Light

Review Question What are Newton’s laws of motion?

Review Question How did Newton know that there had to be a force acting on the Moon to keep it in its orbit about the Earth?

Review Question What is Newton’s law of gravity?

Review Question How did Newton know that the force of gravity gets weaker with distance?

Review Question Why are there two high tides each day on Earth?

Review Question What is tidal friction?

Review Question Explain how tidal friction is responsible for the synchronous rotation of the Moon?

Newton and Kepler Newton’s laws of motion combined with Kepler’s laws led directly to the formulation of the universal law of gravity. Using Newton’s laws and the law a gravity you can derive Kepler’s laws of planetary motion. Planetary motion is thus just a special case of ordinary Newtonian motion.

Kepler’s 1st law The planets move in ellipses with the Sun at one focus. Conic section orbits are a natural outcome of the 1/r2 nature of the gravitational force.

Kepler’s 2nd law A line drawn from the planet to the Sun sweeps out equal areas in equal intervals of time. Just another way of saying angular momentum is conserved which comes from Newton’s 2nd law of motion.

Kepler’s 3rd law The square of the sidereal period is proportional to the cube of the semimajor axis of the orbit: p2  a3 Newton’s laws require:

The Scientific method at work Use the simplest model to explain the observations Refine the model, make it more complicated, only if new observations require Explain why the model works

Explanation vs. usefulness Kepler’s laws were adopted because they were useful for predicting planetary motions. Newton’s laws explained why Kepler’s laws worked in a broader context.

Gravity That one body may act upon another at a distance through a vacuum, … and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has … a competent faculty in thinking, can ever fall into it. Isaac Newton 1692

Discovery of Neptune Saturn and Uranus did not follow Kepler’s laws exactly Le Verrier and Adams use these deviations to predict the existence and position of Neptune which was observed in 1846

Light The primary source of information we have about the solar system comes from the light received from space.

Does light travel instantaneously from one place to another? Galileo unsuccessfully tried to measure speed of light between two mountains using lanterns.

Speed of Light Olaus Romer – in 1676 noticed that the timing of eclipses of the moons of Jupiter was dependant on the relative positions of Jupiter and the Earth. The eclipse of a moon of Jupiter was 16.6 minutes later when Jupiter was near conjunction than when Jupiter was near opposition.

Discussion Why would the eclipses happen later when Jupiter is at conjunction, then they did when Jupiter was at opposition?

Romer’s Speed of Light Determination

What is the Nature of Light? Light is a form of electromagnetic radiation, i.e. light is oscillating electric and magnetic fields. An oscillating electric field produces an oscillating magnetic field and an oscillating magnetic field produces an oscillating electric field.

Electric and Magnetic Waves

Water Waves

Frequency Wavelength Relation

Electric Field Waves

Frequency and wavelength Because the speed of light is constant, higher frequency electromagnetic waves have shorter wavelengths and lower frequency electromagnetic waves of longer wavelengths.

We see variations in wavelength or frequency as color White light contains all colors of the rainbow

The energy of electromagnetic waves. E = h  f = h  c/ h is a constant, f is the frequency is the wavelength, and c is the speed of light The higher the frequency, or the smaller the wavelength, the higher the energy.

The Electromagnetic Spectrum

Light interacts with matter in four general ways Emission Absorption Transmission Reflection

Discussion In the movie Superman, the caped wonder claims to be able to tell the color of Lois Lane’s underwear with his X-ray vision. Is this possible, or did Superman rifle though her dresser drawers when she wasn’t home?

There are two types of emission Blackbody or thermal emission which produces a continuous spectrum, i.e. all colors of the rainbow. Line emission which produces only certain wavelengths of light.

Continuous, Absorption and Emission Spectra

Discussion All objects emit thermal (blackbody) radiation. Why? Things to consider: Of what is matter composed? What is electromagnetic radiation? What does it mean to say an object is hot?

Atom

Temperature

Properties of thermal radiation Blackbodies emit radiation at all frequencies. Thus, they emit a continuous spectrum, all the colors of the rainbow.

Properties of thermal radiation 2) Wien’s law – the temperature of the object is directly proportional to the frequency of maximum emission.

Discussion Why do you think the frequency of maximum emission increases with the temperature of the object?

Properties of thermal radiation 3) Stefan-Boltzmann law – the intensity of the emission is proportional to the 4th power of the temperature.

Thermal Spectra

Infrared Image

Line emission

Continuous, Absorption and Emission Spectra

Ionized Hydrogen gas

Discussion An excited electron will drop rapidly back down to ground state, emitting a photon. If the photon absorbed is the same wavelength as the photon that is emitted, why do we see dark absorption line features in the spectrum?

Discussion The Sun emits radiation like a blackbody. However it has an atmosphere of cooler gas surrounding it. What type of spectrum would you expect the Sun to have?

Solar Spectrum

Discussion The Sun emits radiation like a blackbody. However it has an atmosphere of cooler gas surrounding it. What type of spectrum would you expect the Sun to have?

Iron Absorption lines in the Solar Spectrum

Discussion Explain the difference between how thermal or blackbody radiation is created and how line emission spectrum is created.

Molecular emission and absorption Photons can interact with molecules as well as electrons, causing them to vibrate and/or rotate. Most of these molecular emission/absorption bands are lower energy and are thus in the infrared or radio part of the spectrum.

Discussion How does a microwave heat you food?

Discussion What causes the colors of objects in this room?

Colors of objects All objects both absorb and reflect some of the light that hits them. If an object absorbs red and reflects blue it will appear blue, the color of the object being the color that is least absorbed and best reflected.

Spectrum from Mars

Difference between a Mirror and Scattered Reflection

Discussion A mirror consists of a glass substrate onto which a thin layer of metal is deposited. Why do you think metals are good reflectors of light?

Atmospheric scattering Nitrogen molecules in Earth’s atmosphere scatter blue light more effectively than red, so the sky appears blue.

Discussion Why does the sun appear reddish at sunrise or sunset?

Pleiades reflection nebula

Trifid nebula (M20) in Sagittarius