Transcript Liad Elmelech 7.1-7.3 The Nature of Light, Atomic Spectroscopy

By Liad Elmelech (Mr. Elmelech)
The nature of Light
• Light has properties of both waves and particles
• Light is electromagnetic radiation
• Energy of fluctuating electric and magnetic fields
• Magnetic field - area were magnetic particles
experience a force
• Electric field – are regions were electrically
charged particles experience a force
• Travels at 3.00*10^8 m/s (on reference table)
Amplitude
• Height of crest or depth of trough
• Determines brightness
• Greater amplitude increases intensity
Wavelength (λ)
• Length between crests
• Determines color of visible light
• Colors in order of increasing wavelengths
• Red, orange, yellow, green, blue, indigo, violet
Frequency (v)
• Number of crests that pass through a point at in a given
period of time
• Units: cycles per second (cycle/s or s^-1)
• Hertz(Hz) = 1 cycle/s
Equation for frequency and wavelength of light
c = λv (on reference table)
c is speed of light
Calculate the wavelength, in nm, emitted by a barcode scanner
that has a frequency of 4.62 * 10^14 s^-1.
Nanometer = 10^-9 meters
λ= c/v
λ= (3.00*10^8m/s) / (4.62*10^14/s)
λ= 6.49*10^-7m
λ= 6.49*10^-7m * 1nm/10^-9m = 649nm
In order of decreasing wave lengths / increasing frequency –radio,
microwaves, infrared, visible light, ultraviolet, X-ray ,Gamma Ray
Ultraviolet radiation, X-rays, and gamma rays can damage
biological molecules
http://www.ces.fau.edu/nasa/module-2/radiation-sun.php
Infrared radiation
• Associated with transitions in molecular vibrations
• Can be used to detect presence of different types of bonds
Ultraviolet / visible radiation
• Associated with transitions in electronic energy levels
• Can be used to probe electronic structure
Interference
Constructive interference
• When two waves of equal amplitude are in phase
when they interact a wave with twice the
amplitude results (in phase means that they align
with overlapping crests)
Destructive interference
• If two waves that are out of phase interact the
waves cancel
Photoelectric effect
• Many metals emit electrons when light shines on them
• If frequency is too low than no electrons are emitted
Einstein: “light energy must come in packets”
• E=hv (on reference table)
• E is the amount of energy in a light packet (also
called a photon, or quantum)
• v is frequency
• h is the Plank’s constant
• 6.626*10^-34 Js (on reference table)
• v =λ/ c, therefore, E = hc / λ
Diffraction
• Not purely a wave
• Light is a shower of particles
•
A nitrogen gas laser pulse with a wavelength of 337 nm
contains 3.83 mJ of energy. How many photons does it
contain? (number of photons = Epulse / Ephoton)
λ= 337nm*(10^-9m/1nm) = 3.37*10^-7m
E Photon = hc / λ = ((6.626*10^-34 J*s)(3.00*10^8 m/s)) / (3.37*10^-7m)
=5.8985*10^-19 J
3.83 mJ *(10^-3 J / 1 mJ) = 3.83*10^-3 J
Epulse / Ephoton = (3.83*10^-3) / (5.8985*10^-19)
=6.49*10^15 photons
More about the photoelectric effect - Emissions of electrons from
metal depends on whether a photon has enough energy to dislodge
a single electron. This energy is ionization energy.
• Threshold frequency reached when energy of a photon (hv) =
binding energy(φ)
• hv = φ
• Low frequency light does not eject electrons because no single
photon has enough energy to dislodge
• Energy of a photon that is beyond what is needed to dislodge an
electron is transferred to the electron in the form of kinetic
energy
• KE = hv – φ
Wave – particle duality of light
• Sometimes light behaves like a particle, at other times like a
wave
Atomic Spectroscopy
• The study of electromagnetic radiation absorbed and emitted
by atoms
• When an atom absorbs energy it often reemits that energy as
light
• Different elements emit light of different colors
• Emission spectrum
• A series of lines created by passing light through a
prism
• The emission spectrum of a particular atom is always
the same
• Can be used to identify an element
• Niels Bohr attempted to create a model of the atom that
explains the atomic spectra
No, they travel in
there orbitals