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Infrared Radiation
Introduction
Infrared radiation is the radiation from the part of the electromagnetic spectrum just below visible light.
We feel the energy from this radiation as heat. We can feel this quite strongly when we face the sun but perhaps not so much in winter. We can also feel it from objects that are nowhere near as hot, such as a central heating radiator—which, due to the heating fluid being water, can never be more than 100 oC. A black surface is a better emitter and absorber than a white surface, which is one reason why tarmac can get very sticky in the summer sun and we readily feel the heat coming off a hot tarmac road. The tarmac is absorbing large amounts of the solar radiation and emitting it back to us.
The objective
To determine which surface from a selection of surfaces emits the most energy.
The apparatus
• A Leslie cube; this is a cube that can be filled with hot water and has four different surfaces
• An infrared detector; an infrared blood temperature thermometer that does not need to be held against a person can be used
• A ruler
Arrange the glass block to be in the path of the light from the light box, both placed on top of a large sheet of graph paper so that the light ray is traveling along one of the axes of the graph paper.
The apparatus should be set up so that the thermometer points to the center of one side of the Leslie cube. The Leslie cube can optionally have a thermometer inserted so that the water temperature can be monitored.
The variables
The independent variable is the type of surface that we are measuring the radiation from. The dependent variable is the temperature reading from the thermometer, which is proportional to the energy being emitted from the surface of the cube.
The Physics
All objects emit and absorb infrared radiation. The hotter the object, the more it will radiate. When an object is hotter than its surroundings, it will emit more radiation than it absorbs and therefore it will cool down; that is, its temperature will drop. Objects will either heat up or cool down until they are in equilibrium with their surroundings; at this point, their temperature becomes stable and they will be emitting the same amount of energy as they are absorbing. Energy is emitted and absorbed from the surface of a body, and the nature of the surface has a big impact on how efficient the body is at being an absorber and an emitter.
The method
Fill the Leslie cube with hot water and wait a few minutes until all the surfaces have reached the same temperature. This can be checked by holding a thermometer against each surface. Place the thermometer at a fixed distance—say, 10 centimeters from the surface of the Leslie cube—and read the temperature. Note the surface facing towards the thermometer and the temperature. Repeat this for the other three surfaces as quickly as possible to prevent the water in the Leslie cube from cooling by more than a very small amount.
The Video
Watch a video of the Infrared Radiation e-practical here.
This shows how to use it and how to collect the data.
The e-Practical
Perform the experiment yourself, collect your own data, make mistakes and be able to correct them. The e-practical requires that your browser canrun WebGL 2 (usually found on Windows browsers, safari on iOS, and various Mobile browsers, test with https://get.webgl.org/webgl2/). This link is for students and evaluation only, schools should purchase a site licence.
The e-practical will run on laptops and desktops for PCs and Apple computers and will run on mid to high spec tablets and 'phones.
On a portable device, make sure you click on 'Toggle onscreen controls'. The left joystick controls movement, the right joystick controls direction and where you are looking.
Move the thermometer on the ruler to the 10-cm mark; there is a pointer on the bottom of the wooden base that is positioned at the same horizontal position as the sensor to aid this. Position yourself so that you can see the thermometer readout; you may need to zoom in to get a good enough view. You need to click on and hold the red button for several seconds to take a reading. Click on the Leslie cube to rotate it.
The Results
You should find that the matte black surface is the best emitter. The worst surface at emitting is the metallic surface.
Further Discussion
Can you think of any way to compensate for the temperature of the water dropping slightly between the readings for each surface?
Find out what is meant by black body radiation.
Why do you think there is no such thing as a perfect black body in nature?
How do we achieve a good approximation to a perfect black body?
This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.