Light and the Electromagnetic Spectrum

1. Light and the Electromagnetic Spectrum

2. Light as Energy There is much evidence in our world that light is a form of energy.

3. Electromagnetic Spectrum • Electromagnetic waves include visible light and several other types of waves. • Arranged in order, they form the electromagnetic spectrum.

4. Electromagnetic Spectrum • Waves with shorter wavelengths have higher frequencies and greater energies. • Radio waves are the least energetic; gamma waves are the most energetic.

5. Radio Waves • used in TV and radio transmissions • used in communications • microwaves

6. Infrared Waves • produced by the thermal motion of atoms • all matter emits infrared waves • have many commercial uses

7. Visible Light Waves • narrow band • 3.9 × 1014 to 7.7 × 1014 Hz • λ = 770 nm to 390 nm • deep red to deep violet • a continuous spectrum

8. Ultraviolet Waves • greater energy and higher frequency than visible light • three levels

9. X-rays • produced when high-energy electrons strike atoms and suddenly decelerate • penetrate solid matter • medical and industrial diagnostics

10. Gamma Rays • produced by high-energy changes in subatomic particles • stopped only by very thick or dense materials • high doses can cause damage to living things

11. Sources and Propagation of Light

12. Incandescent • Incandescent sources are objects that are heated until they glow. • The frequency and color of the light are related to the object’s temperature.

13. Gas-Discharge • consist of a sealed glass tube containing a gas and fitted with electrodes • current flowing through the tube generates visible light • type of gas determines color of light

14. Gas-Discharge • Fluorescent lights emit UV which strikes phosphors on the inside of the glass tube. • Phosphors glow when struck by high-energy EM radiation.

15. Lasers • light at a single frequency • single, energetic EM wave • extremely intense • many practical uses, but not suitable for area lighting

16. LED’s • light-emitting diodes • solid-state electronic component that emits monochromatic light when a small potential difference is established across it

17. LED’s • wide variety of applications • have become practical for illumination • use low power and are very efficient

18. Cold Light • generate light with minimal heat through chemical reactions • chemiluminescent • bioluminescence—produced by living things • very efficient

19. The Speed of Light • Many have tried to calculate the speed of light. • Galileo • Ole Rømer • Armand Fizeau • Léon Foucault • Albert Michelson

20. The Speed of Light • The currently accepted value for the speed of light is exactly 299,792,458 m/s. • We usually round this to 3.00 × 108 m/s. • This is the speed of light in a vacuum (c).

21. Light Waves • Light travels outward in concentric spherical waves. • Light waves travel at equal speeds through a uniform medium. • plane waves • wave fronts

22. Light Waves • Huygens’s principle postulates how light waves propagate. • wavelets • envelope

23. Light Waves Mathematical Description • The magnitude of the electric field strength (E) and the magnitude of the magnetic field vector (B) both act as sine waves. E = Emax sin ωt B = Bmax sin ωt The electric field and the magnetic field are in phase.

24. Light Waves Mathematical Description • James Clerk Maxwell related electricity, magnetism, and light.

25. Reflection and Mirrors

26. Ray Optics • Light can be regarded as a group of rays. • Light travels in reasonably straight lines. • Reflection: light waves change direction

27. Ray Optics • Diffuse reflection: light waves reflect in random directions • Regular or specular reflection: light waves reflect predictably

28. Ray Optics • normal = perpendicular • angle of incidence (θi) • angle of reflection (θr)

29. Ray Optics Law of Reflection • The incoming ray, the normal, and the reflected ray all lie in the same plane. • The angle of incidence equals the angle of reflection.

30. Albedo • Visible-light albedo is a ratio of the reflected light to the incident light. • All light is reflected: albedo = 1.00 • All light is absorbed: albedo = 0.00

31. Albedo • geometric albedo: sun is directly behind the observer relative to the observed object • bond albedo: no regard to the position of the sun

32. Plane Mirrors • The image we “see” in a mirror is called a virtual image. • In a plane mirror, it appears that the left and right sides are reversed.

33. Plane Mirrors • By using multiple plane mirrors at various angles, we can see multiple images • 90° → 3 images • 60° → 5 images • 45° → 7 images

34. 360° n = - 1 θ Plane Mirrors • The number of images (n) for a given angle θis determined by this formula:

35. Curved Mirrors • concave mirrors • convex mirrors • Spherical concave mirrors produce spherical aberration. • not an issue with parabolic mirrors

36. Concave Mirrors • principal focus or focal point (F) • distance from F to mirror is the focal length (f) • radius of the mirror (R) is important for spherical concave mirrors

37. Concave Mirrors • center of a spherical mirror (C) is the center of the spherical surface • line through F and C intersects mirror at its vertex (V); called the principal or optical axis

38. R f = 2 Concave Mirrors • On a spherical concave mirror, the focus (F) is midway between V and C.

39. Concave Mirrors

40. Concave Mirrors • object distance (dO) is the distance of the object from the mirror • image distance (dI) is the distance of the image from the mirror

41. Concave Mirrors • There are six possible cases with the object located on the optical axis. • A real image is one which can be focused on a screen. • “in front of” the mirror

42. Concave Mirrors • Case 2 (dO > R)

43. Concave Mirrors • Case 4 (f < dO < R)

44. Concave Mirrors • Case 3 (dO = R)

45. Concave Mirrors • Case 1: “infinite” distance from mirror

46. Concave Mirrors • Case 5 (dO = f)

47. Concave Mirrors • Case 6 (dO < f)

48. 1 1 1 + = dO dI f Finding Image Position • The mirror equation: • Distances behind the mirror are assumed to be negative.

49. HI dI = - HO dO Magnification • For all spherical mirrors, the height of the image (HI) relates to the height of the object (HO) by:

50. HI m = HO Magnification • The magnification of the image is the absolute value of the image height to the object height: