Astronomy Homework 3

[QUESTION] [PART A] Study the graph of the intensity of light versus wavelength for continuous spectra, observing how it changes with the temperature of the light bulb. Recall that one of the laws of thermal radiation states that a higher-temperature object emits photons with higher average energy (Wien's law). This law is illustrated by the fact that for a higher temperature object, the graph peaks at __________. [A] A longer wavelength. [B] A shorter wavelength. [C] A higher intensity. [PART B] Click "show" for the emission line spectrum, then click "choose gases" and study the emission line spectrum for neon. The neon "OPEN" sign appears reddish-orange because __________. [A] The yellow and red photons emitted by neon travel much faster than the blue and violet photons and so reach our eyes first. [B] Each yellow and red photon emitted by neon carries more energy than each blue and violet photon emitted. [C] Neon atoms emit many more yellow and red photons than blue and violet photons. [D] Neon atoms emit only yellow and red photons. [PART C] The absorption line spectrum shows what we see when we look at a hot light source (such as a star or light bulb) directly behind a cooler cloud of gas. Suppose instead that we are looking at the gas cloud but the light source is off to the side instead of directly behind it. In that case, the spectrum would __________. [A] Appear as a continuous rainbow of colors. [B] Be an emission line spectrum. [C] Still be an absorption spectrum. [D] Appear completely dark. [PART D] What type of visible light spectrum does the Sun produce? [A] A continuous spectrum. [B] An absorption line spectrum. [C] An emission line spectrum.

[ANSWER] [PART A] [B] A shorter wavelength. ~[EXPLANATION]~ Wien's law states that the thermal radiation from a hotter object peak's at a shorter wavelength. [PART B] [C] Neon atoms emit many more yellow and red photons than blue and violet photons. ~[EXPLANATION]~ The many more lines in the yellow and red parts of the spectrum are what make "pure" neon lights look red or orange. (When you see "neon lights" glowing with other colors (besides reddish-orange), it is generally because they contain additional elements (besides neon) making them glow.) [PART C] [B] Be an emission line spectrum. ~[EXPLANATION]~ Because the cloud absorbs light from the hot source, conservation of energy demands that it must re-emit light with the same total amount of energy. However, this re-emitted light is sent in all directions, not just along the direction from which it originally came. Therefore, if we view the cloud from a location from which we can see only the light that the cloud itself emits, we will see an emission line spectrum. [PART D] [B] An absorption line spectrum. ~[EXPLANATION]~ The Sun can simplistically be pictured as a hot interior light source surrounded by a thin, cooler layer of gas (the Sun's photosphere). The interior produces a continuous spectrum, while the overlying gas acts like a cloud to produce absorption lines.

[QUESTION] Which of the following forms of light can be observed with telescopes at sea level? [-] X rays. [-] Ultraviolet light. [-] Visible light. [-] Infrared light. [-] Radio waves.

[ANSWER] [-] Visible light. [-] Radio waves. [EXPLANATION] Both visible light and radio waves pass almost freely through Earth's atmosphere and therefore are easily observed with ground-based telescopes. The only other light that can be observed with ground-based telescopes is infrared, but it can be detected only at high altitudes (such as mountaintops) and even then only in selected portions of the infrared spectrum.

[QUESTION] Compared to the Sun, a star whose spectrum peaks in the infrared is: [A] Cooler. [B] Hotter. [C] Larger.

[ANSWER] [A] Cooler.

[QUESTION] If you wanted a radio telescope to achieve the same angular resolution as a visible-light telescope, it would need to be: [A] Much larger. [B] Slightly larger. [C] In space.

[ANSWER] [A] Much larger.

[QUESTION] A spectral line that appears at a wavelength of 321 nm in the laboratory appears at a wavelength of 328 nm in the spectrum of a distant object. We say that the object's spectrum is: [A] Redshifted. [B] Blueshifted. [C] Whiteshifted.

[ANSWER] [A] Redshifted.

[QUESTION] How much greater is the light-collecting area of a 6-meter telescope than a 3-meter telescope? [A] Two times. [B] Four times. [C] Six times.

[ANSWER] [B] Four times.

[QUESTION] Suppose that two stars are separated in the sky by 0.1 arc-second. If you look at them with a telescope that has an angular resolution of 0.5 arc-second, what will you see? [A] Two distinct stars. [B] One point of light that is the blurred image of both stars. [C] Nothing at all.

[ANSWER] [B] One point of light that is the blurred image of both stars.

[QUESTION] The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is: [A] Larger. [B] Closer to the stars. [C] Above Earth's atmosphere.

[ANSWER] [C] Above Earth's atmosphere.

[QUESTION] If our eyes were sensitive only to X rays, the world would appear __________. [A] Gray, black, and white like a medical X-ray. [B] Brighter than normal because X rays carry more energy than visible light photons. [C] Dark because X-ray light does not reach Earth's surface. [D] Green, yellow, and orange, because those are the colors of X rays.

[ANSWER] [C] Dark because X-ray light does not reach Earth's surface. [EXPLANATION] Because X rays from the Sun do not reach Earth's surface, eyes that were sensitive only to X rays would have nothing to see.

[QUESTION] The James Webb Space Telescope is designed primarily to observe __________. [A] Gamma rays. [B] Visible light. [C] Infrared light. [D] X-rays. [E] Light of all wavelengths.

[ANSWER] [C] Infrared light. [EXPLANATION] Its location in space allows it to observe infrared wavelengths that do not penetrate our atmosphere to the ground.

[QUESTION] If you see the color red in an X-ray image from the Chandra X-Ray Observatory, it means: [A] The object really is red in color. [B] The red parts are hotter than the bluer parts. [C] It depends: the colors are chosen arbitrarily to represent something about the X rays recorded by the telescope.

[ANSWER] [C] It depends: the colors are chosen arbitrarily to represent something about the X rays recorded by the telescope.

[QUESTION] Why is a sunflower yellow? [A] It emits yellow light. [B] It absorbs yellow light. [C] It reflects yellow light.

[ANSWER] [C] It reflects yellow light.

[QUESTION] Blue light has a higher frequency than red light. Thus, blue light has: [A] Higher energy and shorter wavelength than red light. [B] Higher energy and longer wavelength than red light. [C] Lower energy and shorter wavelength than red light.

[ANSWER] [C] Lower energy and shorter wavelength than red light.

[QUESTION] The twinkling of stars is caused by: [A] Variations in stellar brightness with time. [B] Light pollution. [C] Motion of air in our atmosphere.

[ANSWER] [C] Motion of air in our atmosphere.

[QUESTION] The set of spectral lines that we see in a star's spectrum depends on the star's: [A] Atomic structure. [B] Chemical composition. [C] Rotation rate.

[ANSWER] [C] Rotation rate.

[QUESTION] If you had only one telescope and wanted to take both visible-light and ultraviolet pictures of stars, where should you locate your telescope? [A] On Earth's surface. [B] On a tall mountain. [C] In an airplane. [D] In space.

[ANSWER] [D] In space. [EXPLANATION] While visible light can be observed from the ground, ultraviolet light can be easily observed only from space. Indeed, the capability of observing ultraviolet light is a major advantage of the Hubble Space Telescope over larger ground-based telescopes.