Optics

Figure 1‑14. Light split into component wavelengths with a prism. Credit: Lucas Vieira. Public Domain

The science of optics (light) led to the development of the first telescopes, and advances in optics led to improvements in image quality and resolution (lenses and mirrors), detection of all the wavelengths of electromagnetic radiation (spectrographs), and detection of extremely faint light of distant stars and galaxies (photographic plates and CCD cameras). Modern telescopes are a window to the universe and the foundation of cosmology.

There were many flawed attempts to explain the properties of light in the ancient world. In 1021 AD, The Egyptian scientist, Alhazen, dispelled the many misconceptions about light when he published his Book of Optics. Once his book was published in Europe, his description of the path of light through lenses led to the invention of reading glasses, spyglasses, and telescopes.

In 1704, Isaac Newton published Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light. Newton divided light into its component colors with a prism (Figure 1‑14). He also found that he could recombine the colors in a second prism and form white light again. Thus, he determined that white light is simply the combination of all the colors and black is the absence of light. Newton’s experiments helped him conceive of a better telescope. He concluded that a telescope with a reflecting surface would produce clearer images than a lens telescope because a mirror reflects all colors at the same angle whereas a lens diffracts each color at a different angle; however, Newton’s concept of a reflecting telescope had to wait until the quality of reflecting surfaces improved.

Spectrographs and spectroscopes precisely divide light into its component colors within narrow bands of light. Scientists attach spectrographs or spectroscopes to telescopes to precisely measure light intensity at different wavelengths from a galaxy or star. The elements in a star form a spectral signature that is unique to that star just as a human signature is unique to a person. Scientists study graphs of the intensity of light vs. wavelength to determine the elements in a star, cluster of stars, or galaxy. The shift in the wavelengths of elements reveals the velocity of a star or galaxy with respect to the earth.

Figure 1‑15. Blueshift and redshift of light waves based on the Doppler Effect. Credit Ales Tosovsky. Used here per CC BY-SA 3.0.

Wavelengths are compressed if a star or galaxy is moving toward us, and they become longer if the star or galaxy is moving away from us (Figure 1‑15). This is the Doppler Effect, which also causes a change in pitch in sound waves (change of wavelength). For example, the sound pitch from a train horn changes as it moves towards, past, and away from an observer. When the train is moving towards you, the sound waves have a higher frequency, and when the train is moving away, the sound has a lower frequency and lower pitch. Similarly, when a galaxy is moving toward us, the light is shifted toward a shorter wavelength (blueshift). When the galaxy is moving away from us, there is a shift toward longer wavelengths (redshift). Scientists can precisely calculate the speed of the galaxy based on the shift in the wavelength of light. Observations of receding galaxies led to the discovery of the expanding universe.

Figure 1‑16. Two spectra with adsorption lines. The left side is the light spectrum from the sun, and the right side is the shifted light spectrum from a galaxy due to redshift. Credit: Georg Wiora. Used here per CC BY-SA 3.0

Atoms capture specific wavelengths of electromagnetic radiation that excite the electrons within an atom. Each element absorbs specific frequencies or wavelengths. The light spectrum from stars has black gaps (absorption lines) at certain wavelengths or colors of light (Figure 1‑16) because the gases around stars absorb those frequencies (wavelengths) of light and do not let those frequencies of light pass through. Thus, studying the absorption lines reveals the elements in a star. There are gaps at hundreds of positions in the spectrum of sunlight (Figure 1‑16). In 1925, Cecilia Payne-Gaposchkin studied the absorption lines of the sun and determined that the sun is 98% hydrogen and helium.

Frequencies shift due to the movement of stars and the Doppler Effect, and the positions of the absorption lines shift. Based on laboratory experiments, Scientists know precisely where the absorption lines should be when there is no Doppler Effect. Scientists calculate the speed of a galaxy by looking at the shifted positions of absorption lines from that galaxy. The positions of the absorption lines on the left side of Figure 1‑16 are from the light of the sun. These positions never change because the sun is not moving away from us or toward us. However, if galaxies are moving away from us, then the absorption lines (and other wavelengths) of the light from those galaxies is shifted upward (right side of Figure 1‑16). The absorption lines on the right side of Figure 1‑16 are shifted toward the red, which means that the galaxy is moving away from us. The farther the lines shift upward, the greater the speed at which the galaxy is moving away from us. If the lines shift downward, then that is a blueshift, and the galaxy is moving toward us. One of the great discoveries in the history of science is that galaxies that are farther away from us generally have a higher velocity than galaxies that are closer to us.

Figure 1‑17. Herschel discovers infrared radiation in 1800 by placing a thermometer beyond the visible colors. (Artist unknown)

In 1800, William Herschel accidentally discovered nonvisible light. Herschel divided sunlight into its colors with a prism. He wanted to know the energy in each color, so he placed a thermometer under each color. However, he also placed an extra thermometer beyond the red light, where there was no visible color. Surprisingly, the extra thermometer heated up more than the other thermometers (Figure 1‑17). Herschel realized that the thermometer was receiving energy from an invisible band of light. The light just beyond the red range is near-infrared (NIR) radiation. One year later, Johann Ritter discovered radiation (UV) at the other end of the visible spectrum, beyond the blue. We now know that 60% of the light from the sun is invisible: ultraviolet (UV), near-infrared (NIR), and infrared (IR).

Visible light is just one small part of the electromagnetic spectrum. Microwaves, X-rays, and infrared light are some of the other ranges of electromagnetic radiation, all of which are invisible to the human eye. Visible light waves are in the intermediate part of the electromagnetic spectrum. They are above the light bulb in Figure 1‑18.

Light waves have the same parameters as ocean waves. The distance between wave peaks is the wavelength and the rate that they hit the shore (ocean waves) is the frequency. Short waves in the sea with high frequency are very energetic. The same principle applies to both visible and invisible light.

In the electromagnetic spectrum (Figure 1‑18), there is nothing special about visible light except that the light receptors (cones) in the retina detect it. Although our eyes cannot detect radio waves, X-rays, and other frequencies of invisible light, scientists build telescopes that detect these frequencies, just as infrared goggles detect people in the infrared range at night.

Within the electromagnetic spectrum, X-rays and gamma rays have short wavelengths and high energy and are on the right side of Figure 1‑18. Radio waves and microwaves have the lowest energy and longest wavelengths and are on the left side. Blue light is the highest energy wavelength of visible light, and it is next to UV (ultraviolet) light. Red light is the lowest energy in the visible light spectrum, and it borders the visible light range on the left.

Figure 1‑18. Electromagnetic spectrum. Credit: United States Department of Energy (DOE), Lawrence Berkeley Laboratory, MicroWorlds

Scientists learn about the Milky Way and other galaxies by viewing different ranges of the electromagnetic spectrum. For example, all the images in Figure 1‑19 are looking at the same part of the Milky Way. The reason that the Milky Way appears as a line in some of the images is that it is a disk, and we are within the disk. Other images have greater sensitivity to the gases that surround the Milky Way. Observations from different parts of the electromagnetic spectrum enabled scientists to map the structure of the arms and core of the Milky Way (Figure 1-2).

Figure 1‑19. Plane of the Milky Way seen at different wavelengths (different parts of the electromagnetic spectrum). Credit: NASA Astrophysics Data Facility at Goddard Space Flight Center.

Electromagnetic radiation shifts from high to low energy in an expanding universe because the expanding universe stretches out the waves and makes them longer. The early hot temperatures of the Big Bang produced extremely high energy gamma radiation, which is on the right side of Figure 1‑18. As the universe expanded, the waves expanded with the universe. They became low energy microwave radiation, which is in the left middle of Figure 1‑18.

The light of stars is extremely faint, and the human eye is not designed for the detection of such low light levels. Nineteenth century astronomers attached photographic plates to telescopes, and modern astronomers use CCD cameras to detect the faint light of distant stars and galaxies.

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Orion Nebula star formation region. Credit: ESO/H. Drass/ALMA (ESO/NAOJ/NRAO)/A. Hacar

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