All bodies, no matter how hot or cold, continuously radiate photons. At a given temperature, the intensities of the electromagnetic waves emitted by an object vary from wavelength to wavelength throughout the visible, infrared, and other regions of the spectrum. Figure illustrates how the intensity per unit wavelength depends on wavelength for a perfect blackbody emitter. Although this figure can strictly be applied only to a black body, yet this will approximately discribe the behavior of many of the self radiating systems. For example, sun has an approximate temperature of 6000K. it is not a black body, it has an emissivity of nearly 0.6 But its peak almost occurs at a wave length that predicted by Wein's law. Suppose we have a bulb of power 100W. It emits only about 5W as visible light, rest is emitted as infrared radiated. Assume that the bulb filament has a surface area of 10mm^(2) (hc=1250 eV-nm) What is the approximate temperature of the filament?