Consider the following statements: $(1)$ Both wave and particle nature are required for a complete understanding of light. $(2)$ Light cannot exhibit both wave and particle nature simultaneously in a single experiment.

The dual nature of radiation is demonstrated by:

One day on a spacecraft corresponds to $2$ days on the earth. The speed of the spacecraft relative to the earth is

According to the particle theory of light (Newton's corpuscular theory),what is the velocity of light in a dense medium compared to that in a rare medium?

Statement $(I)$ : By increasing the potential difference between cathode and anode continuously in a photoelectric experiment,the photocurrent always increases continuously.
Statement $(II)$ : If two photons $A$ and $B$ of energies $2.5 \ eV$ and $3.5 \ eV$ respectively fall on a metal surface of work function $2.0 \ eV$,then the ratio of maximum kinetic energies emitted between $A$ and $B$ is $3$.
Statement $(III)$ : The maximum energy needed by an electron to come out from metal surface is called the work function of the metal.
Which of the following is correct?

Photo cells are used for the: