Mini-Unit: Light Day 1 - Notes. What is light? Light can be described as a ray, a wave, and a...
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Transcript of Mini-Unit: Light Day 1 - Notes. What is light? Light can be described as a ray, a wave, and a...
Mini-Unit: Light
Day 1 - Notes
What is light?Light can be described as a
ray, a wave, and a particle.
Light as a Ray: The Ancient Greeks
• Ray: ray/rā/ (noun.) a straight line moving from one point to another
• Pythagoras (yes, the right triangle guy!) proposed that vision came from light rays coming from a person’s eye and focusing on an object.
• Epicurus believed the opposite: objects produce light rays which then travel to the eye.
• Three well known phenomena shown to prove light’s ray-like tendencies are: reflection, refraction, and scattering.
Light as a Ray: Reflection and Scattering
• Light rays strike smooth surfaces, such as a mirror, and bounces off.
• When light strikes a rough surface, light rays are reflected at many different angles because the surface is uneven. (Called scattering)
Light as a Ray: Refraction
• Occurs when a ray of light passes from one transparent medium to another.
• When this happens, light changes speed and the light ray bends.
• The angle at which the light ray bends depends on the material.
Light as a Wave: James Maxwell
• James Maxwell (1860)• Light is
electromagnetic radiation (EMR): radiation made up of electric and magnetic fields. These fields vibrate at right angles to each other.
The Electromagnetic Spectrum
Properties of Waves
• Wave: a vibrating disturbance by which energy is transmitted
• Wavelength: (λ) is the distance between identical points on a wave (ex: one crest to another crest).
• Frequency: (ν) the number of waves that pass through a point in one second
• Amplitude: the height of the wave from mid-line of the wave to crest or trough
• Node: Point of zero amplitude
Properties of Waves
Speed of Waves• EMR travels at the speed of light (c)• The speed is related to the
wavelength and frequency:
c = speed of light (2.998 x 108 m/s)λ = wavelength (m or nm)ν = frequency (s-1 or Hz)
Example: What is the frequency of light with a wavelength of 550nm?
(2.998x108 m/s)= (550nm)(ν)
550nm x 1m = 5.5x10-
7m109 nm
(2.998x108 m/s)= (5.5x10-7m)(ν)
ν = 5.5x1014 /s
Light as a Particle: Max Planck (1900)
• Discovered that atoms and molecules emit energy in discrete quantities called quanta.• To describe this relationship a
formula is used:
E = energy of a vibrating system (J)h = Planck’s constant = 6.626 x 10-34 J·s ν = frequency (s-1) or (Hz)
Example: What is the energy of red light if its frequency is 4.567x1014 s-
1? E = (6.626x10-34Js)(4.567x1014
s-1)
E = 3.026x10-19 J
Important Relationships
Based on the formulas that incorporate energy, frequency, and wavelength:E=hv, energy and frequency are directly relatedc=v, wavelength and frequency are indirectly related