Ch. 5

Light and Matter Notes

• Light is a wave of energy.

• Light is also composed of particles called photons.

• Each photon of light has a wavelength, frequency, and energy.

• The amount of energy a photon carries is related to its frequency: the higher the frequency, the higher the energy.

• Wavelength and frequency are inversely related.

  • Wavelength is the distance between two identical points on adjacent waves.

  • Frequency is the number of times a wave repeats in one second.

• Electromagnetic radiation is another name for light and encompasses radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

• Matter is composed of atoms.

• Atoms are composed of protons, neutrons, and electrons.

• Protons have a positive charge, electrons have a negative charge, and neutrons have no charge.

• The number of protons in an atom is the atomic number.

• The combined number of protons and neutrons in an atom is the atomic mass number.

• An isotope is a form of an element with the same number of protons but a different number of neutrons.

• Electrons in atoms can only have specific amounts of energy.

• The possible energies of electrons are called energy levels.

• Energy level transitions occur when electrons move between energy levels, either absorbing or emitting energy in the process.

• When an electron moves to a lower energy level, it emits a photon with energy equal to the energy difference between the levels.

• When an electron absorbs a photon, it moves to a higher energy level, with the photon's energy equal to the energy difference between the levels.

• Since each element has a unique set of energy levels, the photons they emit or absorb also have unique wavelengths, creating unique spectral lines for each element.

• These unique spectral lines act as chemical fingerprints that allow scientists to determine the composition of distant objects.

• Thermal radiation is the light emitted by an object due to its temperature.

• All objects emit thermal radiation, and its spectrum depends only on the object's temperature.

• Thermal radiation spectra follow two laws:

  • Hotter objects emit more light at all wavelengths (Stefan-Boltzmann Law): This means a hotter object will be brighter than a cooler object of the same size.

  • Hotter objects emit photons with a higher average energy (Wien's Law): This means the peak wavelength of a hotter object's thermal radiation spectrum will be at a shorter wavelength than a cooler object's.

• The Doppler effect causes the wavelengths of light from moving objects to shift.

  • If an object is moving towards us, its light is blueshifted (shifted to shorter wavelengths).

  • If an object is moving away from us, its light is redshifted (shifted to longer wavelengths).

• Spectral lines are used to measure Doppler shifts, as we know the wavelengths of spectral lines from stationary sources.

Telescopes Notes

• Telescopes are instruments that collect light and allow us to see fainter objects and more detail than the naked eye.

• The two key properties of a telescope are its light-collecting area and its angular resolution.

  • Light-collecting area determines how much light a telescope can collect at one time. A larger light-collecting area means the telescope can see fainter objects.

  • Angular resolution is the smallest angle over which two points can be distinguished. A smaller angular resolution means the telescope can see finer detail.

• There are two basic designs for telescopes: refracting and reflecting.

  • Refracting telescopes use lenses to collect and focus light.

    • A refracting telescope uses a convex lens as its objective to gather and focus light.

  • Reflecting telescopes use mirrors to collect and focus light.

• Reflecting telescopes are more common for research today because they have several advantages over refracting telescopes:

  • Mirrors can be made larger than lenses without distorting the image.

  • Mirrors do not suffer from chromatic aberration, a problem where different colors of light are focused at different points by a lens.

• Telescopes can be designed to observe light across the entire electromagnetic spectrum.

• Interferometry is a technique that allows multiple telescopes to work together to achieve the angular resolution of a much larger telescope.

• Space telescopes have several advantages over ground-based telescopes:

  • They are not affected by Earth's atmosphere, which can blur images and absorb some wavelengths of light.

  • They can observe wavelengths of light that are blocked by Earth's atmosphere.

• Radio telescopes are used to study radio waves from space. They are often very large because radio waves have long wavelengths, which require large dishes to achieve good angular resolution.

• Infrared telescopes are used to study objects that are too cool to emit much visible light, such as planets and dust clouds. They must be cooled to very low temperatures to minimize their own thermal radiation.

• Ultraviolet, X-ray, and gamma-ray telescopes are used to study high-energy phenomena such as supernovae and black holes. These telescopes must be located in space because Earth's atmosphere blocks these wavelengths of light.

Summary

• Light is both a wave and a particle, carrying energy and information about the objects it comes from. By understanding the interactions between light and matter, we can use spectra to determine the composition, temperature, and motion of distant objects. Telescopes allow us to collect more light and see more detail, and telescopes designed for different wavelengths of light have opened new windows on the universe.