Light Pollution: Definition, Causes & Effects

Lesson Transcript
Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

Expert Contributor
Mark Wilson

Mark has taught college-level Human Anatomy & Physiology. He has a Doctor of Chiropractic degree and a PhD in Public Health

Back in the day, before the light bulb was ever invented, everybody - no matter where they were - could look up and see the Milky Way and a multitude of stars in the sky. Not any more. Learn why in this lesson. Updated: 12/26/2020

What Is Light Pollution?

Do you live in the city or a suburb? If so, what do you see when you look up at the sky at night? Do you see the Milky Way behind a blanket of stars? Do you see the sky filled with twinkling stars and other celestial lights? The answer, sadly, for most people, is they don't. They may see a handful of bright stars and a few other stars sprinkled in between and maybe a few prominent constellations like the Big Dipper, but they don't see the starry night sky anywhere close to what it really is.

Images of two night skies

Can you see the difference in the two photos? The top picture shows the night sky in a rural setting while the bottom picture shows the night sky in a city. You can clearly see the Milky Way in the top picture while the bottom picture looks like it's day time. This is actually due to all the external lights that are always lit in cities.

This unwanted extra light from artificial sources is referred to as light pollution. Light pollution exists in four different forms:

  1. Glare: This is when a light that is too bright often causes glare, or a visual discomfort, when looking at the light.

  2. Skyglow: this is when too many external artificial lights cause the night sky to glow. You can see this when looking down at a well-lit city. The sky above the city is brightened.

  3. Light trespass: This is when a big, bright light shining on one area usually trespasses and shines on unwanted areas. For example, if you live in the middle of the city and you're trying to go to sleep, the lights coming from outside artificial lights can prevent you from going to sleep because the light trespasses into your room, making it look almost like daytime. And finally,

  4. Clutter: This is when you have too many lights bunched up together or when the lights are grouped in confusing ways, and it can make you dizzy or confused looking at them.

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  • 0:04 What Is Light Pollution?
  • 1:52 Causes of Light Pollution
  • 2:37 Effects of Light Pollution
  • 3:18 Examples of Light Pollution
  • 4:20 Lesson Summary
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Causes of Light Pollution

What causes light pollution is not only the street lights that are always on at night but also lights that people keep on at their homes. For example, some people have security floodlights that come on at night so it's not completely dark around their home. Also, some businesses with large windows purposely leave all their lights on at night. Another source of light pollution comes from advertising lights. Think of a large, well-known store at night, and you'll probably notice that its store name is lit up. Sometimes, these store names also use blinking lights. Still another source is the bright light from stadiums and other sporting locations. If you've seen one at night, then you'll see just how much light these places give off. The light is so bright, it can often be seen from miles away and can be distracting to drivers at night.

Effects of Light Pollution

According to the International Dark-Sky Association, in 2016, 80 percent of the world's population has been affected by skyglow. This unwanted light from artificial sources does negatively impact people. For example, people trying to sleep in areas exposed to a lot of light pollution will find it more difficult. This light pollution creates so much artificial light that it practically tricks your brain into thinking that it is still daytime, disrupting your body's circadian rhythm.

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Additional Activities

Light Pollution Activity:

This lesson discussed a number different sources which light pollution may came from. Our own activities contribute quite a bit to light pollution. Urban areas have streetlights, businesses light up signs as well as lights in their parking lots at night, and porch lights shining at night contribute to light pollution. One of reasons why this is a problem is that it disturbs our normal circadian cycle of sleeping and being awake. A small gland in the brain, called the pineal gland, releases a hormone called melatonin, which helps us sleep. The signal for this hormone to be released is darkness. Before we had electricity and light bulbs, the darkness after sunset was a signal to sleep. Now, we are able to have artificial lighting well after sunset.

For this activity, identify different sources of light pollution and think of ways to help reduce them.

Sources of Light Pollution:

Identify different sources of light pollution around you by the different categories of light pollution

  1. Glare: Are there any bright lights, which are disturbing nearby? Include any sources inside, as well as outside.
  2. Business lights: Do you live near businesses which have lights on at night?
  3. Street lights: Where are the nearest streetlights to where you live?
  4. Floodlights: Are any floodlights present on your residence? When are they turned on?
  5. Stadium lighting: Do you live near a stadium or school with an outdoor playing field? How bright are the lights when they are turned on?

Preventing Light Pollution:

Once you have identified the various sources of light pollution, think of strategies to help decrease light pollution.

Some examples of decreasing light pollution:

  • Window treatment - blinds, curtains, or window shades can help reduce the amount of outdoor light which enters your home.
  • Timers - in order to prevent indoor lights from shining too long, plugging lamps into a timer which shuts the light off at a certain time may help.
  • Motion activated floodlights - rather than having floodlights turned on for a long period of time, a motion sensor will turn the light on only when it is needed.
  • What else can you think of to help reduce the amount of light pollution in your area?

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