The wavelength of a $1 \ keV$ photon is $1.24 \times 10^{-9} \ m$. What is the frequency of a $1 \ MeV$ photon?

If $f(0)=0, f(1)=1, f(2)=2$ and $f(x)=f(x-2)+f(x-3)$ for $x=3, 4, 5, \ldots$, then $f(9)$ is equal to

The potential difference between two parallel plates is $10^4 ~V$. If the plates are separated by $0.5 ~cm$,the force on an electron between the plates is

There are $3$ sections in a question paper and each section contains $5$ questions. $A$ candidate has to answer a total of $5$ questions,choosing at least one question from each section. Then the number of ways,in which the candidate can choose the questions,is

In the final answer of the expression $\frac{(29.2-20.2)(1.79 \times 10^5)}{1.37}$,the number of significant figures is:

$A$ parallel plate capacitor with oil (dielectric constant $K = 2$) between the plates has capacitance $C$. If the oil is removed,the capacitance of the capacitor becomes: