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What is light? Baby don’t Hertz me…

The Electromagnetic Spectrum

Unit 10: Light & Lenses

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What is Light? Baby don’t Hertz me!

What is Light?

  • Throughout the ages there has been a great debate on whether light was a particle (like an atom) or a wave
  • This debate raged in the 17th century when Christiaan Huygens, a Dutch physicist, disputed the work of Isaac Newton
  • Huygens thought that light was a wave, and that’s why it could travel through the vacuum of space

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What is Light? Baby don’t Hertz me!

  • Newton claimed light was a particle, and that’s why it bounced off surfaces in straight lines

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  • The scientific community trusted Newton for many years until other scientists, including Max Planck, Albert Einstein, and Niels Bohr, proved that all particles have a wave-nature

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What is Light? Baby don’t Hertz me!

  • Verdict: light is a wave but can act like a particle
    • all particles can act like waves

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What is Light? Baby don’t Hertz me!

What is Light?

  • Light travels in packets of energy called photons
    • Photons give light particle-like qualities
  • λ and f are wave properties but also used to describe light

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Electromagnetic Waves

  • EM waves are transverse waves that can travel without a medium!
    • They can travel through a vacuum (empty space)

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  • EM waves travel VERY FAST – 3x108 m/s (300,000 km/s) 🡪 the speed of light in a vacuum

At this speed they can go around the world 8 times in one second

The wave equation still applies!

v = f λ

3x108 = f λ

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Electromagnetic Spectrum

The range of wavelengths or frequencies over which electromagnetic radiation extends

**Reminder**

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  • Wavelength = the distance between two crests of a wave

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  • Frequency = the number of waves occurring in a unit of time

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From LONGEST to SHORTEST Wavelength

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Radio Waves

  • Uses:
    • TV broadcasting
    • AM and FM broadcast radio
    • Heart rate monitors
    • Cell phone communication
    • MRI (MAGNETIC RESONACE IMAGING)
      • Uses Short wave radio waves with a magnet to create an image

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Radio Observatories/Telescopes

  • Can see incredibly small details and faint signals
  • Can use multiple radio telescopes in conjunction with each other (arrays)

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Microwaves

  • Uses:
    • Microwave ovens
    • Bluetooth headsets
    • Broadband Wireless Internet
    • Radar
    • GPS

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Microwave Observatories/Telescopes

  • Used to study the Big Bang

The Cosmic Background Explorer (COBE)

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Infrared Radiation – Invisible Heat

  • Uses:
    • Night vision goggles
    • Remote controls
    • Heat-seeking missiles

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Infrared Observatories/Telescopes

  • Ground-based infrared telescopes must be at high altitudes to minimize water vaper, which absorbs infrared waves
  • Space-based observatories must be beyond Earth’s atmosphere

James Webb Telescope

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Visible Light

  • Only type of EM wave able to be detected by the human eye
  • Violet is the highest frequency light
  • Red light is the lowest frequency light
  • Roy G. Biv
    • Red, Orange, Yellow, Green, Blue, Indigo, Violet

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ROY G BIV and the Order of the Rainbow

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Visible Light Observatories/Telescopes

  • Light is distorted as it travels through the atmosphere, so visible light telescopes work best on mountaintops and in deserted areas
    • Space-based optical telescopes don’t have this issue

Keck Telescopes

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Ultraviolet

  • Uses:
    • Black lights
    • Security images on money
    • Harmful to living things
      • Used to sterilize medical equipment
      • Too much causes sunburn
      • Extremely high exposure can cause skin cancer

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Ultraviolet Observatories/Telescopes

  • Earth’s atmosphere absorbs ultraviolet light, so ultraviolet telescopes MUST be in space
  • The Hubble Telescope is able to observe both visible and ultraviolet wavelengths

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X-Rays

  • Uses:
    • Medical imaging
    • Airport security
      • Moderate dose can be damaging to cells

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X-Ray Observatories/Telescopes

  • Must be space-based as Earth’s atmosphere absorbs x-rays
  • Have to be very large to capture very small wavelengths

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Gamma Rays

  • Highest energy EM waves
  • Uses
    • Sterilizes medical equipment
    • Cancer treatment to kill cancer cells
      • Kills nearly all living cells

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Gamma Ray Observatories/Telescopes

  • Wavelength is so small, no telescope has been able to focus
  • Generally used to detect gamma rays, then another type of telescope is used to create images

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What the Electromagnetic Spectrum Shows Us

SPIRAL GALAXY M83

Spitzer Space Telescope

Infrared

Hubble Space Telescope

Ultraviolet

Allen Telescope Array

Radio Waves

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A helpful little mnemonic…

  • Raging Martians Invaded Venus Using X-ray Guns
    • Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma

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Mauna Kea Observatory

James Clerk Maxwell Telescope (radio)

Subaru Telescope (infrared)

Keck Telescopes

(visible and infrared)

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Keck Telescope

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Lesson Check

1. Which type of wave has the longest wavelength?

radio waves

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2. Which type of wave has the shortest wavelength?

gamma rays

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3. What are the seven colors of visible light IN ORDER?

red, orange, yellow, green, blue, indigo, violet (roy g biv)

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4. Which type of wave is considered invisible heat?

infrared

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Example

The waves used by a microwave oven to cook food have a frequency of 2.45 gigahertz (2.45 x 109 Hz). Calculate the wavelength of this type of wave.

Givens

v = 3x108 m/s

f = 2.45x109 Hz

v = f λ

3x108 = 2.45x109λ

λ = 0.12 m

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10.1 Lesson Check