Solar radiation, once it approaches or enters the Earth's atmosphere, can either be absorbed by the gases in the atmosphere, scattered (reflected) or transmitted.
Solar radiation, known as albedo, warms the earth when it is absorbed, which influences the climate and contributes to rising temperatures. It is possible to understand albedo by experiment and how it affects life on earth.
Albedo experiment 1: reflected radiation and temperature
The objective of this experiment is to understand the impact of the scattering of the sun's rays (albedo) on the temperature on Earth. In order to bring this into play, the experiment will consist of compare the temperature increase of a white and a black surface when heated by light.
To carry out the experiment in the best possible way, the equipment needed is :
- 2 flat metal surfaces in black and white,
- Two identical bulbs
- Two thermometers
- And a phone to time it
The procedure for the experiment is simple:
- Start by gluing or laying the two metal surfaces onto the polystyrene.
- Then measure the temperature of both surfaces.
- Then record the temperature of both surfaces as the temperature at time 0.
- Now switch on both lamps and point them at one of the surfaces.
- You should now measure the temperature of each surface every minute for 5 minutes. Make a small table to organise the data with the temperature of the black surface, the white surface and the time of measurement.
Now you just have to analyse your data:
- What has happened? How did the temperatures change?
- Did the temperatures change in the same way between the two surfaces?
- If yes, or if no, how can you explain it?
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Albedo experiment 2: reflected rays and light
Experiment 1 showed that not all surfaces rise in temperature identically when exposed to a lamp of the same power. Here, it will be a question of understand why by determining the influence of the colour and texture of a surface on its ability to reflect light.
This experience requires :
- Cardboard of different colours and textures (corrugated or flat)
- A lamp
- A light meter to calculate illuminance (some phone applications have this option)
- A calculator
To carry out this experiment, you need :
- Place a paper on a table and light it.
- Place the light meter in front of the paper to capture the rays reflected from the paper
- Repeat this process for each box, measuring the proportion of reflected light with the light meter each time.
- Finally, measure the illuminance of the lamp with the luxmeter (shine the luxmeter directly on the lamp). This will allow you to calculate the percentage of reflected rays in relation to the total rays supplied by the lamp.
To analyse the results, ask yourself these questions:
- Which textures and colours reflect the most? The least?
- What influence does colour have on the proportion of reflected radiation?
- What influence does the texture have on the proportion of reflected radiation?
Reflected radiation or albedo
Albedo is a value that characterises the ability of an element or system to reflect or not reflect radiation. The albedo therefore influences the tendency of this element or system to warm up, since the more it reflects, the less it warms up, since it captures less radiation. The albedo is calculated as follows: A = (reflected radiation/incident or received radiation) *100.
An albedo of 100 % will therefore reflect all the incident light. An albedo of 0 % will absorb all the incident light. Each element on Earth has a different albedo depending on its colour and texture. It is the average albedo of all these that will give the albedo of the earth-atmosphere system and thus allow us to know how much of the radiation is reflected. On average, The Earth has an albedo of 30 % mainly due to the atmosphere.
To say that colour affects albedo is both true and false. In reality, it does, but the reflection is reversed. It is not the colour that directly affects the radiation, but it is precisely because the element absorbs one part and reflects another that we see it in that colour. On the one hand, of all the radiation, only visible light (hence the name) is seen over a wavelength range of approximately 400 nm to 800 nm. An element will absorb and reflect (or scatter) a specific wavelength range. In this sense, for example, it is because an object only reflects red, and absorbs everything else, that it is seen as red. A perfect black body absorbs everything and a perfect white body reflects everything. It is therefore according to the ranges of absorbed and reflected wavelengths that a body will have a stronger or weaker albedo and a stronger or weaker heating.
Therefore, in experiment 1, some of the light radiation is absorbed by the plates as heat energy. The temperature rise is faster and greater for the black plate than for the white plate, because the white surface reflects more light, while the black one absorbs more of it.
Finally, here are some albedo values to compare your results from Experiment 2:
- Black colour: 0 % ;
- Forest: 5 to 10 % ;
- Concrete: 17 to 27 % ;
- Ice: 30 to 50 % ;
- Fresh snow: 80 to 90 % ;
- Perfect mirror: 100 %.