Light as a Wave                                            SPH4U Young Star Cluster NGC 7129

What is Light?                                                 All charged particles have an electric field. When they move, they change the electric field (and create a magnetic field). These field distortions propagate through space as an electromagnetic wave, aka light. Light is a charged particle saying, “Hey, I just moved.”

Electromagnetic Waves The electric and magnetic field distortions are perpendicular to each other and to the direction of propagation:

Wavelength The wavelength is determined by the scale of the charged particle’s displacement: A displacement within an atomic nucleus (10-12 m) will create light with a wavelength of 10-12 m. A displacement within a radio antenna (1 m) will create light with a wavelength of 1 m.

Wavelength The wavelength is determined by the scale of the charged particle’s displacement: A displacement within an atomic nucleus (10-12 m) will create light with a wavelength of 10-12 m. A displacement within a radio antenna (1 m) will create light with a wavelength of 1 m. gamma rays radio waves

Light Production by Wavelength Big Metal Antennae: Radio Vibrating Molecules: Infrared Excited Electrons: Visible Light Chemical Bonds: UV Ionized Electrons: X-rays Atomic Nuclei: Gamma-Rays (Note that there is more energy involved as you go to shorter wavelengths.)

The Electromagnetic Spectrum

The Electromagnetic Spectrum Visible light is a very small part of the spectrum: between 700 nm (red) and 400 nm (violet).

Human Perception of Light The colour detectors on our retinas (cones) are sensitive to red, green, and blue.

Light Speed                                                      Electromagnetic waves travel at: in a vacuum.

Light and Radiation                                                      Astronomical distances are measured in light travel time: Planets/Sun: Light minutes away Nearest Stars: Light years away Nearest Galaxies: Millions of Light years The Edge of the Observable Universe: 14 Billion Light years (light from further away hasn’t had time to reach us yet)

Frequency Frequency is how many crests arrive each second:                                                                                   

The Wave Equation

Example What is the frequency of a light wave with a wavelength of 420 nm?

Example What is the frequency of a light wave with a wavelength of 420 nm?

Example What is the frequency of a light wave with a wavelength of 420 nm?

Blackbody Radiation Objects emit radiation (lose energy from particle motion) at wavelengths related to their temperatures: Blackbody curve

Blackbody Radiation Wien’s Law

What is your peak wavelength?

What is your peak wavelength? Body temperature: 310 K

What is your peak wavelength? Body temperature: 310 K too long to be visible: peak is in the infrared

The light seen here is reflected visible light.

This is the emitted infrared. T (Fahrenheit)

This is the emitted infrared. Cold-blooded

Emitted Energy This light is emitted in all directions: L (Luminosity): total light energy/time (Watts) F (Flux): light energy/time/unit area (Watts per m2)

Emitted Energy This light is emitted in all directions: L (Luminosity): total light energy/time (Watts) F (Flux): light energy/time/unit area (Watts per m2)

Doppler Shift Light also exhibits other wave behaviours, e.g., the Doppler Effect

Doppler Shift For an unmoving source of light, the waves arrive with the same spacing in all directions. The wavelength is simply the space between each wave.

Doppler Shift For a moving source of light the waves in front bunch up – the wavelength gets shorter! Blue light is shorter wavelength: we call this a blue shift.

Doppler Shift For a moving source of light the waves behind spread out – the wavelength gets longer! Red light is longer wavelength: we call this a red shift.

Doppler Shift Red-shifted absorption lines in the spectrum of a galaxy moving away from ours.

(Non-relativistic) Doppler Shift l wavelength of signal f frequency of signal v velocity of recession (away) c speed of signal

Example A source’s blue hydrogen line is shifted from 486.1 nm to 537.4 nm. What is the speed of the source relative to us?

Example A source’s blue hydrogen line is shifted from 486.1 nm to 537.4 nm. What is the speed of the source relative to us?

Example A source’s blue hydrogen line is shifted from 486.1 nm to 537.4 nm. What is the speed of the source relative to us?

Example A source’s blue hydrogen line is shifted from 486.1 nm to 537.4 nm. What is the speed of the source relative to us? [away from us]

More Practice “Light as a Wave Homework Questions”