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

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 10 8 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? (2.998x10 8 m/s)= (550nm)(ν) 550nm x 1m = 5.5x10 -7 m 10 9 nm (2.998x10 8 m/s)= (5.5x10 -7 m)(ν) ν = 5.5x10 14 /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 = x J·s ν = frequency (s -1 ) or (Hz)

Example: What is the energy of red light if its frequency is 4.567x10 14 s -1 ? E = (6.626x Js)(4.567x10 14 s -1 ) E = 3.026x J

Important Relationships Based on the formulas that incorporate energy, frequency, and wavelength: E=hv, energy and frequency are directly related c= v, wavelength and frequency are indirectly related