtual)

  11. Basic terminology for curved mirrors • Basic Terminology • f = focal length of (curved) mirror • do = distance from mirror to object (or p) • di = distance from mirror to image (or q) • Mirror Equation

  12. Reflection – Plane mirrors • The image is behind the mirror (virtual) • The image is upright • The image is an equal distance behind the mirror as the object is in front. Image Object Mirror

  13. Reflection - Converging Mirrors • Consider 3 object placements (d0) with respect to the converging mirror center • Object inside the focal length • Object at the focal length • Object outside the focal length 1 3 2 Note: Radius of curvature = 2 x focal length

  14. Reflection – Converging (concave) mirrors Object • Object is positioned between the mirror and the focus (inside) • Image is behind the mirror (virtual), upright and enlarged Image

  15. Reflection– Converging (concave) mirrors • Object is positioned at the focus • Image does not exist due to the parallel nature of the reflected rays – they do NOT intersect. Object

  16. Reflection– Converging (concave) mirrors Object Image • Object is positioned outside the focus • Image is real, inverted and its size will vary depending on the object location

  17. Reflection– Diverging (convex) mirrors • Object is in front of the mirror (do) • Image (di) is always behind the mirror (virtual) • Focus (f) is always behind the mirror Image Object

  18. Convention for Mirror Equation

  19. Magnification • Relationship between the object and image size • M = - di/do (or M = |di/do| • - means image is real (in front of mirror) • + means image is virtual (behind mirror) • M = hi/ho • hi = height of image • ho = height of object

  20. Examples of Mirror Equation • Example: What is the image distance (di) from a diverging mirror, given the focal length is 20 cm and the object distance is 12 cm? What is the magnification (M)? • Solution for di • Refer to diverging mirror characteristics • f = -20 (diverging) • do = 12 cm • di =? • Solve using mirror equation • 1/-20 = 1/12 + 1/di • di = -7.5 cm (image is virtual – behind the mirror) • Solution for M • M = -di/do = -(-7.5/12) = 0.625

  21. Practice • When an object is placed 30 cm in front of a concave mirror, a real image is formed 60 cm from the mirror. What is the focal length? What is the magnification? • Solve the mirror equation for f • 1/f = 1/do + 1/di • 1/f = 1/30 + 1/60 • 1/f = 3/60 • f = 20 cm • Solve M = -di/do for M • M = |-60/30| = 2

  22. Practice • A fortune teller is polishing her crystal ball that is so shiny she can see her reflection when she gazes into it from a distance of 15 cm. • What is the focal length of the crystal ball if she can see her reflection 4 cm behind the surface of the ball? • Solve the mirror equation for f • 1/f = 1/do + 1/di • 1/f = 1/15 + 1/-4 = 1/15 – ¼ = (4–15)/60 = -11/60 • f = -60/11 = -5.5 cm • What is the magnification of the image? • Solve M = -di/do for M • M = -(-4/15) = 4/15 = 0.267

  23. Homework • EM spectrum • 449/1-4 • Reflection • 462/1-4 • 466/1-3,5,6

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