Which one of the following oxides is reduced by water gas to obtain the metal during its extraction?

The first minima due to a single slit diffraction is at $\theta = 30^o$ for a light of wavelength $5000\,\mathring{A}$. The width of the slit is:

The plane faces of two identical plano-convex lenses,each having a focal length of $40 \, cm$,are placed against each other to form a biconvex lens. The distance from this lens at which an object must be placed to obtain a real,inverted image with magnification $-1$ is ...... $cm$.

Light enters from air into a given medium at an angle of $45^{\circ}$ with the interface of the air-medium surface. After refraction,the light ray is deviated through an angle of $15^{\circ}$ from its original direction. The refractive index of the medium is:

If the roots of the equations $px^2 + 2qx + r = 0$ and $qx^2 - 2\sqrt{pr}x + q = 0$ are real,then

The energy spectrum of $\beta$-particles [number $N(E)$ as a function of $\beta$-energy $E$] emitted from a radioactive source is: