Understanding Gold, Copper, and Light Absorption

The color we perceive from an object depends on which wavelengths (colors) of light it reflects or transmits and which ones it absorbs. White light is made up of all the colors of the visible spectrum: Red, Orange, Yellow, Green, Blue, Indigo, and Violet (often remembered with the acronym ROYGBIV).

When white light hits an object, some colors are absorbed by the material, while others are reflected back into our eyes. The color we see is the color (or combination of colors) that are reflected or transmitted.

How Gold and Copper Get Their Yellow Color

Gold and copper are unique among most metals because they are not silver-colored. Their characteristic yellowish or reddish-yellow color comes from how they interact with light.

Most metals reflect a wide range of wavelengths across the visible spectrum, making them appear silvery-white. However, gold and copper preferentially absorb light in certain parts of the spectrum. Specifically, gold and copper absorb light in the blue and violet parts of the spectrum.

Consider white light hitting gold or copper:

• Light in the blue and violet range is significantly absorbed by the metal.
• Light in the green, yellow, orange, and red range is strongly reflected.

When the blue and violet light is removed from white light by absorption, the remaining reflected light is dominated by the longer wavelengths – primarily yellow and some green, orange, and red, resulting in the perceived yellow (or reddish-yellow) color of gold and copper.

Analyzing the Options

The question asks which color gold and copper absorb, leaving yellow light. Based on the explanation above, gold and copper absorb blue and violet light.

• Option 1: green - If green were absorbed, the reflected light would lack green, altering the perceived color, but blue/violet absorption is key to the yellow color.
• Option 2: blue - Absorption of blue (and violet) light leaves the longer wavelengths (green, yellow, orange, red) to be reflected, resulting in a yellow appearance. This aligns with the known properties of gold and copper.
• Option 3: red - If red were absorbed, the object would likely appear greenish or bluish.
• Option 4: orange - If orange were absorbed, the yellow color might be less vibrant, but absorbing blue is the primary reason for the shift from a silvery color to yellow.

Therefore, the absorption of blue (and violet) light is what causes gold and copper to appear yellow.

Conclusion

Gold and copper absorb blue and violet light from the white light spectrum. This absorption causes the light that is reflected back to appear yellow. Thus, blue is one of the colors they absorb that results in their characteristic color.

Revision Table: Gold, Copper, and Color

Review the key concepts about how gold and copper get their color:

• Material: Gold and Copper
• Typical metal color: Silvery
• Actual color of Gold/Copper: Yellow/Reddish-Yellow
• Mechanism: Selective Absorption
• Colors Absorbed: Blue, Violet
• Colors Reflected: Green, Yellow, Orange, Red
• Result: Perception of Yellow

Additional Information: Selective Absorption

Selective absorption is a fundamental concept in understanding the color of many materials, not just metals. When light strikes a substance, electrons in the atoms or molecules of the substance can absorb energy from certain wavelengths of light. The energy of the absorbed light matches the energy difference between electron energy levels.

• If a substance absorbs all wavelengths, it appears black.
• If it absorbs no wavelengths (and reflects all), it appears white or colorless/transparent (depending on form).
• If it absorbs specific wavelengths, it appears the color of the wavelengths that are *not* absorbed, but are reflected or transmitted. For example, a red object absorbs most colors but reflects red light. A blue object absorbs most colors but reflects blue light.
• Gold and copper are examples where the electronic structure leads to significant absorption in the blue-violet range, giving them their unique non-silvery metallic color.