Key concepts
Physics
Light
Reflection
Vision
Introduction
You might have heard the saying “not everything is as it seems.” Something faraway might look great, but when you take a closer look it might not turn out to be as you thought it would be! In this activity you will turn a spoon blackened with soot into shiny silver. Even though it is only an illusion, you will not know it when you see it. Try it out for yourself!
Background
How do we see what we see? Vision is based on light—without light we would not be able to see anything. Light travels in waves that eventually reach our eyes, where signals are sent to our brains, which translates these signals into information such as the color, location or appearance of an object we are looking at. When light waves strike an object, they can interact with it in several ways: They can be reflected back like those from a mirror; they can be absorbed, which means they are captured by the object; they can be transmitted and pass through it; or they can be scattered and deflected in different directions. Additionally, light that passes through a transparent object such as air, water or glass can also be refracted, which means the light wave bends when it passes from one transparent substance into another. Without this bending of light due to refraction we would not be able to use lenses or magnifying glasses.
How light interacts with an object depends mostly on the properties of its material. Objects that absorb most of the light appear dark or opaque. If an object has a very smooth surface, the light can be reflected and bounces off of the material’s surface. One example is a mirror’s smooth surface, which reflects light at an angle equal to that of the incoming light wave. This is the reason why we see ourselves in the mirror—the image reflected by the flat surface is reproduced. Rough surfaces reflect light as well, but they reflect it in more than one direction. The incoming light wave is scattered from the surface in all directions because the surface is uneven. In this activity you will see for yourself how a material's surface can determine how light interacts with it. The result will surprise you as the exact same object can appear in very different lights—literally!
Materials
- Adult helper
- Candle
- Matches or a lighter
- Two metal spoons (that you have permission to use)
- Glass
- Water
- Paper towels
- Black permanent marker
- Nail polish remover
Preparation
- Fill the glass halfway with water and place it next to your candle.
- Paint the back of one spoon with the black permanent marker.
Procedure
- Take the metal spoon that you have painted with black permanent marker. Can you clearly see the black color on the back of the spoon?
- Put the painted metal spoon into the glass of water and look at it from all angles. Is the spoon still black underwater? Does the appearance change from different viewing angles?
- Next take the other metal spoon and observe its appearance. How does it look? Is it very shiny?
- Ask an adult helper to light the candle and observe the burning candle. What does the flame look like? Do you see any soot coming from the flame?
- Next, hold the back of the metal spoon in the top part of the yellow flame. Does the appearance of the spoon change? If yes, how does it change? Can you explain what happened?
- Once your spoon is covered in soot, let it cool for a minute. Observe the spoon’s surface. Does its black surface look rough or smooth?
- After the spoon has cooled slowly submerge it into the glass of water. Then look at it from different angles. How does the back of the spoon look? Can you see the black sooty surface under the water? Does its appearance change with different viewing angles?
- Finally, slowly take the spoon out of the water and look at it again. Does the spoon look different? Is the soot still black? Does it look wet?
- Extra: Test if the activity works with objects other than a spoon. Make sure, however, that the objects you choose are heat-resistant and you have permission to test them. Try to cover a heat-resistant plate or an eggshell with soot. Can you create the same illusion with different objects?
Observations and results
A metal spoon looks very shiny because its surface is very smooth, which is why it acts like a mirror and reflects light very well. If you paint it with black paint, it looks black. This doesn’t change when you put the painted spoon in water. It still looks black from all angles.
When you put the metal spoon into the flame, however, you should have observed that after awhile the spoon turned black. This is because the candle produces soot from its wax that is made up of carbon and hydrogen. The yellow region of the flame contains tiny carbon particles that are deposited on the metal surface. Under a microscope you would see the soot on the spoon looks very bumpy and rough, almost like a mountain landscape. The surface looks black because, due to its roughness, any light that reflects from one particle will hit another one and get absorbed.
Once you put the black spoon into the water, however, you should have observed the previously black surface suddenly looked shiny and silver. Why is that? The answer is that due to the surface roughness of the soot, tiny air bubbles get stuck on its surface when you submerge the spoon in water. That means, on top of the soot there is a very thin layer of air bubbles. This air layer creates a very smooth surface on top of the rough soot surface. When light hits the boundary between the water and air layer, it is no longer absorbed but reflected in the same way as a mirror—the surface looks shiny. This phenomenon is also called “total internal reflection,” which can only happen at the border between two different materials the light passes through—in this case water and air. When you take the spoon out of the water the spoon looks black again because the light once more is absorbed by the rough, sooty surface. With the painted spoon, there is no water–air boundary underwater because no air bubbles get stuck to the paint. That is why you do not see this phenomenon with the painted spoon.
Cleanup
Remove the soot from the spoon using a paper towel. Use nail polish remover to remove the permanent marker from the other spoon. Make sure to extinguish the candle flame and clean your work space.
More to explore
Light Absorption, Reflection and Transmission, from the Physics classroom
Refraction of Light, from Science Learning Hub
Now You See It…Testing Out Light Refraction, from Scientific American
Science Activity for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
