With the help of a suitable diagram show how white light gets split into its constituent colours.

Four cells each of emf $4 \mathrm{V}$and internal resistance $0.1 \mathrm{ohm}$ are connected in series. The combination in series is joined to an ammeter of negligible resistance, of $1.6 \mathrm{\Omega }$ resistor and an unknown resistor ${\mathrm{R}}_1$. The current in the circuit is $2 \mathrm{A}$. Draw a labelled diagram for the above given arrangement and calculate:
$1$. The total resistance in the circuit.
$2$. The total emf.
$3$. The value of ${\mathrm{R}}_1$.
$4$. The potential difference across ${\mathrm{R}}_1$.