The radiation emitted by a star $A$ is $10000$ times that of the sun. If the surface temperatures of the sun and the star $A$ are $6000 \ K$ and $2000 \ K$ respectively,the ratio of the radii of the star $A$ and the sun is : (in $: 1$)

Explain the physical significance of Van der Waals parameters.

If an organism is in the same $Class$ but not in the same $Family$, then it may belong to the same:

The point of intersection of the diagonals of a square is at the origin,and the coordinate axes are drawn along the diagonals. If the side length of the square is $a$,which of the following is $NOT$ a vertex of the square?

Energy is being emitted from the surface of a black body at $127^{\circ}C$ temperature at the rate of $1.0 \times 10^6 \ J/\text{s} \cdot \text{m}^2$. The temperature of the black body at which the rate of energy emission is $16.0 \times 10^6 \ J/\text{s} \cdot \text{m}^2$ will be ......... $^{\circ}C$.

If the acute angle between the lines $ax^2+2hxy+by^2=0$ is $\frac{\pi}{4}$,then $4h^2=$