How many integuments surround the ovule in $Pinus$ and $Cycas$?

In the sequence of sets $(1,2,3), (4,5,6), (7,8,9,10), \ldots$,the sum of elements in the $50^{th}$ set is

In a Young's double-slit experiment,the fringe width is $0.2 \, mm$. If the wavelength of light used is increased by $10\%$ and the separation between the slits is also increased by $10\%$,the fringe width will be.......$mm$.

Which of the following is incorrect with respect to $Chondrichthyes$?

In a Young's double slit experiment,$12$ fringes are observed to be formed in a certain segment of the screen,when light of wavelength $600 \, nm$ is used. If the wavelength of light is changed to $400 \, nm$,the number of fringes observed in the same segment of the screen will be

The ionization energy of $He^+$ is $19.6 \times 10^{-18} \ J \ atom^{-1}$. What is the energy of the first stationary state $(n = 1)$ of $Li^{2+}$?