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