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.
Need a full question paper?
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.
Explore more
Similar questions
A square wire loop with sides $0.5 m$ is placed with its plane perpendicular to a magnetic field. The resistance of the loop is $5 \Omega$. Find at what rate the magnetic induction should be changed so that a current of $0.1 A$ is induced in the loop.
→What are a and ẞ parameters for a transistor? Obtain a relation between them.
A gas with adiabatic constant $\gamma=1.4$, expands adiabatically so that the final pressure becomes half the initial pressure. If the initial volume of the gas $1 \times 10^{-2} m ^3$, find the final volume.
→In a biprism experiment, a source of light having wavelength $6500 A$ is replaced by a source of light having wavelength $5500 A$. Calculate the change in the fringe width, if the screen is at a distance of $1 m$ from the sources which are $1\ mm$ apart.
→A u-tube is made up of capillaries of bore $1\ mm$ and $2\ mm$ respectively. The tube is held vertically and partially filled with a liquid of surface tension $49\ dyne/cm$ and zero angles of contact. Calculate the density of the liquid, if the difference in the levels of the meniscus is $1.25\ cm$. take $g = 980\ cm/s^2$
→Green light of wavelength $5100 \mathring A$ from a narrow slit is incident on a double-slit. If the overall separation of $10$ fringes on a screen $200 \ cm$ away from it is $2 \ cm$, find the slit-separation.
→A loop-the-loop cart runs down an incline into a vertical circular track of radius 3 m and then describes a complete circle. Find the minimum height above the top of the circular track from which the cart must be released.
When $2 \times 10^{10}$ electrons are transferred from one conductor to another, a potential difference of 20 V appears between the conductors. Find the capacitance of the two conductors.
→Find the principal specific heats of helium and hence the universal gas constant. Given : $C_p=$ $20.81 \mathrm{~J} / \mathrm{mol} . \mathrm{K}, \mathrm{C}_{\mathrm{V}}=12.5 \mathrm{~J} / \mathrm{mol} . \mathrm{K} . \mathrm{M}_0(\mathrm{He})=4 \times 10^{-3} \mathrm{~kg} / \mathrm{mol}$.
→State Ampere's circuital law. Using Ampere's circuital law, obtain an expression for magnetic induction at any point due to a straight conductor carrying current.