
where $\sigma$ is the Stefan's constant.

$Reason :$ Air surrounding the fire conducts more heat upwards.
If heat flows through them from $x = 0$ to $x = 2L$ at a steady rate and conductivities of the metals are $K_{cu}$ and $K_{steel}$ $(K_{cu} > K_{steel}),$ then the temperature varies as (convection and radiation are negligible)