VISUAL PHYSICS ONLINE

7.1 THE NATURE OF LIGHT

ELECTROMAGNEITC WAVES

P71 004

(A)

The family of curves below shows the relationship between the intensity of blackbody radiation and its wavelength for various Kelvin temperatures. Who was the first to correctly explain this relationship? What information do the graphs give you?

(B)

Scientists tried to explain observations of blackbody radiation using classical wave theory and then quantum theory.

(B1) What is meant by the term blackbody radiation?

(B2) What was a limitation of classical wave theory that could not explain blackbody radiation?

(B3) How does quantum theory satisfactorily explain blackbody radiation?

View solution below only after you have completed the answering the question.

Solution

(A)

Max Planck was the first person to give a qualitative explanation of the curves for the radiation emitted by a blackbody. He assumed that energy is quantized. The change in energy of an oscillator is a nhf where n is an integer.

The total area under the curves represents to energy emitted by the blackbody. The higher the temperature, the more energy radiated.

Wien’s Displacement Law l_peak T = constant

The higher the temperature, the shorter the wavelength of the em radiation emitted by the blackbody.

(B1) Blackbody - object which gives the maximum amount of energy radiated from its surface at any temperature and wavelength and the absorbs all the radiation that falls on it.

(B2) JWS Rayleigh & James Jeans

Math model described blackbody radiation curves at long wavelengths accurately but not at shorter wavelengths in the UV, radiation from cavity absorbed and emitted with ever smaller and smaller l, energy emitted from cavity becoming infinite – UV catastrophe

(B3) Max Planck was the first person to give a qualitative explanation of the curves for the radiation emitted by a blackbody. He assumed that energy is quantized. The change in energy of an oscillator is a nhf where n is an integer.