Intro

Explore the visible light spectrum and simple colour filtration as you decode a secret message with some coloured cellophane “science goggles”!

Volunteers: 2-4

1: to discuss the procedure/background science.
1-3: to hands-on assist with the procedure.

Equipment

• Crayons (all colours)
• Flashlights (every student will need one)
• Tape
• Scissors
• **Garbage bags, paper towels/surface cleaner (or other safe disposal method)
• ***Room that can be darkened

Materials

• Cellophane (yellow, blue, red, purple, and orange)
• Cardstock (or firmer construction paper, to mount the cellophane on)
• Poster board (must be white, having multiple is a good idea)

Procedure

1. Prepare your cellophane filters before your event (steps 2-6)
2. Cut out circular pieces (big enough to cover the front of the flashlight) of all 5 colours of cellophane, make multiple of each colour
3. Mount these circular cut-outs on a border or frame of firmer cardstock, something that will hold the cellophane flat and give the students something to hold onto
4. Alternatively you may make cellophane “goggles” by mounting two cellophane pieces onto makeshift cardstock glasses
5. Put together multiple kits or packages of all 5 coloured filters
6. Students will experiment with all 5 coloured filters, but red will be the one that reveals the secret message (you may make another colour be the one that works, you will just need to draw your secret message correctly, using the information provided below)
7. Prepare you secret message poster board before your event (steps 8-11)
8. Using a white piece of poster board, write your secret message and the additional distraction letters/lines in different colours of crayon, according to the instructions below
9. The red cellophane filter will not let purple/blue/green/brown through, these colours will appear dark behind the red cellophane filter (so the secret message should be written, creatively hidden, in these colours)
10. The red cellophane filter will let red/pink/orange/yellow colours through, these colours will seem to disappear behind the red cellophane filter (so the poster board should have distractor letters/lines drawn in these colours, jumbling up the secret message to the naked eye)
11. Make multiple secret message poster boards, with different secret messages (and possibly different levels of difficulty) to post on the wall around a darkened room
12. EVENT DAY: students will be given a flashlight and a kit of 5 differently coloured cellophane filters
13. With help from volunteers (to avoid stumbling or injuries) students will go around to the various poster boards and experiment with their 5 coloured cellophane filters until discovering red reveals the secret message(s)
14. To use a mounted filter all a student must do is hold the mounted filter in front of the flashlight (so that all light coming from the flashlight passes through the filter) and then point the flashlight at the secret message poster board(s)
15. To use filter goggles all a student must do is where the filter goggles (as close to their eyes as possible) and then shine the flashlight on the secret message poster board(s)

Background Science

Learning Question: What portion of the electromagnetic spectrum is visible light? What are the optics behind prisms and more simple colour filters?

The electromagnetic radiation spectrum, pictured below, is a representation of all wavelengths of light (or radiation) that arrive on earth from the sun. The visible spectrum, the portion of the spectrum that our naked eyes can see, forms only a small portion of this spectrum, covering wavelengths from 400nm (the colour purple) to 700nm (the colour red). Other wavelengths of light (including gamma rays (10-5 nm), x-rays, ultraviolet light, infrared light, microwaves, and radio waves (1012 nm)) are not visible to the human eye.

According to this spectrum, sunlight contains all known colours, the full rainbow. All of these colours blend to form white light while a lack of any of these colours produces black light. All of the colours of a rainbow can be separated out from white light via filtration of their different wavelengths. This is because different wavelengths of light will bend (or refract) differently through a filter. Some colours will deviate more than others (purple deviates the most, while red deviates the least). Filters that can do this include prisms (which can separate out the entire rainbow) and more simple filters, such as coloured cellophane, that can filter out a single chosen colour of the rainbow.

A clear unspecialized filter, like a piece of glass, will let all wavelengths of light (therefore all colours) through. A coloured cellophane filter, like we use in this activity, will only let certain wavelengths (colours) of light through and not others. What our eyes end up seeing are the wavelengths (colours) of light that were allowed through. The wavelengths (colours) of light that were not allowed through were physically blocked and instead absorbed by the filter, our eyes do not see these colours.

For example, a red cellophane filter will let red light through (a 700nm wavelength). A green filter will let green light through, a purple filter, purple light (a 400nm wavelength) through etc. So when you look through a red cellophane filter (by placing it over your flashlight or putting on the “goggles”) your entire field of view appears slightly tinged red.

In this particular activity, when the red cellophane filter (simple filter) lets the red wavelength of light through, your entire field of view will appear red and, as a consequence, any red lettering or lines on the secret message poster board will seem to fade away or even disappear. Simple red filters also tend to let through similar colours such as pink, orange, and even a bit of yellow. So, any lettering or lines in these colours on the secret message poster board will also appear to fade away.

Alternatively, when the red cellophane filter (simple filter) does not let other colours through because they are blocked and absorbed by the filter (including purple, blue, green, brown, and black), any lettering or lines in these colours on the secret message poster board will appear dark brown or black.