Properties of Light

Planck Spectrum

• As an object is heated, the radiation it emits peaks at higher and higher frequencies.
• Shown here are curves corresponding to temperatures of 300 K (room temperature), 1000 K (glow dull red), 4000 K (red hot), and 7000 K (white hot).

“Red Hot”

• As something begins to heat-up, there probably isn’t any visible information to tell you it is warming up.
• Once it starts to glow red, you have learned it’s hot – don’t touch.
• Like the stove burners.
• As it continues getting hotter, it changes to orange, then yellow, green, blue and white.

Wien’s Law

• The Sun and stars emit energy that approximates the energy from a blackbody.
• It is possible to estimate their temperatures by measuring the energy they emit as a function of wavelength - that is, by measuring their color.
• The wavelength at which a blackbody emits its maximum energy can be calculated by

• max 

• where the wavelength  max is in nanometers (10-9 m)
• and the temperature T is in kelvin.

• This relationship is known as Wien’s law.

• Effect of Temperature

• Hotter objects are brighter and “bluer” than cooler objects.

Getting Warmer

Electromagnetic Radiation

Problem - Wien’s law

• The average surface temperature of the Sun is about 5800 K. At what wavelength is maximum energy emitted from the Sun?
• If T = 5800 K
• and max = 3,000,000 / T ,
• then max = 3,000,000 / 5800 = 520 nm.
• 520 nm is at the middle of the visible light portion of the electromagnetic spectrum.
• The human eye is most sensitive to the wavelengths at which the Sun puts out the most energy.