What makes an object sparkle and shine? How can that be measured? Why does a mirror reflect imaged? There is one simple answer to all those questions. Emissivity. Emissivity is defined as the ability of a surface to emit radiant energy compared to that of a black body at the same temperature and with the same area. In more simple terms, emissivity is a measurement of how well something radiates absorbed energy. There is a scale to compare emissivity, and that scale explains why some things are more reflective then others.
To determine how well an object reflects off of something else there has to be a scale. On this scale there has to be a control. The emissivity scale goes from zero to one. To give an example, a mirror is a zero on the scale. The higher the rate on the scale, the less reflective the object is. The control is a one on the scale. It’s called a black body and it doesn’t reflect anything at all.
No surface can radiate perfectly. The human skin is a .98 on the scale. That means it is not very reflective at all because it is close to the black body. Shiny aluminum is a .10 on the emissivity scale. It is a smaller number that is close to zero, so like a shiny mirror it reflects a lot off it. There are ways to test the emissivity at home.
By taking an object and making it about 18 degrees warmer then room temperature the process of measuring emissivity begins. Using masking tape or painting the surface of the object is how the number is determined. By taking the new temperature of the now top surface (the tape or the paint) the number compared to the room temperature is the sum of the emissivity. That number determines where the object falls on the scale as well as how reflective the object is.
An object sparkles and shines because of light reflecting off or it. This is called emissivity which can be measured on a scale of zero (mirror) to one (black body). There are ways to measure an objects emissivity with the simple use of in home objects to get a rough estimate. All of this is what makes an object sparkle and shine!
Questions:
- What are more examples of items on the scale?
- Why can’t an object radiate perfectly?
- Are there any exceptions to the scale?
I think it’s really amazing how any object can(or can’t) shine or reflect light. I didn’t know that reflection had anything to do with THERMAL energy! I found on this (http://www.thermoworks.com/emissivity_table.html) website a whole list of other examples on the emissivity scale.