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P6 I) Infrared Radiation
P6 I) Infrared Radiation
Different surfaces emit and absorb different amounts of infrared radiation. In this section, we are going to look at two different experiments that investigate the amount of infrared radiation that different surfaces emit and absorb.
Infrared Radiation Emission
The amount of infrared radiation that an object emits is dependent upon the temperature and material on the surface of the object. If we had two identical objects at different temperatures, the hotter object would emit more infrared radiation than the cooler object. Darker surfaces emit more infrared radiation than lighter surfaces. Also, matted surfaces emit more infrared radiation than shiny surfaces.
We can carry out an experiment to show that darker matted surfaces emit more infrared radiation than lighter shiny surfaces. The experiment uses a piece of equipment known as Leslie Cube, which is a hollow cube that has different surfaces for the 4 vertical faces/ sides. The surfaces on the 4 vertical faces are usually something like:
There is a hole at the top of Leslie Cube. We pour boiling water into the cube through this hole. We put the cube on a heat proof mat before we pour in the boiling water (this is for safety reasons; it would be dangerous to transport the cube when it is full of boiling water). A diagram of the experiment is shown below.
The amount of infrared radiation that an object emits is dependent upon the temperature and material on the surface of the object. If we had two identical objects at different temperatures, the hotter object would emit more infrared radiation than the cooler object. Darker surfaces emit more infrared radiation than lighter surfaces. Also, matted surfaces emit more infrared radiation than shiny surfaces.
We can carry out an experiment to show that darker matted surfaces emit more infrared radiation than lighter shiny surfaces. The experiment uses a piece of equipment known as Leslie Cube, which is a hollow cube that has different surfaces for the 4 vertical faces/ sides. The surfaces on the 4 vertical faces are usually something like:
- Dark matt surface – usually matt black paint
- Light matt surface – usually matt white paint
- Dark shiny surface – usually a dull metal
- Light shiny surface – usually a shiny metal
There is a hole at the top of Leslie Cube. We pour boiling water into the cube through this hole. We put the cube on a heat proof mat before we pour in the boiling water (this is for safety reasons; it would be dangerous to transport the cube when it is full of boiling water). A diagram of the experiment is shown below.
After we have poured the boiling water into the cube, we wait for a few minutes so that the cube warms up. We then hold a thermometer outside each of the 4 vertical faces. The temperature outside each of the 4 vertical faces will be the same (if we have waited long enough).
The next step is to place an infrared radiation detector a set distance (such as 10 cm) away from the 4 faces on the cube. The set distance needs to be the same for all of the 4 faces. We can draw a line or use tape on the heat proof mat to ensure that the distances are the same. We then measure the amount of infrared radiation that is being emitted from each of the 4 vertical faces.
We repeat the process of getting infrared readings from the 4 faces for a few different temperatures. We get a few different temperatures by waiting for the cube to cool down. Also, we would repeat the whole experiment and find and the mean values for infrared readings.
From the experiment, we will observe that more infrared radiation is emitted from dark surfaces compared with light surfaces. Also, more infrared radiation is emitted from matt surfaces compared with shiny surfaces. The dark matt surface will emit the most infrared radiation, and the light shiny surface will emit the least infrared radiation.
We will also notice that all 4 of the surfaces will emit more infrared radiation when the temperature of the cube is the hottest. As the temperature of the cube decreases, the amount of infrared radiation that each of the 4 faces emits will decrease.
The next step is to place an infrared radiation detector a set distance (such as 10 cm) away from the 4 faces on the cube. The set distance needs to be the same for all of the 4 faces. We can draw a line or use tape on the heat proof mat to ensure that the distances are the same. We then measure the amount of infrared radiation that is being emitted from each of the 4 vertical faces.
We repeat the process of getting infrared readings from the 4 faces for a few different temperatures. We get a few different temperatures by waiting for the cube to cool down. Also, we would repeat the whole experiment and find and the mean values for infrared readings.
From the experiment, we will observe that more infrared radiation is emitted from dark surfaces compared with light surfaces. Also, more infrared radiation is emitted from matt surfaces compared with shiny surfaces. The dark matt surface will emit the most infrared radiation, and the light shiny surface will emit the least infrared radiation.
We will also notice that all 4 of the surfaces will emit more infrared radiation when the temperature of the cube is the hottest. As the temperature of the cube decreases, the amount of infrared radiation that each of the 4 faces emits will decrease.
Infrared Radiation Absorption
Different materials absorb different quantities of infrared radiation. Dark matt surfaces absorb more infrared radiation than light shiny surfaces. We can undertake an experiment to prove that this is the case. The setup for the experiment is shown below.
Different materials absorb different quantities of infrared radiation. Dark matt surfaces absorb more infrared radiation than light shiny surfaces. We can undertake an experiment to prove that this is the case. The setup for the experiment is shown below.
The Bunsen burner will emit infrared radiation. There are two metal plates that are identical in width, height and thickness. The only difference between the two metal plates is the surfaces that are facing the Bunsen burner. One of the metal plates will have a dark matt side facing the Bunsen burner, and the other metal plate will have a light shiny side facing the Bunsen burner. Both of the metal plates have a ball bearing attached with some wax to the side that is facing away from the Bunsen burner. We place the metal plates the same distance away from the Bunsen burner, so that both of the metal plates receive the same quantity of infrared radiation.
As the metal plates absorb infrared radiation, energy will be transferred to the thermal energy stores of the metal plates, which will then be transferred to the thermal energy stores of the wax. Eventually the wax will melt, and the ball bearing will fall to the floor.
When we complete this experiment, we will observe that the ball bearing on the dark matt metal plate will fall off before the ball bearing on the light shiny plate. This is because dark matt surfaces are better at absorbing infrared radiation compared to light shiny surfaces.
As the metal plates absorb infrared radiation, energy will be transferred to the thermal energy stores of the metal plates, which will then be transferred to the thermal energy stores of the wax. Eventually the wax will melt, and the ball bearing will fall to the floor.
When we complete this experiment, we will observe that the ball bearing on the dark matt metal plate will fall off before the ball bearing on the light shiny plate. This is because dark matt surfaces are better at absorbing infrared radiation compared to light shiny surfaces.
Real Life Examples
Dark surfaces being good absorbers of infrared radiation is why your phone’s screen feels hot when it is left in the sun on a sunny day. Also, on sunny days, the interiors of dark coloured cars are hotter than the interiors of light coloured cars; this is why the majority of cars in very sunny/ hot countries (such as Dubai) are white.
Dark surfaces being good absorbers of infrared radiation is why your phone’s screen feels hot when it is left in the sun on a sunny day. Also, on sunny days, the interiors of dark coloured cars are hotter than the interiors of light coloured cars; this is why the majority of cars in very sunny/ hot countries (such as Dubai) are white.