Reflection And Refraction Of Light

1. Reflection And Refraction Of Light Chapter 22

2. Introduction • Light is necessary for life on this planet. • It is our source of energy. • It enables us to see and to communicate.

3. The Nature of Light • Scientists have always been intrigued by light. • Much time was spent arguing over the exact nature of light. • Was it a wave or a particle?

4. The Corpuscular Theory • The Greeks believed that light was made up of particles (corpuscles). • Newton used the corpuscular theory to explain the reflection and refraction of light.

5. The Wave Theory • Christian Huygens believed that light traveled as waves and used this concept to describe many of light’s properties.

6. The Visible Spectrum

7. Thomas Young showed that light exhibits interference. • This gave support to the wave theory.

8. Maxwell proposed that light was an electromagnetic wave.

9. Back to the Corpuscular Theory • Just as scientists began to accept the idea that light was a wave, Max Planck and Albert Einstein returned to the corpuscular theory to explain thermal radiation and the photoelectric effect.

10. Is light a wave ora particle?

11. Today, scientists view light as being both a wave and a particle at the same time.

12. The Nature Of Light • Until the beginning of the 19th century, light was believed to be a particle • Newton strongly believed this • He used the particle theory to explain the laws of reflection and refraction • Most scientists agreed with him B14

13. Another theory was proposed during Newton’s lifetime. • Christian Huygens believed that light was a wave. • His wave theory also verified the laws of reflectionand refraction.

14. Huygen’s Principle was not readily accepted because all known waves required a medium. • There was no sign of diffraction of light waves. • Newton’s explanation was preferred for a century.

15. Demonstrating the Wave Nature of Light • Thomas Young demonstrated the interference of light and gave support to the wave theory • James Maxwell predicted that light was a high frequency electromagnetic wave that traveled at 3 x 108 m/s

16. The Photoelectric Effect • It was discovered that clean metal surfaces emit charges under UV light. • This is called the photoelectric effect.

17. The photoelectric effect could not be explained by the wave theory.

18. Einstein • Einstein explained the photoelectric effect in terms of corpuscles or quanta of energy. • He received the Nobel Prize in 1926.

19. Einstein’s formula: h is Planck’s Constant 6.63 x 10-34 J.s

20. The Photoelectric Effect • In the photoelectric effect: • Photons transfer energy to electrons. • Particle nature • Photons have frequency and wavelength. • Wave nature

21. Measurements Of The Speed Of Light • Galileo was unsuccessful in his attempt to measure the speed of light. • Two observers, with lanterns, in towers five miles apart

22. The Ray Approximation In Geometric Optics • Light travels in a straight line path until it encounters a boundary between two different materials. • Rays approximate beams of light.

23. Wavefronts • A wavefront is a surface passing through the points of a wave that have the same phase and amplitude. • Rays are perpendicular to wavefronts.

24. Spherical Wavefronts • The wavefront produced by a point source is spherical.

25. Reflection • Rays are reflected at the same angle as their incident angle upon the surface. • Reflected rays are parallel to each other. 235

26. Types of Reflection • Specular reflection • Reflection from a smooth surface • Wet highways • Mirrors • Diffuse reflection • Reflection from a rough surface • Dry highways • Paintings 22.2

27. The Law of Reflection • The angle of reflection always equals the angle of incidence. • Red eyes in photographs • retroreflection

28. Refraction • Refraction of light • Light rays bend when they enter a different medium. • Part of the light is reflected from the surface. • The type of medium affects the angle of refraction. 22.7, 238, 22.6a, 29-3

29. Refraction at a Boundary • Wave speed changes after refraction.

30. Reversibility • The paths of light rays are reversible. • Light rays which pass through a slab of transparent material emerge parallel to the original path. 241, 29-4, 29-5, 62, 73

31. The Law Of Refraction • The index of refraction (n) is the ratio of the speed of light in a vacuum to the speed of light in the medium

32. Formulas: • n = 1 for a vacuum, >1 for other media Table 22.1 (Pg. 738)

33. Snell’s Law • Snell’s Law

34. Total Internal Reflection • This only occurs when light tries to go from a medium with a high index of refraction to one with a lower index of refraction. • Example: light going from water to air or glass to air 22.25, 244

35. Total Internal Reflection • At the critical angle (qc), the refracted ray is parallel to the boundary • qr = 90o • qi = qc 27.35

36. Total internal reflection only occurs at angles greater than the critical angle.

37. Applications of Total Internal Reflection • Diamonds • Periscopes • Fiber optics (using lasers) • Communications • Medicine • (Fiberscopes) • Stomach • Colon • Knee joints • Entertainment equipment

38. Dispersion And Prisms • The index of refraction in a medium depends upon the wavelength of light. • Verified in our lab by using Snell’s Law • Greater for red or for violet? 22.13, 246

39. Prisms • A prism can be used to separate white light into the visible spectrum. • ROYG. BIV( ~ 650 nm to 400 nm) 22.15a

40. The angle of deviation (d) is measured between the original path and the new path 22.15a, b

41. A Prism Spectrometer • A prism spectrometer • Used to study wavelengths emitted by a light source • We’ll use one in a later lab. • Elements may be identified by comparing with known elements.

42. Rainbows • When can you see a rainbow? • How must you be positioned with respect to the sun? • How are rainbows formed? • Is it possible to see a complete circle? • Niagara Falls ---Rainbow Falls 242, 22.19, 404, 61

43. Rainbows are formed because of refraction, reflection and dispersion.

44. Huygen’s Principle • Huygens believed that light was composed of waves. • He stated that all points on a given wavefront are sources of new spherical secondary waves.

45. Questions 1 - 4, 7, 9 - 13 Pg. 753