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
  lights 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 Newtons
  lifetime.
• Christian Huygens believed that light was a wave.
• His wave theory also verified the laws of
  reflectionand refraction.

14

• Huygens Principle was not readily accepted
  because all known waves required a medium.
• There was no sign of diffraction of light waves.
• Newtons 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

• Einsteins formula
• h is Plancks 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, gt1 for other media
• Table 22.1 (Pg. 738)

33
Snells Law

• Snells 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 Snells 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.
• ROY G. 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
• Well 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
Huygens 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