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Question
Explain the construction and working of Ferry’s black body.
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Solution
Construction:
- Ferry’s perfectly blackbody consists of a double-walled hollow sphere having a tiny hole or aperture, through which radiant heat can enter.
- The space between the walls is evacuated and the outer surface of the sphere is silvered.
- The inner surface of a sphere is coated with lampblack.
- There is a conical projection on the inner surface of the sphere opposite the aperture. The projection ensures that a ray travelling along the axis of the aperture is not incident normally on the surface and is therefore not reflected back along the same path.
Working:
- A heat ray entering the sphere through the aperture suffers multiple reflections and is almost completely absorbed inside. (about 98% of heat is absorbed.)
- Thus, the aperture behaves like a perfect blackbody.
- The effective area of a perfect blackbody is equal to the area of the aperture.
- This gives a greater effective area as a perfect black body.
- Similar working can be achieved using a cavity radiator that consists of a block of material with an internal cavity.
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Answer in brief:
What is a perfect blackbody? How can it be realized in practice?
Show the graphical representation of radiant power of a black body per unit range of wavelength as a function of wavelength.
Draw a neat labeled diagram of Ferry’s black body.
Compare the rates of emission of heat by a blackbody maintained at 627°C and at 127°C, if the black bodies are surrounded by an enclosure at 27°C. What would be the ratio of their rates of loss of heat?
Why is a conical projection in front of the hole i.n Fen-y's blackbody?
The energy spectrum of a black body exhibits a maximum around a wavelength '`lambda`'. The temperature of a black body is now changed such that the energy is maximum around a wavelength 3`lambda`/4. The power radiated by a black body will now increase by a factor of ______.
A black body has maximum wavelength `lambda_"m"` at temperature 2200 K. wavelength at temperature 3300 K will be ______.
For a perfectly black body, the graph is plotted between the frequency of the radiation with maximum intensity (vm) and the absolute temperature T. Out of the following, which is the correct graph?
A blackbody at 327 °C, when suspended in a black enclosure at 27 °C, loses heat at a certain rate. Find the temperature of the body at which its rate of loss of heat by radiation will be half of the above rate. Assume that the other conditions remain unchanged.
Draw a neat labelled diagram of Ferry’s perfectly black body.
