Re: When a beam of light falls on red cellophane (color filter film), why can we see it as red?

The question: "I have a science question about which I am very confused. I shine a beam of white light on a piece of red cellophane (color filter film) in a dark room, and I can see that the piece of cellophane is red instead of a black object by looking at the cellophane itself. I thought the red color filter film was supposed to absorb all the colours of light except red and transmit only the red light. Then why do I still see the red colour of the color filter film? Does the red color filter film also reflect some red light to my eyes as well as transmit some red light?? Kindly enlighten me. Thank you and I hope to hear from you soon."

You have actually answered your question! Yes, some light is reflected to your eyes. All materials other than vacuum reflect a portion of incoming light, and we call this "partial reflection". For optical materials such as glass and filter cellophane (or other filter plastics) the amount reflected is about 4% for each surface. But read on below!

We think of light being partially reflected from surfaces such as your filter's front and back surfaces, and in many cases we can calculate the amount of light that does get reflected by assuming that the reflection occurs at the surfaces. But in reality the reflection occurs throughout the bulk of the filter material. The assumption of reflection at the surfaces is a convenient tool, though, and works for calculations of many everyday phenomena. Richard Feynman puts it this way in his book QED: The Strange Theory of Light and Matter¹, page 101 : "Light is really not affected by surfaces. An incoming photon is scattered by the electrons in the atoms inside the glass, and a new photon comes back up...". And on page 107: "Thus we can get the correct answer for the probability of partial reflection by imagining (falsely) that all reflection comes from only the front and back surfaces...partial reflection is the scattering of light by electrons inside the glass." I recommend the entire book to you, but especially chapter 3. (The book is a popularization of QED [quantum electrodynamics] but is very very good at explaining the basics of QED without doing all the math. If you want the mathematical details you can find technical books about the subject.)

Thus, except for a tiny miniscule of light that might be reflected back to your eyes by the actual tiny layer of filter atoms at the actual front surface, any other light that is reflected back to your eyes will have been selectively absorbed according to "color" by the filter's atoms so that you obtain almost all "red" light. The bulk of the filter absorbs some "colors" as well as reflecting some of the light to your eyes, so the light that does get back to your eyes is the same "color" as what goes through the filter to the other side.

John Link, MadSci Physicist

¹ Feynman, Richard, QED: The Strange Theory of Light and Matter, Princeton University Press, Princeton NJ, 1985 (ISBN 0-691-08388-6).