Question

Two identical metal bars are heated up until they are both glowing. One of them is "red hot" and the other is "blue hot." Which one is hotter, the one that glows red or the one that glows blue?A) the red oneB) the blue oneC) We cannot tell without knowing more about the two bars.

Answer 1

Answer:

B) the blue one

Explanation:

We can assimilate each metal bar to a black body. The peak wavelength of the radiation emitted by a blackbody is given by Wien's displacement law:

$\lambda = \frac{b}{T}$ (1)

where

b is the Wien's displacement constant

T is the absolute temperature of the object

In this case, we have one object hotter and the other one colder. We see from (1) that the peak wavelength is inversely proportional to the temperature: therefore, the hotter object will have shorter peak wavelength, while the colder object will have longer peak wavelength.

Since red light has longer wavelength than blue light, we can conclude that the object that glows blue is hotter than the one that glows red.