Question

Explain whether a red apple is opaque or transparent and why it appears red.

Answer 1

A red apple is effectively opaque (you cannot see through a whole apple). Its surface and flesh contain cells, air pockets and other structures that strongly scatter and absorb light, so very little directional light is transmitted through it. (If you slice an apple very thinly it can be translucent for some wavelengths, but a whole apple blocks transmission and is not transparent.)

Why it appears red
- The apple’s skin contains pigments (mainly anthocyanins and sometimes carotenoids) that absorb much of the incoming visible light at short and middle wavelengths (blue and green) but absorb less of the long wavelengths (red).
- Light that is not absorbed is scattered by the rough, turbid tissue and returned to your eye. Because mostly the red wavelengths survive this selective absorption+scattering, the reflected light is enriched in red and the fruit looks red.
- You may also notice a white shiny spot on the apple: that is specular reflection from the smooth surface, which reflects the full spectrum and so looks whitish.

In short: a whole red apple is opaque because its tissue scatters and absorbs transmitted light, and it looks red because pigments preferentially absorb other colors and reflect/scatter red light to your eye.