Download Particle Nature of Light Reading

Particle Nature of Light
1)
We have already explained light as a wave. Iron metal is normally gray in color. If you place a piece of iron in a
fire, it glows red hot. As the iron gets hotter, it gains energy and changes color. The wave model of light could not
explain this phenomenon. Scientists had to come up with a different explanation of light.
stove burner exhibiting the
photoelectric effect
2)
In 1900, Max Planck began working on this new explanation of light. When he shined a high frequency light on a
metal surface, he noticed that electrons are given off, or emitted. This phenomenon is called the photoelectric effect.
His conclusion was that matter can gain or lose energy in small, specific amounts called quanta. A quantum is the
minimum amount of energy lost or gained by an atom. Think of it as a packet of energy. He then proposed that the
energy of a quantum is directly related to the frequency with the following equation:
E = h●ƒ
E = Energy of a quantum or photon in Joules (J)
h = Planck’s constant  6.63 x 10-34 J●s
1
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ƒ = frequency in Hertz (Hz or )
In other words, the energy of radiation increases as the frequency increases.
3)
In 1905, Albert Einstein proposed the idea that light behaves as both a wave and a particle. In other words, a
beam of light has many wavelike properties but can also be thought of as a stream of tiny particles, or bundles of light
energy, called photons. A photon is a particle of light with no mass that carries a quantum of energy. The energy of the
photon must have a minimum value in order to release, or eject, the electrons. The energy of a particular photon
depends on the frequency of the radiation. If the frequency is high, then the energy is also high.
Atomic Emission Spectra
4)
Have you ever wondered how a neon sign works? Have you ever been to a 4th of July fireworks celebration? We
can excite atoms by forcing them to absorb energy. Atoms have electrons that exist at normal, or “ground state”.
Electrons are happiest and the most stable closest to the nucleus. After these atoms gain extra energy, the electrons are
said to be in the “excited state.” For example, they will actually jump from energy level 2 to energy level 4 further away
from the nucleus.
Excited neon atoms emit
light when electrons fall
back to the ground state.
5)
Once they are there, they realize that they aren’t supposed to be in such a faraway place and they need to go home.
Whenever an excited hydrogen atom falls back from an excited state to its ground state, the electron drops down to its
lower energy level and emits a photon (packet of light energy) of radiation. When atoms release energy, they each
release a unique “fingerprint” of light that is different from every other element. The further they need to jump, the
more energy that is lost. The color of light emitted is directly related to this distance. This unique fingerprint is called an
atomic emission spectrum – the set of frequencies emitted by atoms of the element. For example, if we hold strontium
nitrate into a direct flame, the strontium atoms release a characteristic red color. Red is equivalent to low energy
meaning that the electrons only jump a short distance between energy levels. These specific wavelengths may be seen
through a special device called a spectroscope, a device used to see each elements unique fingerprint. An atomic
emission spectrum can be used to identify an unknown element. Scientists use spectroscopes to look at galaxies moving
away from us that look red, known as “red-shift.” Galaxies moving toward us look blue because of shorter wavelengths,
called “blue-shift.” Spectroscopes also help us identify the elements on the surface of the sun: hydrogen and helium.
Spectra or “fingerprint” for Hydrogen
Proper use of a spectroscope
Spectra for Calcium’s excited electrons