PhysicsSolids and Light – Introduction to Light

Light – Wave or Particle?

Evidence for Particle• “White” light can be separated into

colors, but colors cannot be separated any more.

Newton's Experiment• Colors of light can be mixed to

produce additional colors of light and “white” light

Primary Colors of Light

Light – Wave or Particle?

Evidence for Particle• Light travels in straight lines

(doesn’t bend around corners like sound waves)

• Reflection – Light bounces off an object’s flat surface just like a ball bounces off a wall. Reflected

Light

Incident Light

Angle of Incidence

Angle of Reflection

reflection incidence

Evidence for Waves• Light diffuses outward as it travels

away from the source (like water ripples)

• Refraction – bending of light waves when entering or leaving a medium of different density

Refraction of Light

Light – Wave or Particle?

Light – Wave or Particle?Evidence for Waves• Diffraction – Light bends (slightly)

around objects or when it passes through narrow slits

Diffraction of Water Waves• Interference – “Overlapping” light

waves can add together to create bright bands and cancel out to create dark bands.

Interference of Water Waves

Light - Duality

• Light has characteristics of both waves and particles

• The “wave” model of light is used to describe light propagation (how it moves)

• The “particle” model of light is used to describe how light interacts with matter

• Explains experimental phenomenon like:Blackbody Radiation and The Photoelectric Effect

Wave Characteristics of Light• Light is actually made up of oscillating

electric and magnetic fields. This is why light is referred to as Electromagnetic Waves or Electromagnetic Radiation

• Electromagnetic waves transfers energy from one place to another. When we arrange electromagnetic waves according to the energy they transfer, an Electromagnetic spectrum is produced. EM Spectrum includes: Radio waves, Microwaves, Infrared, Visible (ROYGBIV), Ultraviolet, Soft and Hard X-rays, Gamma Rays and Cosmic Rays.

Wave Characteristics of Light

• Frequency – The number electromagnetic waves that pass a point in one second. Frequency is measured in units called Hertz (Hz).

• The energy the photon transfers, determines the frequency of the EM wave. As the energy of the EM photons increases, the frequency of the EM waves also increases.

Wave Characteristics of Light

• Wavelength – The length of one complete oscillation of an EM wave

Propagation of EM wave• As the energy and frequency of the

EM wave increase, the wavelength decreases.Wavelengths of Various EM waves

Wave Characteristics of Light

• Amplitude - The strength of the electric and magnetic fields making up the EM waves

• The intensity of light is related to the amplitude of the waves. As amplitude increases, intensity of the light also increases.

Speed of Light• Regardless of wavelength or frequency, all

EM waves travel at the same speed• The speed of light is 3.0 x 10 8 m/s• The speed of light is considered to be a

universal constant. Nothing can go faster than the speed of light

• The speed of light(c) is equal to the wavelength() of the wave times its frequency(f)

fc

Planck’s Constant

• Max Planck showed that the energy radiated by an object was related to the frequency of the particles vibration times a constant.

• Planck’s constant was determined to be 6.626 x 10 -34 Js

Energy of a Photon

• Einstein used Planck’s constant to connect the frequency of EM wave to the energy of the photons.

• Energy of Light(Elight) is equal to the frequency(f) of the wave times Planck’s constant(h)

fhElight

Voltage

• The minimum amount of electrical energy needed to light the LED is called the Threshold Voltage

• Threshold Voltage (Vthreshold) times the charge of an electron (e) equals the energy of the light(Elight) emitted by the LED. eVE Thresholdlight