Chapter 27, Problem 59A

(a)

To determine

The bar which is hotter.

Explanation of Solution

Introduction:

On heating one bar glows dark red and other glows bright orange. As temperature increases, wavelength decreases and hence color changes from infrared to visible range (VIBGYOR) to ultraviolet rays.

In the VIBGYOR, red has least frequency and hence longest wavelength. As one increases the temperature, first red color is reached with least energy and when it is heated more, the color reaches bright orange and hence the temperature is higher and rod is hotter.

Conclusion:

The rod which glows bright orange is hotter.

(b)

To determine

The bar which radiates more energy.

Explanation of Solution

Introduction:

On heating one bar glows dark red and other glows bright orange. According to radiant heat energy formula

  $E\propto T^4$

Energy is proportional to $4^{th}$ power of temperature and as temperature of rod with bright orange color is higher, it is radiating more energy than the one with deep red.

Conclusion:

The rod with bright orange color radiates more energy.