The term “light pollution” means exactly what it sounds like. It is artificial light that is wasted. It is caused by poorly installed light fixtures, over powered lights such as those typically in a shopping mall parking lot and inefficient lighting fixtures. This wasted light can single-handedly destroy an enjoyable view of the night sky.
The most notable disruption to astronomy that is a result of light pollution is known as “sky glow”. This is the bubble of light that surrounding a city that can typically be seen for dozens of kilometres. It is caused primarily by bad lighting practices such as over powered lights and bad light planning. The effect it has on astronomy is nothing to shrug at. It causes the sky to appear to glow in a way. This light that is wasted upwards washes out all but the brightest objects in the sky. It all comes down to contrast. Astronomical objects are typically faint, so with a sky that is brightened it makes it harder, if not impossible to see certain objects even with large telescopes and proper filters. The easiest way to understand sky glow would be to think of the daytime ironically. The Sun is so bright that it causes the background to fade away. While only the sky is affected, not space, you cannot see through this light to the fainter objects you wish to see.
Another very common and frustrating aspect of light pollution to astronomers is a more localised issue. Street lights, poor yard lighting and a generally uneducated public on this topic causes light to shine directly into the area that is being used to observe the night sky. All of this creates a local yet lesser version of the sky glow effect as well as effects dark adaption. Dark adaption is what it’s called when your eyes adapt to low light levels. When it is dark, your pupils will expand to allow increased amounts of light in, thus making it easier to see after you have been in the dark for a certain amount of time. To achieve full dark adaption takes roughly 20-30 minutes. Now if you introduce say a motion sensor yard light in the neighbour’s yard, and this shines directly over to your yard when a cat moves past it, your pupils will contract because you do not need to gather as much light to see adequately. This takes roughly 0.2-1.3 seconds depending on the degree to which your pupils contract. This effect makes it just as difficult as sky glow to see faint objects, since your eyes are no longer sensitive to faint objects.
These are just a couple of effects of light pollution in astronomy. If you wish to learn more about light pollution please visit The International Dark Sky Association or ask a local astronomy club.