In photo-emissive cell, with exciting wavelength λ, the fastest electron has speed v. If the exciting wavelength is changed to 3 λ /4, the speed of the fastest emitted electron will be:

Einstein's photoelectric equation is given byEk=E−w but Ek=12mv2 and E=chλ∴ 12mv2=ch(3λ/4)−w or 12mv2=43hcλ−w… (2) Dividing Eq. (ii) by Eq. (i), we getv2v2=43chλ−wchλ−w=43chλ−43w+13wchλ−w=43+w3chλ−w>43∴    v2v2>43  or v′>43v