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### IITJEE Problems: Modern Physics

Q 1. What is the Kinetic energy of the photoelectrons ejected from a tungsten surface by UV light of wavelength 1940Å ? Express the result in eV. The photoelectric threshold of tungsten is 2300Å .
Ans. K=1 eV
Q 2. The work function of sodium is 2 eV.
(a) Find the maximum energy of the photoelectrons ejected when a sodium surface is illuminated with light of 3105Å.
(b) Calculate the maximum wavelength of the light that would produce photoelectric effect.
Ans. 2 eV , 6210Å
Q 3. A small plate of metal (work function 1.17 eV) is placed at a distance 2 m from monochromatic light source of wavelength 4.8x10-7and power 1.0 watt. The light falls normally on plate. Find the number of photons striking the metal plate per square meter per second. If a constant uniform magnetic field of strength 10-4 tesla is applied parallel to the metal surface , find the radius of the largest circular path followed by the emitted photoelectrons.
Ans. 4.82 x1016 , 4.0 cm
Q 4. Calculate the wavelength of the wave associated with :
(a) 1 MeV photon
(b) 1 MeV proton
Ans. 1.242x10-10 cm, 2.84x10-12 cm
Q 5. The molecules of a certain gas at 320 K have a rms speed of .499 Km/sec. Calculate the De Brogli wavelength of these molecules. What is the gas?
Ans. 0.25Å , O16