All living things require food and water. For plants, light is food. They use it in an important process known as photosynthesis, in which the energy of light is captured by chloroplasts, sparking multiple metabolic responses-- one of these being creating sugars (food) for plants. Sugars fuel plant growth, so the more light a plant is exposed to, the more energy it will make and the faster it will grow.
Know these pairs:
These traits of light are essential in knowing how and why a plant will behave in your home.
Quality is based on the color and type of light. Light is energy that comes in varying wavelengths, each with a corresponding color. Colors we can see with our own eyes may not be helpful for plants, because they 'see' light differently than we do. Plants look green because they reflect green light, so green light is useless to plants. Rather, plants need light they can absorb and utilize like yellow, orange, red, blue and violet, in addition to invisible light like UV light from the sun and some infrared.
Quantity of light is based on the intensity or the brightness of light that hits the leaves. The more light photons that hit the leaf = the more energy captured and faster growth. Begonias or Oxalis, for example, depend on strong light to maintain their fast-paced growth. Any type of plant that makes flowers or fruits depends on intense light too. These plants are working with basic ingredients like water, CO2, sugars and nutrients that are chemically constructed into complex molecules, like flower pigments, but only when the ideal light intensity conditions are met. Some plants have self-regulating systems and will even refuse to blossom or will try to but fall short halfway through the process if there isn't sufficient intense light.
In and Out
Outdoors, even in the shade, light is bouncing from all angles-- from 360 degrees around and from the 180 degree arc above in the sky. When a plant is inside, light typically only comes from one source, like your sunny window, greatly reducing the angles light is bouncing off from, and the amount of light and vital photons a plant needs. When we bring a plant indoors, we literally invoke something called exponential reduction in photon exposure. The poetic quote above helps us remember this fact a little more easily.
You may have heard the terms "bright light" and "low light" plants, but what does that really mean?
"Bright light" or "full sun" means there is no obstruction (drapes or blinds, a tall tree or building that produces shade) between the plant and the source of light (a sunny window). This is where your plant will get the most bright or direct light while indoors. Ficus, succulents and Monstera are sun worshipping plants and should be positioned directly in or no more than 2-3 feet from a window. Typically speaking, you would want to put them in the brightest spot in the room.
"Medium light" or "filtered sunlight" is light that's been diffused (sheer curtains) between the plant and the source of light (a sunny window). Some refer to it as "dappled sunlight". Anything partially obstructing the path between your plant and the source of light creates this medium light. Ferns and aroid plants (ZZ and Philodendron) have evolved to survive on the forest floor, so they are used to being shaded from the sun. They have not evolved to manage the strong rays of direct sunlight so they prefer medium light conditions.
"Low light" means no direct sunlight will reach your plant. It is probably a few feet away from your source of light (sunny window) or any kind of space where it can see outside but can not see the sky. Low light means less energy and also less food. Some plants can survive in low light conditions but they will not thrive.
Keep in mind that the sun changes places in the sky depending on time of day and season, affecting how much light your plant will receive. Monitor how the light changes throughout the year and change your plants placement as necessary.