Wave Behavior and The electromagnetic Spectrum

What is Light?

• We call light “Electromagnetic Radiation”. Or EM for short

• It’s composed of both an electrical wave and a magnetic wave.

Wave or particle?

• Just like Dr. Jekyll and Mr. Hyde, light can be thought of both as a wave and as a particle (called a photon or a quantum).

• In its particle nature, light behaves like a bouncing ball – it can reflect off a surface.

Wave or particle?

• In its wave nature, light can bend, just like when sound waves bend and spread out when passing through an open door. We call this bending refraction and diffraction.

• We’re going to concentrate on light’s wave nature.

First, the terminology

• Wavelength – the distance from one wave top or crest to the next crest

• The waves of light are so small that they’re measured in nanometers (1 x 10-9 meters)

• The symbol for wavelength is λ, the Greek letter “lambda”.

wave crest wave trough node

Describing Light Waves

• 3 things are needed to adequately describe a light wave: – its wavelength (λ) in nanometers – its frequency (f) in “waves per second”, also

known as Hertz (Hz)

– its speed (c) in meters per second.

Wave frequency • Frequency is the number of waves that

pass a given point in 1 second. The unit is also known as a Hertz (Hz), or “cycles per second.”

• Your car’s radio dial is marked in kHz (kiloHertz) for ‘am’ stations and MHz (MegaHertz) for ‘fm’ stations.

• 97.9 FM means 97.9 million waves are being broadcasted each second.

More about Frequency

• The symbol for frequency is the Greek letter “nu” (f).

• Frequency and wavelength are inversely related to each other. As the wavelength gets shorter, more waves pass in 1 second (the frequency increases.)

How are they related? • Frequency and wavelength are inversely

related through the equation:

λ x f = c “lambda times nu = c”

“wavelength times frequency = c” where “c” is the speed of light.

C - the speed of light

• c is the speed of light, a constant in a vacuum or in air.

• c = 3.00 x 108 meters / second (300,000,000 m/s or 300,000 km/s.)

• This ‘c’ is the same thing in Einstein’s famous E = mc2 equation.

Let’s try some calculations

• Example 1: What is the frequency of light, if the wavelength is 500 nanometers (nm)?

• Step 1: convert nanometers to meters.

500 nm = 500 x 10-9 m = 5.00 x 10-7 m

• Step 2: re-arrange the equation

If λ x f = c, then f = c / λ • Step 3: insert the numbers

frequency = 3.00 x 108 m/s

5.00 x 10-7 m = 6.00 x 1014 Hertz

You try this one…

• What is the frequency of light that has a wavelength of 400. nm?

If you answered 7.50 x 1014 Hz, you’re right!

3.00 x 108 m/s / 4.00 x 10-7 m = 7.50 x 1014 Hz

Finding wavelength from frequency

• Finding the wavelength is similar to finding the frequency:

If λ x f = c, then λ = c / f

• What is the wavelength of light that has a frequency of 2.5 x 1014 Hz?

If you said 1.2 x 10-6 m, you were right!

Now convert this answer into nanometers.

If you said 1.2 x 10-6 m = 1200. x 10-9 m, and this is 1200. nm, you were right again.

Types of light & Astronomy

• Low energy radio waves can be used to vibrate the nuclei of atoms.

• If you’ve ever had an MRI (magnetic resonance imaging), radio waves were shot into your body. The atoms of your body responded by giving back a “radar image” of your organs, bones, etc.

• One of only two types of the EM spectrum that can make it through our atmosphere without being absorbed.

The Electromagnetic Spectrum The electromagnetic spectrum represents

the range of energy from low energy, low frequency radio waves with long wavelengths up to high energy, high frequency gamma waves with small wavelengths.

Visible light is a small portion of this spectrum. This is the only part of this energy range that our eyes can detect. What we see is a rainbow of colors.

RedOrangeYellowGreenBlueIndigoViolet

ROY G BIV

Frequency Ranges of Visible Light Red light has a frequency of roughly 4.3 × 1014 Hz, and a wavelength of about

7.0 × 107 m (700nm). Violet light, at the other end of the visible

range, has nearly double the frequency—7.5 × 1014 Hz—and (since the speed of light is the same in either case) just over half the wavelength—

4.0 × 107 m (400nm).

The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about

5.5 x 10-7m (550 nm), in the yellow-green region of the spectrum.

It is no coincidence that this wavelength falls within the range of wavelengths at which the Sun emits most of its electromagnetic energy—our eyes have evolved to take greatest advantage of the available light.

Microwaves and Infrared

• Microwaves and Infrared (IR) light can be used to examine the chemical bonds between atoms in a molecule.

• Lots of chemical reactions also give off IR light – it’s the same as heat!

• A lot of the infrared radiation is absorbed by carbon dioxide (CO2), water (H2O) and methane in our atmosphere.

Chemical bonds

A reaction giving off lots of infrared and visible light.

Ultraviolet light

• Ultraviolet (UV) light can cause lots of chemical reactions.

• 2 chemical reactions that are harmful are the tanning of your skin and the formation of cataracts in the lenses of your eyes.

• Most of this type of radiation is absorbed by (O3) Ozone, oxygen and nitrogen in our atmosphere.

X-rays and Gamma rays

• The last 2 types of light can (again) be used to look into the nuclei of atoms, and to strip electrons from atoms (ionization).

These types of EM rays are absorbed by Nitrogen and Oxygen in our atmosphere

Study Guide page

• Wave Behavior and the Electromagnetic Spectrum.