The electromagnetic waves constituting light are refracted when crossing the boundary from one transparent medium to another because of their change in speed. Thus, sounds, such as voices and music, can be heard much farther across water at night than in the daytime. The speed of sound waves is greater in warm air than in cold at night, air is cooled at the surface of a lake, and any sound that travels upward is refracted down by the higher layers of air that still remain warm. For example, waves in deep water travel faster than in shallow if an ocean wave approaches a beach obliquely, the part of the wave farther from the beach will move faster than that closer in, and so the wave will swing around until it moves in a direction perpendicular to the shoreline. The reflectivity of a surface material is the fraction of energy of the oncoming wave that is reflected by it.Ģ) refraction: the change in direction of a wave passing from one medium to another caused by its change in speed. Reflection at rough, or irregular, boundaries is diffuse. The angle between the direction of motion of the oncoming wave and a perpendicular to the reflecting surface (angle of incidence) is equal to the angle between the direction of motion of the reflected wave and a perpendicular (angle of reflection). Regular reflection, which follows a simple law, occurs at plane boundaries. At least part of the oncoming wave disturbance remains in the same medium. In particular, our atmosphere is opaque to certain wavelengths (good for us) meaning that they can only be observed from space (expensive for astronomers).ĭue to its wave-like nature, light has three properties when encountering a medium:ġ) reflection: the abrupt change in the direction of propagation of a wave that strikes the boundary between different mediums.

However, note that observing at different wavelengths requires vastly different technology and conditions. There are different physics at different wavelengths, for example, high energy magnetic fields are seen in the x-ray, radiation from heat is seen in the infrared (more at bottom of page).

Our eyes only see over the following range of wavelengths:Īstronomy is a passive science, but observing phenomenon at different wavelengths has several advantages to overcome the lack of making experiments. The whole range of wavelengths is called the electromagnetic spectrum (more at bottom of page).

For example, short wavelengths are high energy gamma-rays and x-rays, long wavelengths are radio waves. The wavelength of the light determines its characteristics. (More on Roemer and Maxwell at bottom of pages) Maxwell (1850’s) showed that light is energy carried in the form of opposite but supporting electric and magnetic fields in the shape of waves, i.e. Roemer (1680’s) was the first to measure the speed of light using Jupiter’s moons -> c=299,790 km/sec or about 185,000 mi/sec How do you transport energy? Answer = wavesĮlectromagnetic Radiation (a.k.a. The law of the conservation of energy means that energy can neither be created or destroyed, only transformed from one form to another. to change the system) and is measured in ergs. Instead, it was defined that energy is a measure of the ability of a physical system to perform work (i.e.

energy does not have size, shape or occupy space.

Huygens (1650’s) was the first to develop the terminology, stating that: It took over 150 years to develop the concept of energy (more at bottom of page). (More on these astronomers at bottom of page) Lockyer (1890’s), discovered an unknown element in the Sun, later named helium. Kirchhoff (1850’s), who developed the three laws of spectroscopic analysis which, in turn, is used to determine the chemical composition of the Sun and stars. The two discoveries combined to produce a new field called spectroscopy, and allowed astronomers to measure the chemical composition of planets and stars for the first time.įraunhofer (1810’s), who magnified the Sun’s spectrum and discovers spectral lines produce a unique chemical fingerprint in the spectrum.

The discovery that elements emit a unique spectrum, i.e.

The invention of the spectroscope, a device that separates white light into component colors called a spectrum (more on spectrum at bottom of page).

There were two major developments which started this process: The 19th century sees a movement from celestial mechanics to understanding the physical nature of the Solar System through passive observations Light and Matter (Optics and Quantum Physics):

De circuli magnitudine inventa lens