Homework 3.

  1. Thought # 11, p. 104.

    Consulting table 4.1, one finds:


  2. Thought #12, p.104.

    Since visible light cannot penetrate your skin (you make a better door than a window!), it follows that longer wavelength, less energetic radio waves cannot either. But X-rays, as you know from doctor visits, do penetrate your skin.

    Like the Sun's photons hitting the outside of your body can kill your skin cells (you know a sunburn causes dead skin cells to peel away) and can cause skin cancer, any photons that make it into your body can kill cells and cause cancer there, too.

  3. Thought # 15, p. 104.

    Both Jupiter and Mars emit thermal radiation, because they're above absolute zero. But both are too cold to EMIT in the visible range; rather, their emission is in the infrared. (As table 4.1 shows, objects with temperatures in the range 10-1000 K emit in the infrared wavelength range.) So the light we see from these planets has nothing to do with their temperature.

    Why do they look different? They REFLECT visible light from the Sun in different visible wavelengths, because they're made of different materials. The substances of which Mars is made absorb all but reddish wavelengths, so only reddish colors are reflected. Similarly, Jupiter's substances absorb all but yellow, and therefore reflect yellow.

  4. Thought #17, p. 104.

    Assume the Earth, the Sun, and the star are all in a plane. Then, as the Earth circles the Sun, sometimes the Earth will approach the star and sometimes it will move away from it. (If we did not assume all three to lie in a plane, the star could be in the direction of either celestial pole, and the Earth's orbit would never carry it toward or away from the star.) By measuring the amount of Doppler shift in the star's light as the Earth orbited, one could use the last equation on page 101 to figure out the Earth's orbital velocity.

  5. Problem #19, p.104.

    a) RADIUS (or R, same as distance) = 2 times as much (larger),
    so AREA (or A) = 4 times as much (larger),
    BRIGHTNESS (or B) = 1/4 as much (smaller) (FAINTNESS (or F) = 1/B = 4 times as much)

    b) R = 10 times as much (larger),
    so A = 100 times as much (larger),
    so B = 1/100 times as much (smaller) (so F = 100 times as much)

    c) R = 1/2 times as much (smaller),
    so A = 1/4 times as much (smaller),
    so B = 4 times as much (larger) (so F = 1/4 as much)

  6. Name two spectral ranges (besides visible) that make it to the Earth's surface, and two that do not. Why do astronomers put telescopes in Earth orbit?

    Figure 4.5, p.88, illustrates the fact that radiation in some spectral ranges does not penetrate the Earth's atmosphere (the pink arrows don't go all the way to the ground). The three highest energy bands -- gamma, X-ray, and UV -- don't make it all the way down, and neither does IR. The other bands all make it down.

    To make observations in spectral regions that do not penetrate the Earth's atmosphere, high-energy (gamma, X-ray, UV) and IR telescopes are placed in orbit. Further, to make observations in all spectral ranges without atmospheric affects, astronomers also use orbiting telescopes -- e.g., the Hubble Space Telescope takes better images in the visible range than ground based telescopes can.

  7. Review #3, p.128.

    More photons will enter a telescope with a larger aperture. More photons means better resolution, which makes a better telescope.

  8. Review #5, p.128. The eyes have good color resolution, but are not permanent. Photographic plates are permanent, but not good in red and IR (infrared) bands. CCD's are permanent, and handle red and IR bands well, but suffer toward the blue, higher energy end of the EM (elcetromagnetic) spectrum. CCD's are also most easily used with computers, as information stored in CCD's pixels closely matches the structure of computers' processors.

  9. Radio and radar observations are often made with the same antenna, but otherwise they are very different techniques. Compare and contrast radio and radar astronomy in terms of the methods used.

    Radar measurements are called active: signals are sent out, bounced off objects, and their reflections back at Earth are studied. Radio observations, however, are passive: signals emitted by objects themselves are detected at Earth, and studied.