U3L1 - Lecture Notes - Light and Quantized Energy

Lesson 1
Light and Quantized Energy

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​Unit 3
Electrons in Atoms

Essential Questions

•How do the wave and particle natures of light compare?

•How is a quantum of energy related to an energy change of matter?

•How do continuous electromagnetic spectra and atomic emission
spectra compare and contrast?

Light and Quantized Energy
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The Atom and Unanswered Questions

• Recall that in Rutherford's model, the atom’s mass is concentrated in the nucleus
and electrons move around it.

• The model doesn’t explain how the electrons were arranged around the nucleus.

• The model doesn’t explain why negatively charged electrons aren’t pulled into
the positively charged nucleus.

• In the early 1900s, 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.

Light and Quantized Energy
Copyright © McGraw-Hill Education

Multiple Choice

True or False: Rutherford's model doesn’t explain how the electrons were arranged around the nucleus.

• True

• False

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 (ν) is the number of waves that pass a given point per second.

• The amplitude is the wave’s height from the origin to a crest.

Light and Quantized Energy
Copyright © McGraw-Hill Education

Multiple Choice

The wavelength of a wave is measured from

• 1. crest to crest

• 1. crest to trough

• 1. origin to crest

• 1. origin to trough

The Wave Nature of Light

• The speed of light (3.00 × 108 m/s) is the product of it’s wavelength and
frequency c = λν.

Light and Quantized Energy
Copyright © McGraw-Hill Education

Multiple Choice

The speed of light is the product of it’s wavelength and __________.

• amplitude

• frequency

• crest

• duration

The Wave Nature of Light

• 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.

Light and Quantized Energy
Copyright © McGraw-Hill Education

The Particle Nature of Light

• The wave model of light cannot explain all of light’s characteristics. Some
examples include:

– Why heated objects emit only certain frequencies of light at a given temperature.

– Why some metals emit electrons when light of a specific frequency shines on them.

Light and Quantized Energy
Copyright © McGraw-Hill Education

Multiple Choice

True of False: Heated objects emit only certain frequencies of light at a given temperature

• True

• False

The Particle Nature of Light

The Quantum Concept

• In 1900, German physicist Max Planck (1858-1947) began searching for an
explanation of this phenomenon as he studied the light emitted by heated
objects.

• Planck’s study led him to a startling conclusion:

–Matter can gain or lose energy only in small, specific amounts called quanta.

–A quantum is the minimum amount of energy that can be gained or lost by an atom.

–Planck’s constant has a value of 6.626 × 10–34 J ● s.

Multiple Choice

Matter can gain or lose energy only in small, specific amounts called ______.

• quanta

• hertz

• millimeters

• ions

The Particle Nature of Light

The Photoelectric Effect

• The photoelectric effect is when electrons are emitted from a metal’s surface
when light of a certain frequency shines on it.

Multiple Choice

The photoelectric effect is when ________ are emitted from a metal’s surface
when light of a certain frequency shines on it.

• protons

• electrons

• neutrons

• ionss

The Particle Nature of Light

Light’s Dual Nature

• Albert Einstein proposed in 1905 that light has a dual nature.

–A beam of light has wavelike and particle-like properties.

–A photon is a particle of electromagnetic radiation with no mass that carries a
quantum of energy.

Multiple Choice

A ______ is a particle of electromagnetic radiation with no mass that carries a
quantum of energy

• proton

• neutron

• photon

• ion

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 return to their stable state by emitting light to release energy.

Light and Quantized Energy
Copyright © McGraw-Hill Education

Multiple Choice

How do the excited Neon atoms of Neon lights return to their stable state

• by emitting matter to release energy.

• by emitting light to gain energy.

• by emitting matter to gain energy.

• by emitting light to release energy.

Atomic Emission Spectra

• 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 emission spectrum is unique.

Light and Quantized Energy
Copyright © McGraw-Hill Education

Multiple Choice

True or False: Each element’s atomic emission spectrum is same.

• True

• False