This article was written and reviewed by Serge, MSc. I hold degrees in Plant Biology, Environmental Biology and Biogeochemistry, with research experience in plant physiology, ecosystem science, and field-based environmental studies. Every article on this site is grounded in real academic training and genuine scientific research.
Not all light is equal for plants. A plant sitting under a yellow incandescent bulb grows completely differently from one under a blue-white LED. The color of light hitting a leaf determines which photosynthetic pigments activate, how fast the plant grows, and even the shape and structure it develops.
I find this one of the most visually compelling experiments you can run at home because the results appear within weeks and connect directly to real decisions about grow lights, indoor gardening, and plant placement near windows.
My plant biochemistry training covered photosynthetic pigments in detail. Chlorophyll a absorbs light most efficiently in the red and blue parts of the spectrum. Chlorophyll b extends absorption into different wavelengths. Carotenoids capture additional wavelengths and transfer energy to chlorophyll. The color of light you provide determines how efficiently these pigments work and therefore how fast photosynthesis runs.
This experiment makes that abstract biochemistry visible and measurable.
What You Need
4 identical seedlings in identical pots with identical soil
4 different light sources or colored cellophane to filter white light:
Red cellophane or red LED
Blue cellophane or blue LED
Green cellophane or green LED
White light control, standard daylight LED or bright windowsill
A ruler
A notebook
A dark room or box to isolate each plant under its specific light color
Colored cellophane from a craft shop works perfectly for this experiment. Wrap it around a cardboard box, cut a hole in the top for a standard white bulb, and you have a colored light chamber for under €5.
Setting Up the Experiment
Place each seedling in its own light chamber or under its own colored light source. Keep all other conditions identical — same temperature, same watering schedule, same distance from the light source.
Run lights for 12 hours daily on a consistent timer.
Plant 1
White light (control) Standard daylight LED or bright windowsill. Full spectrum light providing all wavelengths.
Plant 2
Red light Red cellophane over white light or red LED. Red wavelengths around 660 nm drive photosynthesis strongly and promote stem elongation and flowering responses.
Plant 3
Blue light Blue cellophane over white light or blue LED. Blue wavelengths around 450 nm drive compact vegetative growth, stomatal opening, and chlorophyll production.
Plant 4
Green light Green cellophane over white light or green LED. Plants reflect most green light rather than absorbing it. This group tests whether plants can use green light at all and how they grow under predominantly reflected wavelengths.
What to Measure and Record
Measure every 5 days for 4 weeks:
Stem height from soil surface to growing tip
Internode length, distance between leaf attachment points on the stem
Leaf color, dark green, pale green, yellow
Leaf size, measure the largest leaf
Overall plant health and appearance
Photograph all four plants from the same angle weekly.
What Your Results Will Show
White light plant (control):
Grows normally with balanced development. This is your reference point for comparing all other groups.
Red light plant:
Likely shows strong stem elongation but possibly pale, thin leaves. Red light drives photosynthesis efficiently but without blue light the plant lacks the signal to develop compact, thick leaf structure. This is called etiolation-like growth — lots of height, less structural density.
Blue light plant:
Likely shows compact, dark green, dense growth. Blue light promotes chlorophyll production and compact vegetative development. Many professional grow light systems use blue-heavy spectra for vegetative growth phases for exactly this reason.
Green light plant:
This is the most interesting result. Most people expect the green light plant to fail completely since plants reflect green light. In reality plants absorb a small proportion of green light through carotenoids and can survive under green light alone, though growth is significantly slower and leaves often appear pale. The result challenges the common assumption that plants simply cannot use green light at all.
Frequently Asked Questions
What color light is best for plant growth?
Full spectrum white light covering red and blue wavelengths produces the best overall plant growth. Red light around 660 nm drives photosynthesis most efficiently. Blue light around 450 nm promotes compact vegetative development and chlorophyll production. Most quality grow lights combine both spectra for this reason.
Can plants grow under green light?
Yes, slowly. Plants reflect most green light but absorb a small proportion through carotenoid pigments. Growth under green light alone is significantly slower and plants develop pale, thin tissue compared to those under full spectrum or red and blue light. Green light is not useless but it is far less efficient for photosynthesis than red or blue wavelengths.
Does light color affect plant shape?
Yes significantly. Blue light promotes compact, bushy growth with thick dark leaves. Red light without blue promotes elongated stems and thinner leaves. This is why indoor plants grown under warm yellow incandescent bulbs stretch toward windows, they respond to the lack of blue wavelengths by elongating their stems to reach a better light source.
How does light color affect photosynthesis?
Chlorophyll a absorbs red light most efficiently. Chlorophyll b extends absorption into blue wavelengths. Carotenoids absorb additional wavelengths and transfer energy to chlorophyll. Green light is mostly reflected which is why leaves appear green. The efficiency of photosynthesis depends directly on how well the available light wavelengths match the absorption peaks of these pigments.
What experiments can I do with plant lights at home?
Light color comparisons, light intensity comparisons, photoperiod experiments testing how hours of light affect flowering and growth, and grow light comparisons between different bulb types all make excellent home experiments. The light color experiment in this article produces visible, measurable results within 2 to 3 weeks.
See the Biology of Light With Your Own Eyes
Set up your four light chambers this week. Within two weeks you will see measurable differences in growth, leaf color, and plant structure between groups.
The results you observe explain directly why grow light spectrum matters for indoor gardening, why plants stretch toward windows, and why professional growers use specific light recipes for different growth stages.
Understanding light is one of the most practical things you can learn as a grower. This experiment gives you that understanding from direct observation.
