CK-12-Chemistry Intermediate

5.1. Light http://www.ck12.org

Convert the wavelength to m, then apply the equationc=λνand solve for frequency. Dividing both sides of the
equation byλyields:

ν=

c

λ

Step 2: Calculate.

620 nm×

(

1 m

109 nm

)

= 6. 2 × 10 −^7 m

ν=

c

λ

=

3. 00 × 108 m/s

6. 2 × 10 −^7 m

= 4. 8 × 1014 Hz

Step 3: Think about your result.

The value for the frequency falls within the range for visible light.

Practice Problems

1. What is the frequency of radiation that has a wavelength of 2.7 × 10 −^9 m? In what region of the
   electromagnetic spectrum is this radiation?


2. Calculate the wavelength in nm of visible light that has a frequency of 6.80× 1014 Hz.

Quantum Physics

German physicist, Max Planck (1858-1947), studied the emission of light by hot objects. You have likely seen a
heated metal object glow an orange-red color (Figure5.4).

Classical physics, which explains the behavior of large, everyday objects, predicted that a hot object would emit
electromagnetic energy in a continuous fashion. In other words, every possible wavelength of light could potentially
be emitted. Instead, what Planck found by analyzing the spectra was that the energy of the hot body could only be
lost in small discrete units. Aquantumis the minimum quantity of energy that can either be lost or gained by an
atom.An analogy is that a brick wall can only undergo a change in height by units of one or more bricks, so not
all height changes are possible (assuming the bricks have a uniform size and cannot be broken into smaller pieces).
Planck showed that the amount of radiant energy absorbed or emitted by an object is directly proportional to the
frequency of the radiation.

E=hν

In the equation,Eis the energy, in joules, of a single quantum of radiation,νis the frequency, andhis a fundamental
constant called Planck’s constant. The value of Planck’s constant is 6.626× 10 −^34 J•s. The energy of any system
must increase or decrease in units ofhν. The emission or absorption of a single quantum of low-frequency radiation
will cause less of a change in energy than emission or absorption of a quantum of high-frequency radiation.

The Photoelectric Effect and the Particle Nature of Light

In 1905, Albert Einstein (1879-1955) proposed that light could also be described as quanta of energy that behave as
particles. Aphotonis a particle of electromagnetic radiation that has zero mass and carries a single quantum of