Intro: Gummy Bears and Lasers!
Gummy Bears, it turns out, are great for demonstrating the otherwise abstract concepts that describe the most basic of light-matter interactions: Absorption, Transmittance and Reflection.
Step 1: Materials
Step 2: Background
A common misconception, is that color is a property of matter. This underlying misconception leads to all manner of confusion – like the idea that when white light passes through a green object to cast a green glow on the paper, the object is adding color to the otherwise clear, “natural” light. No matter how many times you think about it, its hard to grasp the idea that whenShine the white LED towards the students and remind them that white light is composed of many (“all”) wavelengths or frequencies of light. Explain that the LED is a light source – and that they can see directly the light that is emitted by the LED.
Ask… what happens when white light interacts with “something”?
Facing students, shine the LED at the white paper and ask – what’s happening to the light?
Repeat with the waxed paper and the plastic baggie.you see an object as red, what’s really happening is that most of the wavelengths that make up white light are being absorbed by the object and only the wavelength we know as red is being reflected. What you “see” is the red light transmitted to their eyes.
When LASER light, which is monochromatic (composed of a single color or wavelength) hits something, it is absorbed, converting light energy into heat, it is reflected, or it is transmitted. But most of the light you are familiar with is white light, composed of many colors, or wavelengths. When white light hits an object, the object selectively absorbs, reflects or transmits certain wavelengths. The way that light interacts with an object depends on the wavelength(s) of light and the nature of the atoms in the object.
A material will absorb frequencies of light that match the frequency at which electrons in the atoms that make up a material vibrate. Because different materials are made up of atoms whose electrons vibrate at different frequencies, different materials absorb different frequencies of light. This breaks up the notion that the reason light passes through a material or not because of its “thickness”.
Light that a material does not absorb is either reflected, or transmitted.
The way in which we see color is due in large part to the way light interacts with matter. So the color was never in the object…only in the light that shines upon it and ultimately is reflected to our eyes.
Step 3: White Light
Take a look at the white LED - recall that white light is composed of many (“all”) wavelengths or frequencies of light.
Shine the LED at the white paper and ask – what’s happening to the light? Most of it is "blocked" right?
Repeat with the waxed paper and the plastic baggie. More light passes through.
Now shine the white LED at the green gummy - see the green "shadow"? Only green light can pass through the green gummy - all other wavelengths are being absorbed.
Repeat with the white LED and the red gummy - see the red "shadow"? Only red light can pass through the red gummy - all other wavelengths are being absorbed.
Step 4: Terminology
Transmittance is the fraction of incident light at a specified wavelength that passes through something (like a gummy bear). Mathematically, transmittance is the ratio of the intensity of the light that passe through a sample to the intensity of the light when it entered the sample or T = I out / I in
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.
Absorbance is the measure of the quantity of light that a sample neither transmits nor reflects.
Step 5: Transmission
Now shine the green Laser through the green gummy bear. The bear "glows" green, and you can see that the green light is passing through the gummy bear... the green light is being transmitted.
Repeat with the red laser and the red gummy bear. As you might predict, the bear glows red, and you can see that the red light is passing through the red gummy bear... red light is being transmitted.
Now... what might happen if you shine the green laser at the red gummy and/or the red laser at the green gummy? Will the gummy glow? What color? Let's see!
Step 6: Absorption
When you shine the red laser at the green gummy - virtually NO light passes through! Why? Because the green gummy absorbs the red light. The light is blocked by the color of the gummy - not the "thickness".
Same when you shine the green laser at the red gummy! Virtually no light passes through - again because the red gummy absorbs the green light!