Question
Explain the construction and working of Ferry’s black body.
✓
Answer
i. Ferry's perfectly blackbody consists of a double-walled hollow sphere having a tiny hole or aperture, through which radiant heat can enter.
ii. The space between the walls is evacuated and the outer surface of the sphere is silvered.
iii. The inner surface of a sphere is coated with lampblack.
iv. 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.
Ferry's blackbody
v. A heat ray entering the sphere through the aperture suffers multiple reflections and is almost completely absorbed inside.
vi. Thus, the aperture behaves like a perfect blackbody.
vii. The effective area of a perfect blackbody is equal to the area of the aperture.
ii. The space between the walls is evacuated and the outer surface of the sphere is silvered.
iii. The inner surface of a sphere is coated with lampblack.
iv. 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.
Ferry's blackbody
v. A heat ray entering the sphere through the aperture suffers multiple reflections and is almost completely absorbed inside.
vi. Thus, the aperture behaves like a perfect blackbody.
vii. The effective area of a perfect blackbody is equal to the area of the aperture.
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