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What is a black body?
What is a black body?
A black body is a perfect absorber and radiator of radiation.
A perfect black body exists in nature.
A perfect black body exists in nature.
False
Who devised a black body that approximates closely to the properties of a perfect black body?
Who devised a black body that approximates closely to the properties of a perfect black body?
Fery
Fery's black body consists of a metallic cavity in the form of a double walled hollow copper sphere with a ___ coating from inside.
Fery's black body consists of a metallic cavity in the form of a double walled hollow copper sphere with a ___ coating from inside.
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What happens to the radiation falling on the hole in Fery's black body?
What happens to the radiation falling on the hole in Fery's black body?
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What is the radiation coming out of a black body when heated at constant temperature called?
What is the radiation coming out of a black body when heated at constant temperature called?
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What was the focus of the experiments conducted by Lummer and Pringsheim between 1893 to 1897?
What was the focus of the experiments conducted by Lummer and Pringsheim between 1893 to 1897?
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According to Stefan's Law, what is the relationship between the area under the curve of radiation energy and the fourth power of the absolute temperature?
According to Stefan's Law, what is the relationship between the area under the curve of radiation energy and the fourth power of the absolute temperature?
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What does Wien's displacement law state about the wavelength corresponding to maximum energy?
What does Wien's displacement law state about the wavelength corresponding to maximum energy?
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What did Max Planck propose in 1900 regarding radiation?
What did Max Planck propose in 1900 regarding radiation?
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What is the formula for the energy values of Planck's oscillator?
What is the formula for the energy values of Planck's oscillator?
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In the average energy of Planck’s oscillator, E is calculated as E = ΣE/N, where N is the total number of ___ .
In the average energy of Planck’s oscillator, E is calculated as E = ΣE/N, where N is the total number of ___ .
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What is the Planck constant represented by?
What is the Planck constant represented by?
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Study Notes
Black Body
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A black body is a perfect absorber and radiator of electromagnetic radiation
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A black body does not exist in nature, but Fery’s black body is a good approximation
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Fery's black body design:
- A double-walled hollow copper sphere
- Coated with lamp black on the inside and nickel polished on the outside
- A narrow conical hole acts as an entry and exit point
- A projection within the cavity prevents radiation from escaping
- The space between the walls is evacuated to minimize heat loss
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Fery's black body acts as a perfect absorber:
- Incoming radiation undergoes multiple reflections before escaping, ensuring full absorption
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Fery's black body acts as a perfect radiator:
- When heated to a constant temperature, it emits blackbody radiation
Blackbody Radiation Spectrum
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Lummer and Pringsheim experimented with blackbody radiation from 1893 to 1897
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They measured the energy distribution of the spectrum using:
- A heated hollow enclosure acting as a blackbody
- A thermocouple to measure the blackbody's temperature
- Concave mirrors to direct the radiation
- A fluorspar prism to disperse the radiation into a spectrum
- A bolometer to detect the radiation intensity at different wavelengths
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Key observations:
- Energy distribution in the spectrum is not uniform
- Energy intensity rises with wavelength, reaches a maximum, and then decreases
- Total energy emitted at a given temperature is proportional to the fourth power of the absolute temperature (Stefan's Law)
- The peak wavelength (wavelength corresponding to maximum energy) shifts to shorter wavelengths as the temperature increases (Wien's Displacement Law)
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Existing theoretical formulas, like Wien's and Rayleigh-Jeans, couldn't fully explain the experimental blackbody radiation curves
Planck’s Quantum Hypothesis
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Max Planck proposed his radiation law in 1900
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Planck's assumptions:
- The cavity contains oscillators of molecular dimensions vibrating at different frequencies
- These oscillators emit and absorb radiation at their own frequencies
- Oscillators can only exchange energy in discrete packets called quanta (photons)
- The energy of a quantum is dependent on the oscillator's frequency: E = hv, where h is Planck's constant
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Planck's explanation:
- Oscillators have discrete energy levels: En = nhv (n = 0, 1, 2, 3,...), where n is an integer
- Energy exchange is limited to multiples of hv (0, hv, 2hv, 3hv, ..., nhv)
Average Energy of Planck's Oscillator
- N is the total number of oscillators
- E is the total energy
- The average energy is: E = ΣE / N
- No, N1, N2, Nr, ... represent the number of oscillators with energies 0, hv, 2hv, ... nhv
- The number of oscillators with energy rhv is: Nr = No e-rholky
- The total number of oscillators is: N = No + N1 + N2 + ...
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Description
Explore the concepts of black bodies and blackbody radiation through this quiz. Learn about Fery's black body design, its properties, and the historical experiments conducted by Lummer and Pringsheim. Test your understanding of how black bodies absorb and emit electromagnetic radiation.
