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Chapter 1 Light rays –Laser beam –Ray propagation Waves –Rope waves Wave medium Wave propagation –Other waves –Wave properties –Electromagnetic waves Light waves (EM waves) –Speed of light –Wavelength and frequency –Amplitude The Electromagnetic Spectrum –Visible light –Radio waves –Microwaves –Xrays

Light Rays Rays are a convenient descriptive tool Indicate the direction the light is traveling

Light Rays We only see light when a ray enters our eye

Light Rays: Bending Light rays bend when they pass from one material to another This is the principle behind lenses (chapter 3) Air Glass or water

Light Rays: Reflection Mirror Specular reflectionDiffuse reflection

Waves A wave is a propagating disturbance or oscillation The medium does not move very far, but the wave disturbance does

Rope Waves Disturbance has some speed, but rope segments do NOT move in direction of wave Rope segments move up and down, not in the same direction as the disturbance moves

Rays, Waves and Wavefronts The wavefront is defined as a curve or surface perpendicular (at a 90 o angle) to all the rays The wavefront is defined as a curve or surface perpendicular (at a 90 o angle) to all the rays OR, rays are drawn at right angles to the wavefront OR, rays are drawn at right angles to the wavefront

Wavefront Examples Let’s draw wavefronts for these rays: What type of light source might produce these rays? A laser Two light bulbs

Wave Properties Aperiodic wave Periodic wave

Periodic Wave Properties (Vocabulary) Wavelength (λ) Amplitude Ray: Direction of wave motion

Periodic Wave Properties (Vocabulary) Frequency (f) is the number of oscillations per second at a fixed point in space Frequency (f) is the number of oscillations per second at a fixed point in space Period (T) is the inverse of frequency, is the time it takes for one wavelength to pass a fixed point Period (T) is the inverse of frequency, is the time it takes for one wavelength to pass a fixed point Ray: Direction of wave motion

Clicker Question: Wavelength What is the wavelength of the red wave? A)1m B)2m C)3m meters

Clicker Question: Period meters What is the period of the red wave? A)2 milliseconds B)2 seconds C)2 minutes

Periodic Wave Properties: Speed For periodic waves, we can identify a speed (v) speed = distance/time speed = wavelength/period speed = Wavelength x frequency v = λf

Light Waves: Properties What do these properties mean for light waves? The amplitude of a light wave is related to its brightness Light waves are special. They have a constant speed over all colors, over billions of years, and over all brightness. We use the letter ‘ c ’ to denote the speed of light in a vacuum

Light Waves: Properties Recall: v = λf for light waves we use c = λf and ‘c’ never changes. Thus the wavelength and the frequency of light waves have a fixed relationship: λ = c/f or f = c/λ

Wavelength and Color The wavelength or frequency of light waves is related to the colors we perceive when the waves enter our eyes The wavelengths of light that our eyes and brain “see” are in the nanometer (nm) range. One nanometer is meters

Clicker Question: Wavelengths The wavelength of green light is around 500 nm. How many wavelengths of green light fit into one centimeter (0.4 inches, about a fingertip)? a)20 thousand b)50 thousand c) Two million d)Two billion e)5 billion

Wavelength and Color Light with wavelength of 650 nm appears red when it enters a viewers eye Light with wavelength of 520 nm appears green when it enters a viewers eye Light with wavelength of 470 nm appears blue when it enters a viewers eye

Clicker questions Which of the light waves has the longest wavelength? Which of the light waves is brightest? Which of the light waves has the highest speed in empty space? a) b) c) d) They all have the same speed a) b) c)

Clicker question What does the viewer see when the wave at left with wavelength 650 nm goes by him? a)Red b)Blue c)Green d)White e)Nothing

What happens when two or more waves with different wavelengths reach your eye? Light with both wavelengths 650 nm and 520 nm appears yellow when it enters a viewers eye Light with only wavelength 580 nm also appears yellow when it enters a viewers eye

What is white light? Light which is a mixture of 650, 520 and 470 nm wavelengths (and possibly more wavelengths) appears white when it reaches your eye No single wavelength (mono-chromatic) wave appears white when it reaches your eye!

White Light through a Prism A prism spreads out the over- lapping wavelengths in white light into different spatial locations where they can be seen as colors. 400 nm 700 nm