What are the effects of "light pollution" on our ability to view the stars? Especially, how are the new city lights helping or hurting the situation? Do sodium vapor lamps really help? What is it about mercury vapor lamps that hurt our ability to see through the atmoshere? How bad is it today from existing observatories to get good viewing conditions as compared to say the last 50 years? Is there any effort to correct this problem? Is there corrective lens that can be used to help as was done with the Hubble telescope? What do you see as the future of land based, visible light telescopes concerning this specific problem?
There are two ways that light pollution interferes with our ability to study the sky. The first is simply that unshielded lights send their light in all directions, including straight up. This sets the sky aglow, in much the same way that the sun sets the sky aglow during the day. Now, the sky does not glow as brightly at night as it does during the day, but the increase in sky glow caused by cities is enough to make it difficult to see dim objects in the sky. When we try to take a picture of a very dim object, sometimes the glow from the sky is too bright to ever see the object clearly. You can help reduce this problem by making sure that lights around your street are shielded so that most of the light is pointed downwards. This is where you want the light anyway, and by shielding the light you will both reduce light pollution and reduce the wattage of bulb required for the same amount of ground lighting, thus saving energy as well!
The second way city lights interfere with astronomy is much more insidious. Often, astronomers want to take the spectra of an object, splitting the light from the telescope into its component colors. When you take a spectrum of fluorescing objects like galaxies, you see that the spectrum is not smooth, but made up of a number of lines. Each line is a unique indicator of the presence of a certain chemical. By studying the strengths of these lines, astronomers can deduce the chemical composition and temperatures of the objects they observe. By noting the redshift of the lines (how far to the red side of the spectrum they are shifted), astronomers can determine how fast the object is moving. Spectroscopy is probably the most valuable tool in the astronomers' toolbox. Unfortunately, city lights play havoc with spectrographs. Have a look at the below image.
This is a spectrogram of a galaxy in the constellation Hercules, taken by the 200 inch telescope at Palomar. Redder colors are on the left, and bluer colors are on the right. The bright horizontal line through the middle is the light of the galaxy we observed that night. The bright vertical lines that you see are not from the galaxy: they are lines from mercury vapor lamps in San Diego! The lines from the galaxy are actually dark against the light of the galaxy, but they are very difficult to find (try it!) because they are lost in the bright lines from the city.
Mercury vapor lamps have an enormous number of these spectral lines in all parts of the spectrum, and interfere with astronomical observations from the infrared to the ultraviolet. This is why astronomers encourage the use of sodium vapor lamps instead. Sodum vapor has only two lines in the entire optical spectrum! So if cities used sodium vapor instead of mercury, we would have a much easier time analyzing astronomical spectra. Already, some parts of the spectrum cannot be observed from palomar, because the mercury vapor is just too bright.
Unfortunately, there is no kind of corrective lens that we can use to filter out all this light pollution. Any filter that we used to filter out the undesirable light would also filter out the light we want to see. The effort has to come from the cities themselves and the voluntary efforts of their citizens to use shielded sodium vapor lights when they live near observatories.
The situation at Palomar now is getting quite grim. Today, the city lights can be directly seen through gaps in the mountains, meaning that city light is making its way directly into the telescopes, without first even being reflected by the sky. Many observers have given up looking at objects in the southwestern sky, because the light pollution is so bad in that direction. The City of San Diego once required the use of sodium vapor lights on city streets to reduce the light pollution at Palomar, but as the city grew, that law was allowed to expire. With the growth of the city and the use of more and more mercury lamps, Palomar is becoming more and more polluted. Some Palomar users estimate that if things keep going the way they have been, then in the next ten years or so, Palomar will be useless for deep-sky astronomy.
There is still hope for land-based astronomy, though. Palomar is hurt particularly bad by the light pollution problem because of its proximity to the heavily populated areas of southern California. Other telescopes, such as the Keck telescopes in Hawaii are faring extremely well. There are no bright city lights near these observatories to get in the way of a good night of observing. Some day in the next couple of decades, Palomar may have to be decommissioned (like the Mount Wilson telescope near Pasadena) in favor of telescopes at more remote sites. Cornell University, for instance, is in the process of building a new telescope far from all civilization in Chile's Atacama Desert. Other telescopes are being built on remote mountaintops, islands, and even the south pole.
Edit by Michael Lam on November 21, 2015: Besides optical telescopes like the ones previously mentioned, radio telescopes suffer the same problem. Instead of optical light sources polluting our view, sources of radio waves, such as cell phones, wireless internet transmitters, GPS satellites, etc., all obscure our view of celestial radio sources. In some cases, they can completely block our ability to observe sources, which is why we try to build radio telescopes in remote locations as well, far away from any interference.