Light and Matter Notes

Light and Matter

• Almost all knowledge of the Universe beyond Earth comes from light.

• Light can tell us about objects in space:

  • Temperature

  • Composition

  • Speeds

  • And more

The Speed of Light is Finite

• Light moves at $300,000 \text{ km/s}$ in a vacuum.

• First measured by Rømer when observing Jupiter’s moons.

• Speed is slower in other materials, e.g., glass.

Light is an Electromagnetic Wave

• Light is a wave of combined electricity and magnetism, called an electromagnetic wave.

• Changing electric and magnetic fields create a self-sustaining electromagnetic wave.

Wave Properties

• Wavelength $\lambda$: length between crests.

• Amplitude: height.

• Frequency $f$: number of waves that pass by each second.

• Period $P$: time to complete one cycle.

Wavelength, Frequency, and Speed

• A long wavelength means low frequency.

• A short wavelength means high frequency.

• The speed of light, $c$, is constant.

• $\text{Speed} = \text{Wavelength} \cdot \text{Frequency}$

• $c = \lambda \cdot f$

Waves in What?

• Water waves, sound waves, and so on, travel in a medium (water, air, …).

• Electromagnetic waves need no medium

• Created by accelerating charged particles:

Mixture of Colors

• Mixed primary PIGMENT colors

  • red+blue = purple

  • blue+yellow = green

  • yellow red = orange

  • red+yellow+blue = black

• Mixed primary LIGHT colours

  • red + blue = magenta

  • blue+green = cyan

  • green + red = yellow

  • red + green + blue = white

Chromatography

• Simple chromatography

Visible Light

• Visible spectrum: small range of wavelengths that humans can see

• Red light = longest wavelength ($\lambda \sim 700 \text{ nm}$)

• Violet light = shortest wavelength ($\lambda \sim 350 \text{ nm}$)

• “Roy G. Biv”

The Electromagnetic Spectrum

• Visible light is just one small part of the electromagnetic spectrum:

  • Gamma rays

  • X-rays

  • UV

  • Visible

  • IR

  • Microwave

  • Radio

  • $\lambda$ increasing

  • $f$ decreasing

Photons

• Photon: particle of light

• Photons with high energy = light with high frequency

• Photons with low energy = light with low frequency.

• $\lambda \times f = c$

  • $\lambda$ = wavelength, $f$ = frequency

  • $c = 3.00 \times 10^8 \text{ m/s}$ = speed of light

• $E = h \times f$ = photon energy

  • $h = 6.626 \times 10^{-34} \text{ joule} \times \text{s}$

The Electromagnetic Spectrum

• Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths.

Lecture Tutorial: EM Spectrum, pp. 47-49

• Work with one or more partners - not alone!

• Get right to work - you have 15 minutes

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one another. Take time to understand it now!!!!

• Come to a consensus answer you all agree on.

• Write clear explanations for your answers.

• If you get stuck or are not sure of your answer, ask another group.

• If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask me for help.

Why is sky blue?

• The blue color of the sky is due to Rayleigh scattering.

• Much of the shorter (blue) wavelength light is scattered by the gas molecules.

• It gets scattered all around the sky.

Why is sunset red?

• At sunset or sunrise, the sunlight we observe has traveled a longer path through the atmosphere than the sunlight we see at noon.

• Therefore, there is more scattering, and nearly all of the light direct from the sun is red.

Additional Notes

• The atmosphere scatters blue light more effectively than red light.

• Hence, mostly red light reaches your eye when you look through a thick slice of atmosphere at the setting Sun.