Hiding heat radiation

This exercise is a part of Educator Guide: Coating Provides Infrared Camouflage / View Guide

Directions for teachers: After your students read “Coating provides infrared camouflage,” ask them to answer the following questions.

1. How do infrared cameras work?

Infrared cameras measure the amount of thermal radiation an object emits within the infrared region. The measurements are used to estimate the object’s temperature.

2. What is the relationship between an object’s temperature and the brightness of the thermal radiation it emits? What do physicists call this relationship?

The hotter an object gets, the brighter its thermal radiation. This relationship is called the Stefan-Boltzmann law.  

3. Name one way you can see this relationship in everyday life?

When you turn on an electric stove, the heating coils glow brighter.

4. In what way does samarium nickel oxide appear to defy this relationship?

When samarium nickel oxide is viewed by an infrared camera in certain wavelengths of infrared light, the material does not get brighter as its temperature increases.

5. What properties of the material explain this defiant behavior?

At the temperatures described, the material switches from an insulator to a metal and its tendency to emit thermal radiation decreases. This decrease in emissivity counteracts the increase in brightness from the Stefan-Boltzmann law.   

6. How did scientists apply samarium nickel oxide? What did they apply it to and what data did they collect?

Scientists made a thin coating of samarium nickel oxide and used it to coat samples of sapphire. The scientists imaged heated samples with infrared cameras to see how brightly the samples glowed.

7. Why does the author describe the coating as a potential “camouflage”?

Camouflage is a disguise that allows people or objects to hide. The material’s ability to maintain a constant brightness despite increasing temperature could be used to hide people or objects from infrared cameras, which image things based on the thermal radiation they emit.

8. Describe two limitations of using samarium nickel oxide as a camouflage.

The temperature range in which the material’s brightness stays mostly constant is from 105° degrees Celsius to 135° Celsius — too high to hide people from detection by infrared cameras. Plus, the camouflage effect applies only at certain infrared wavelengths; cameras looking at different wavelengths would detect the change in brightness. 

9. How does applied physicist Mikhail Kats plan to overcome one of the limitations?

Kats suggests testing alloys of samarium nickel oxide. These materials may have different properties and so show the camouflage effect at lower temperatures.