Section 4.6—Light

Light is Electromagnetic Radiation Electromagnetic energy is energy that has electric and magnetic fields Electromagnetic energy travels in waves at the speed of light “c” (3.0 × 108 m/s)

Types of Electromagnetic Radiation Visible light 400 nm 700 nm 10-12 10-10 10-8 10-6 10-4 10-2 1 102 104 106 Wavelengths (cm) Cosmic Rays Gamma X- Ultraviolet Infrared Microwaves Radio waves Electric Power Notice that LIGHT is only one type of electromagnetic radiation.

Wave Properties—Frequency Frequency () is the number of times a wave completes a cycle in one second (cycles per second is “Hertz” or “Hz” = 1/s) Lower frequency Higher frequency

Wave Properties—Wavelength Wavelength () is the distance from trough to trough of a wave (measured in meters “m” and in metrix prefix units of pm,nm etc.) wavelength

Relationship between wave properties The shorter the wavelength, the higher the frequency The higher the frequency, the higher the energy The speed of light is equal to wavelength (in meters) times frequency (in sec-1 or s-1 or Hz) c =  E= h h is a constant so if  goes up so does E

Visible Range Wavelength increases Frequency decreases Energy decreases Visible light 400 nm 700 nm White light is made of all the colors…a prism can separate white light into a rainbow!

Light is Quantized Light is quantized, which means it comes in packets—you can only have certain amounts of it The “packets” are called “photons” Clicking on the active link above allows you to visualize the photons!! Yeah! 

Section 4.7—Light & Matter

Electrons Absorbing Energy If a Photon collides with an electron on an atom it transfers that energy to the electron. Photons are energy! - +

Electrons Absorbing Energy If a Photon collides with an electron on an atom it transfers that energy to the electron. Photons are energy! Photons can be light energy! So when we shine light on electrons they get excited!!! Notice that the purple light excited it two energy levels. Lower energy, such as blue might only excite it to jump up one level. - + The electron is “excited” to a higher energy level with its newly increased energy from absorbing the photon.

Excitation Excitation Definition: The process of an electron absorbing a photon of light (energy) and being promoted to a higher energy level from its “ground state”

And later… The electron cannot remain in that excited state indefinitely - +

And later… The electron cannot remain in that excited state indefinitely so it falls back down!! - + Energy is released – the electron relaxes

Energy of photon and levels jumped The higher the energy of the photon, the greater the electron jump! A photon of UV light has more energy than a photon of Infrared light The UV photon would cause a higher energy jump (jump up more levels) than the IR photon.

Total energy in = Total energy out However much energy was absorbed must be released again..

Measuring light absorption

Hydrogen Spectrum Hydrogen has a unique spectrum! As do all elements! It is like their fingerprint!!!!  The black bars are the colors that a hydrogen atom absorbs. The other colors pass through the atom un-absorbed.

Ways of producing light Fluorescence: visible light is absorbed and visible light is emitted at the same Phosphorescence: Visible light is absorbed and then a while later is Incandescence: Energy is put in from heat and given off as visible light Chemiluminescence: Energy released during a chemical reaction is absorbed to cause excitation. Relaxation produces visible light Biolouminescence: Chemiluminescence that occurs in a biological organism.