The photoelectric effect is often used in some burglar alarms. A beam of light s

Question

The photoelectric effect is often used in some burglar alarms. A beam of light shines on a metal electrode and causes electrons to be ejected from the metal surface (the cathode). The ejected electrons are attracted to the positively charged screen that is connected with the cell through a battery and a galvanometer. If the light beam is blocked by a burglar’s arm, the electric circuit is broken and the alarm sounds. The tungsten electrode results in the kinetic energy of the ejected electrons to be 8.0 x 10-12 ergs and the wavelength of the incident light is 1.25 x 103 angstroms. Calculate the maximum wavelength of light that could be used for the burglar alarm. Note: h = 6.6 x 10-27 erg sec and the speed of light c = 3.0 x 1010 cm s-1.

Explanation / Answer

Note :- 1 erg = 10-7 J

Energy of incident light = Thresold energy + K.E

Now, h*c/L = Thresold energy + 8*!0-19 J

or, (6.6*10-34*3*108)/(1250*10-10) = Thresold energy + 8*!0-19 J

or, Thresold energy = 7.84*10-19 J

Thus, wavelength of the incident light = h*c/Thresold energy = 2.525*10-7 m = 2525 angstroms

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