<Ul> <Li> A piece of metal heated by a torch first becomes "red hot" as the very longest visible wavelengths appear red, then becomes more orange - red as the temperature is increased, and at very high temperatures would be described as "white hot" as shorter and shorter wavelengths come to predominate the black body emission spectrum . Before it had even reached the red hot temperature, the thermal emission was mainly at longer infrared wavelengths which are not visible; nevertheless that radiation could be felt as it warms one's nearby skin . </Li> <Li> One easily observes changes in the color of an incandescent light bulb (which produces light through thermal radiation) as the temperature of its filament is varied by a light dimmer . As the light is dimmed and the filament temperature decreases, the distribution of color shifts toward longer wavelengths and the light appears redder, as well as dimmer . </Li> <Li> It is easy to calculate that a wood fire at 1500 K puts out peak radiation at about 2000 nm. 98% of its radiation is beyond 1000 nm and only a tiny proportion at visible wavelengths . Consequently, a campfire can keep one warm but is a poor source of visible light . </Li> <Li> The effective temperature of the Sun is 5778 K. Using Wien's law, one finds a peak emission per nanometer (of wavelength) at a wavelength of about 500 nm in the green portion of the spectrum near the peak sensitivity of the human eye . On the other hand, in terms of power per unit optical frequency, the Sun's peak emission is at 343 THz or a wavelength of 883 nm in the near infrared . In terms of power per percentage bandwidth, the peak is at about 635 nm, a red wavelength . Regardless of how one wants to plot the spectrum, about half of the sun's radiation is at wavelengths shorter than 710 nm, about the limit of the human vision . Of that, about 12% is at wavelengths shorter than 400 nm, ultraviolet wavelengths which cannot be seen . It can be appreciated that a rather large amount of the Sun's radiation falls in the fairly small visible spectrum . </Li> </Ul> <Li> A piece of metal heated by a torch first becomes "red hot" as the very longest visible wavelengths appear red, then becomes more orange - red as the temperature is increased, and at very high temperatures would be described as "white hot" as shorter and shorter wavelengths come to predominate the black body emission spectrum . Before it had even reached the red hot temperature, the thermal emission was mainly at longer infrared wavelengths which are not visible; nevertheless that radiation could be felt as it warms one's nearby skin . </Li> <Li> One easily observes changes in the color of an incandescent light bulb (which produces light through thermal radiation) as the temperature of its filament is varied by a light dimmer . As the light is dimmed and the filament temperature decreases, the distribution of color shifts toward longer wavelengths and the light appears redder, as well as dimmer . </Li> <Li> It is easy to calculate that a wood fire at 1500 K puts out peak radiation at about 2000 nm. 98% of its radiation is beyond 1000 nm and only a tiny proportion at visible wavelengths . Consequently, a campfire can keep one warm but is a poor source of visible light . </Li>

Deduction of wien's displacement law from planck's law