Lecture 1

ELECTROMAGNETIC RADIATION

KEY CONCEPTS

• Visible light is just one form of electromagnetic radiation.
• Light (and other forms of electromagnetic radiation) can be thought of as a wave or as a particle.
• Electromagnetic radiation forms a spectrum ranging from short to long wavelength (or from high to low particle energy).
• Electromagnetic radiation always travels at 300,000 km/sec (186,000 miles/sec).

Light Is A Wave

• A wave a fluctuation of some sort which propagates through space.
• Electromagnetic radiation = fluctuation in the electric and magnetic fields which permeate the universe.

A wave is characterized by two numbers:

• Wavelength (lambda) = distance between two wave crests
• Frequency (f) = number of wave crests which pass every second

The speed with which a wave travels is equal to its wavelength times its frequency (c = lambda times f).

The speed of light is always 300,000 kilometers per second.

The color of visible light is determined by its wavelength:

• Blue = 480 nanometers
• Green = 530 nanometers
• Red = 660 nanometers

Visible light is just one type of electromagnetic radiation. Sorted by wavelength, the various types are:

• gamma rays
• X-rays (0.1 to 10 nanometers)
• ultraviolet light (10 to 400 nanometers)
• visible light (400 to 700 nanometers)
• infrared light (700 to 1 million nanometers)
• radio waves

Light is a Particle

• Particle name = Photon
• Photon energy directly proportional to frequency (E=hf)
• Photon energy inversely proportional to wavelength (E=hc/lambda)

One way of creating light:

Heat a black body. A black body is an object which absorbs all the light which strikes it. (A star is an approximation to a black body).

When a black body is heated, it emits a continuous spectrum of electromagnetic radiation.

• Heat is produced by random motions of atoms and molecules.
• Temperature is a measure of the amount of heat.
• No motion = Absolute Zero temperature

Temperature is measured on the Kelvin scale.

• 0 Kelvin = absolute zero
• 273 Kelvin = freezing point of water
• 373 Kelvin = boiling point of water
• 6000 Kelvin = surface of Sun
• 15,000,000 Kelvin = center of Sun

Important Equation:

For a black body, the wavelength at which the spectrum peaks is given by the following formula:

Lambda = 3,000,000/T

The wavelength Lambda is measured in nanometers; the temperature T is measured in degrees Kelvin.

Examples:

• You : T = 310 Kelvin, Lambda = 9700 nanometers (infrared)
• Sun's surface: T = 6000 Kelvin, Lambda = 500 nanometers (visible)
• Sun's center: T = 15,000,000 Kelvin, Lambda = 0.2 nanometers (X rays)

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