Why is a tomato red?

Short answer

A tomato absorbs most of the blue and green part of the white light falling on it and sends back the red part; what reaches your eye is this red-weighted light. An object's color is determined by how much of the light it absorbs, how much it reflects, and how much it transmits through itself. Whether the surface is glossy or matte, meanwhile, changes the direction of this reflection and how we see the color.

When light hits a surface

Three things can happen to light that reaches a surface. Part of it returns from the surface: this is called reflection. Part of it is held inside the material and mostly turns into heat: this is called absorption. Part of it also passes through the material and exits the other side: this is called transmittance. Because energy is neither created nor destroyed, adding up these three shares at each wavelength gives back the entirety of the incoming light.

What matters for color is that these shares change from wavelength to wavelength. A white sheet of paper reflects almost every wavelength at nearly the same rate; that's why it's white. A piece of coal absorbs almost every wavelength; that's why it's black. A tomato, however, behaves selectively: it holds on to some wavelengths and lets others go. In KE-01 we called this kind of selecting selective reflection; here we're making it measurable.

Behind the tomato's selection is a pigment called lycopene. Lycopene is a molecule that holds on to the blue and green part of the spectrum while largely letting long wavelengths go free. Why this molecule absorbs exactly these wavelengths — that is, how the electrons in its chain behave — is the subject of MA-01.

The reflectance curve: a color's fingerprint

Instead of describing a surface's color in words, we can show what percentage of light it reflects at each wavelength with a graph. The horizontal axis holds wavelengths from 400 to 700 nanometers, and the vertical axis holds the reflectance ratio. This graph is called the reflectance curve.

White paper's curve is high and flat from end to end. Black velvet's is low from end to end. A neutral gray is again a flat line, in between the two. A red tomato's curve, though, is characteristic: it runs low in the blue and green region, rises sharply around 600 nanometers, and stays high at the red end. What we call a tomato's "redness" is exactly the location and steepness of this rise. A lemon's curve begins its rise earlier, while passing from green to yellow; a leaf's curve shows a small bump in the green region.

The reflectance curve is the object's own property; it doesn't change no matter what light you view it under. What changes is the light that reaches your eye. The light that reaches your eye is obtained by multiplying the wavelengths contained in the light source by the surface's reflectance curve at each wavelength. A surface cannot reflect a wavelength that isn't in the source. This multiplication rule is also the key to the card's question.

Reflectance curves are measured with devices called spectrophotometers, and they are the language of color quality in industry; how the measurement is done is covered in BI-16, and how two different curves can produce the same color is covered in BI-15.

Transparent, translucent, and opaque

Not every material meets light at its surface. A glass of red fruit juice or a piece of red glass lets most of the light pass through it. The color of these materials is determined not by reflection but by transmittance: blue and green are absorbed inside, and red exits the other side. This is why, when you put a white sheet of paper behind a piece of red glass, the paper looks red.

Opaque materials, on the other hand, don't let light pass through them; they absorb whatever light they don't reflect. But there is a wide translucent region between opaque and transparent, and both a tomato and human skin fall in this region. Light enters up to a few millimeters below the surface, scatters off the structures inside, part of it is absorbed, and part of it exits from a different point. This is why the color of a tomato or a face comes not only from its surface but from within it as well.

This fact has an important consequence for skin tone. Melanin absorbs short wavelengths in particular, and as its amount increases, the skin's reflectance curve drops across the entire visible region. Because of this, the curves of different skin tones can have the same shape but sit at different heights. If a lighting plan or a camera setting is built around only a single curve height, the other tones end up either too bright or too dark; the historical relationship between cameras and this problem is described in FS-09.

Glossy and matte surfaces

On a tomato, where light from a window falls, you see a small white glint. Why is there a white spot on a red fruit? Because two different kinds of reflection come from the surface.

The glint is the part of the light that returns from the surface as if it had struck a mirror, without ever entering the tomato's skin. Because this light never encounters a pigment, it doesn't take on color; if the light source is white, the glint is white too. The tomato's red, on the other hand, comes from light that enters, encounters lycopene, and exits again scattered in every direction. For common transparent materials like glass, specular reflection amounts to about four percent of light striking it head-on, and it rises quickly at oblique angles.

The difference between glossy and matte surfaces arises from the balance of these two reflections. On a glossy surface, specular reflection is concentrated in a single direction: from one angle you see a bright spot, and from other angles the color looks dark and saturated. On a matte surface, specular reflection is also scattered in every direction and spreads over the color like a thin white veil; that's why matte surfaces generally look paler. The same red paint can look dark and deep on a glossy, polished car and lighter on a matte wall.

Example — the tomato under blue light

Imagine you're in a dark room, you turn on a lamp that gives off only pure blue light, and you look at a tomato. Does the tomato look red? Let's apply the multiplication rule. The lamp's light lies only in the blue region. The tomato's reflectance curve is low in exactly this region, because lycopene absorbs blue. The product of the two values comes out very small: almost no light reaches your eye from the tomato. The tomato looks dark, close to black. Perhaps the specular glint on its surface remains as a blue point, because that light returned without ever encountering a pigment.

This small experiment shows most clearly that an object's color doesn't reside in the object alone. The same tomato goes from red to black when you change the light; yet the reflectance curve, the object's own property, hasn't changed at all.

Common misconception

Summary

1. Light falling on a surface is reflected, absorbed, or transmitted; an object's color is determined by how these shares change with wavelength.
2. The reflectance curve is the physical fingerprint of a surface's color; the light that reaches our eyes is the product of the source's light and this curve at each wavelength.
3. On a glossy surface, specular reflection is concentrated in one direction and carries the color of the light, while the color itself comes from diffuse reflection, light that enters and returns scattered; in translucent materials like a tomato or skin, color comes from within as well.

Test yourself

1. (Easy) What is the main pigment that gives a tomato its red color?
2. (Easy) What are the three things that can happen to light falling on a surface?
3. (Medium) What color does a red tomato look under blue light?
4. (Medium) Why is the glint on a red tomato white?
5. (Hard) Why can the same red paint look dark and saturated on a glossy surface and paler on a matte one?
6. (Hard) If different skin tones' reflectance curves can be similar in shape but at different heights, why does a lighting setup built around a single tone cause problems?

Answers

1. Lycopene; it absorbs in the blue and green region and lets long wavelengths go ("When light hits a surface").
2. Reflection, absorption, and transmittance; at each wavelength, these three shares make up the entirety of the incoming light ("When light hits a surface").
3. It looks dark, close to black. The lamp's light is only blue, and the tomato's reflectance curve is low in blue; because the product of the two is very small, almost no light reaches the eye. Only the specular glint on the surface may remain as a blue point ("Example — the tomato under blue light").
4. The glint is the part of the light that returns specularly from the surface without entering the skin; because it doesn't encounter a pigment, it carries the color of the light source, that is, white ("Glossy and matte surfaces").
5. On a glossy surface, specular reflection is concentrated in a single direction and the color looks pure from other angles; on a matte surface, this reflection scatters in every direction and spreads over the color like a thin white veil, fading it ("Glossy and matte surfaces").
6. If the light and camera setting are built around a single curve height, lighter skin tones with higher curves end up too bright and darker skin tones with lower curves end up too dark; the setting must take all tones into account ("Transparent, translucent, and opaque").

Sources

• "Lycopene determination in tomatoes by different spectral techniques (UV-VIS, FTIR and HPLC)," ResearchGate.
• MDPI Applied Sciences, "Current Status of Optical Systems for Measuring Lycopene Content in Fruits: Review," 2021.
• Jacques, S. L., "Optical properties of biological tissues: a review," Physics in Medicine and Biology 58(11): R37–R61, 2013.
• ScienceDirect Topics, "Fresnel Equation — an overview."

Next step

MA-01 goes down to the molecular-level reason for the tomato's selection: it explains energy levels and the relationship between the length of a conjugated chain and the wavelength absorbed, using the example of carrots and tomatoes. BI-05, meanwhile, opens up the other side of light scattering inside a surface: it explains how light scatters differently when it strikes small versus large particles, using the examples of fog and clouds, and showing that the blue of the eye is scattering rather than a pigment.