1.

A certain metallic surface is illuminated with monochromatic light of wavelength lambda.The stopping potentail for photelectric current for this light is 3V_(0).If the same surface is illuminated with light of wavelength 2lambda,the stopping potential is V_(0).The threshold wavelength for this surface for photoelectric effect is

Answer»

`4LAMBDA`
`(lambda)/(4)`
`(lambda)/(6)`
6`lambda`

Solution :According to EQUATION of Einstein,
`eV_(s)=E-phi`
`THEREFORE V_(s)=(hc)/(lambda_(e))-(hc)/(lambda_(0)e)`
`therefore 3V_(0)=(hc)/(lamdbae)-(hc)/(lambda_(o)e)`…..(1) and
`therefore V_(o)=(hc)/(2lambdae)-(hc)/(lambda_(0)e)`…….(2)
By multiplying EQ. (1) with (2) and then substracting from (1),
`3V_(0)=(hc)/(lambdae)-(hc)/(lambda_(0)e)`
`3V_(0)=(3hc)/(2lambdae)-(3hc)/(lambda_(0)e)`
`(--+)/(0=-(hc)/(2lambdae)+(2hc)/(lambda_(0)e))`
`0=-(hc)/(2lambdae)+(2hc)/(lambda_(0))`
`therefore (2)/(lambda_(0))=(1)/(2lambda)`
`therefore lambda_(0)=4lambda`


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