Download Light and Quantized Energy

Learning Targets:
1. Compare wave and particle nature of light.
2. Define a quantum of energy, and explain how it is
related to an energy change of matter.
Main Idea
• Light, a form of electromagnetic radiation, has
characteristics of both wave and a particle.
• Rutherford’s model: the atom’s mass is concentrated in the
nucleus and electrons move around it.
• Drawbacks:
– Does not explain how the electrons were arranged
around the nucleus.
– Does not explain why negatively charged electrons
aren’t pulled into the positively charged nucleus.
• In the early 1900’s, scientists observed certain elements
emitted visible light when heated in a flame.
• Analysis of the emitted light revealed that an element’s
chemical behavior is related to the arrangement of the
electrons in its atoms.
The wave nature of light
• Visible light is a type of electromagnetic radiation, a form
of energy that exhibits wave-like behavior as it travels
through space.
• All waves can be described by several characteristics:
• The wavelength (λ) is the shortest distance between
equivalent points on a continuous wave.
• The frequency (v) is the number of waves that pass a given
point per second.
– Unit: Hz or sec-1
• The amplitude is the wave’s height from the origin to a
crest.
• The speed of light (3.00 x 108 m/s) is the
product of it’s wavelength and frequency,
c = λv
• Sunlight contains a continuous range of wavelengths and
frequencies.
• A prism separates sunlight into a continuous spectrum of
colors.
• The electromagnetic spectrum includes all forms of
electromagnetic radiation.
Shortcomings of the wave model
• Fails to explain why some objects emit only certain
frequencies of light at a given temperature.
• In other words, why some metals emit electrons when light
of a specific frequency shines on them – emissions of
different wavelengths.
The Particle Nature of Light
• Planck’s Conclusion:
matter can gain or lose energy in small, specific amounts called
quanta.
– Quantum: the minimum amount of energy that can be gained
or lost by an atom
• Energy emitted by hot objects was quantized
Photoelectric effect
• The photoelectric effect is when electrons are emitted from
a metal’s surface when light of a certain frequency shines on
it.
– For a given frequency, v, matter can emit or
absorb energy only in whole number multiples.
• Albert Einstein proposed in 1905 that light has a dual nature.
• Light’s dual nature: a beam of light has a wavelike and
particlelike properties.
– Think of it as bundle of energy called photons.
– Photon: massless particle that carries a quantum of energy
Practice…
• Identify the type of radiations for the following:
1. 6.32 x 1020 s-1
2. 9.50 x 1013 Hz
3. 7.230 x 1014 s-1
4. 8.72 x 10-2 m
Ans:
1. gamma radiation
2. infrared radiation
3. Violet
4. microwave
Atomic emission spectra
• Light in a neon sign is produced when electricity is passed
through a tube filled with neon gas and excites the neon
atoms.
• The excited atoms emit light to release energy.
• The atomic emission spectrum of an element is the set of
frequencies of the electromagnetic waves emitted by the
atoms of the element.
• Each element’s atomic spectrum is unique.