Light

Early Ideas About Light Early Greeks thought light came from eye Newton said it was made of particles Huygens said it was a wave Wave-particle debate continued Discovery of light interference in 1801 seemed to confirm wave theory

The Photoelectric Effect Hertz and others noticed electrons are emitted by metal if light was shining on it

The Photoelectric Effect Energy of the emitted electrons didn ’ t depend on intensity of light but on its frequency If frequency too low, no emission occurs no matter how bright the light Could not be explained by wave theory, only particle theory Explained by Einstein (1905)

Modern Theory of Light Light has dual nature, with both wave and particle properties Contains stream of particles called photons, each having energy depending on the frequency of the light wave Experiments can reveal wave or particle nature but never both at same time

Speed of Light Ancients thought light instantaneous Galileo realized it had very fast, but finite speed First measured by Roemer in 1675 by timing orbit of Jupiter ’ s moon, Io Michelson first to measure accurately in 1880

Speed of Light Accepted value is now 299,792,458 m/s ; we will use 3.00 x 10 8 m/s Symbol for speed of light is c Light takes about 8 min to arrive from the sun and 4 years from nearest star Interstellar distances measured in light years, the distance light travels in one year

Electromagnetic Waves Light is energy emitted by vibrating electrons Travels in wave with both electric and magnetic components Visible light is only small portion of broad spectrum of electromagnetic radiation All e-m waves travel at same speed, c

Electromagnetic Spectrum

Light Sources Sources of light are luminous: the sun, stars, light bulbs, candle flames, etc. Objects that reflect light falling on them are illuminated: the moon, the planets, anything we can see that is not luminous

Light Emission Atoms absorb energy from heat or electricity and electrons move to higher energy levels Atom is said to be in an excited state and is unstable Electrons fall back to lower energy levels and emit a packet of light wave energy called a photon Light frequency (and color) depends on amount of energy lost by electron

Continuous Spectrum When atoms are crowded together in a solid or dense gas, available energy levels are so numerous, all light frequencies are emitted White light is seen and when dispersed, all colors are seen

Continuous Spectrum

Atomic Spectra When atoms are not crowded, (low pressure gas), only certain energy levels are available for electrons These energy levels are different for each element Spectroscope: instrument for measuring wavelengths of light emissions

Atomic Spectra Dispersion of the light through prism or diffraction grating reveals a spectrum of discrete lines, a characteristic pattern of colors emitted when atoms excited by heat or electricity Pattern of colored lines is unique to element Called bright line spectrum, can identify elements

Atomic Spectra Examples

Measurement of Light Luminous flux is the rate light energy is emitted by an object, measured in lumens. Illuminance is the rate that light energy falls on a surface, measured in lux (lumens per square meter) Illuminance varies inversely with the square of the distance from the light source

Transparent Materials Transparent materials allow visible light to pass through Energy passed from atom to atom and emitted out other side Slows passage of light through material In water, light travels at 3/4 c in glass at 2/3 c

Transparent Materials Ultraviolet (uv) light causes resonance in glass blocking transmission Infrared (ir) waves vibrate overall structure of glass so it is blocked also Glass is transparent to visible light, opaque to uv and ir waves

Opaque Materials Opaque materials will block transmission of light or other e-m radiation Light either absorbed or reflected Due to free electrons, metals re-emit light and appear shiny Atmosphere is transparent to visible light, ir and low energy uv but opaque to high energy uv, X-ray and gamma ray

Shadows Where light rays don ’ t reach behind opaque objects Sharp shadows caused by small light source (or very distant) Large light source creates shadow with dark central region, the umbra, and surrounding gray region, the penumbra

Shadows From penumbra, part of light source is visible In partial solar eclipse, observer is in penumbra of moon ’ s shadow

Polarization Ordinary light vibrates in all directions Polarized light all vibrates in same plane Usually caused by filters that absorb components of light perpendicular to polarization axis, transmit component parallel to axis

Polarization Two polarizing filters with axes at right angles will block all light transmission Reflection will also polarize light, reflecting mostly light polarized parallel to reflecting surface - glare Polarizing sunglasses have vertical filters that block reflected glare

Polarization by Reflection

Scattering Light is scattered by small particles or molecules with which it interacts High frequency (blue, violet) light interacts and is scattered by air molecules -blue sky Dust particles and water droplets scatter lower frequencies also - white color of clouds and dusty sky

Scattering Sunset and sunrises scatter more blue light because more atmosphere to go through - red-orange sky, yellow sun Light is partially polarized by scattering

Dispersion Dispersion is spreading out wave according to frequency/wavelength Prisms and water disperse light into rainbow Different wavelengths move at different speeds, short waves slowed more and are refracted more