(i) Photo cells are used as switches and sensors.
(ii) Automatic switch on and off of street lights.
(iii) They are used for reproduction of sound in motion pictures
(iv) They are used as timers to measure the speed of athletes during a race.
(v) In photography, they are used to measure the intensity of the given light and to calculate the exact time of exposure.

(i) The photoemission of electrons does not depend on the intensity but it depends on the frequency and hence on the energy of a photon of incident light.
(ii) If the energy of a photon is greater than the work function, the photoemission of electrons results however weak the incident radiation may be.
(iii) The energy of a photon of red light is less than the work function of zinc, so red light cannot emit photoelectrons.
(iv) The energy of a photon of ultraviolet light is greater than the work function of zinc, so ultraviolet light can emit photoelectrons.

\(\frac{\lambda}{2}\)
Reason: de Broglie wavelength associated with the electron is
\(\lambda =\frac { h }{ \sqrt { 2mqV } } \Rightarrow \lambda \alpha \frac { 1 }{ \sqrt { V } } \\ \)
Obviously, when accelerating potential becomes 4 V, the de- Broglie wavelength reduces to half.

Work function W = hv_o = \(\frac{hc}{\lambda_o}\)
\({ \lambda }_{ o }\alpha \frac { 1 }{ W } \)
As W_AB; (\({ \lambda }_{ o }\)) _A > (\({ \lambda }_{ o }\)) _B
i.e., the threshold wavelength of metal A is higher.

From Einstein's photoelectric equation, stopping potential V_o is
eVo = hv - hv_o V_o = \(\frac{h}{e}(v-v_o)\)
Given > v_o, so with increase of frequency v, stopping potential increases.

1) The frequency of incident radiation was kept constant.
2) de Broglie wavelength,
\(\lambda =\frac { h }{ \sqrt { 2mqV } } \alpha \frac { 1 }{ V } \)
If potential difference V is doubled, the de-Broglie wavelength is decreased to \(\frac { 1 }{ \sqrt { 2 } } \) time.

An electron has a larger wavelength.
Reason: de- Broglie wavelength in terms of kinetic energy \(\lambda =\frac { h }{ \sqrt { 2m{ E }_{ K } } } \alpha \frac { 1 }{ \sqrt { m } } \) is for the same kinetic energy.
As an electron has a smaller mass than a proton, an electron has a larger de Broglie wavelength than a proton for the same kinetic energy.

Metal A
Since work function W = hv_o
and v_o > V_o so work function of metal A is more.
Aliter :
On stopping potential axis \(-\frac { { W }_{ o }^{ ' } }{ e } >-\frac { { W }_{ o } }{ e } \)
Hence work function W'o of metal A is more.

we know,
de Broglie wavelength,
\(\lambda =\frac { h }{ p } =\frac { h }{ mv } \Rightarrow \lambda \alpha \frac { 1 }{ v } \)
Also \(v\alpha \frac { 1 }{ n } \)
\(\lambda \alpha n\)
∴ de Broglie wavelength will increase

de Broglie wavelength (\(\lambda\)) is given is \(\lambda\) = \(\frac{h}{m}\)
Given v_p = v_e
where v_p = velocity 
v_e = velocity of electron
Since m_p > m_e
From the given relation 
\(\lambda \alpha \frac { 1 }{ m } ,hence\quad { \lambda }_{ p }<{ \lambda }_{ e }\) 

Laws of photoelectric effect:
(i) For a given surface, the emission of photoelectrons takes place only if the frequency of incident light is greater than a certain minimum frequency called the threshold frequency.
(ii) For a given frequency of incident light, the number of photoelectrons emitted is directly proportional to the intensity of the incident light. The saturation current is also directly proportional to the intensity of incident light.
(iii) Maximum kinetic energy of the photoelectrons is independent of the intensity of the incident light.
(iv) Maximum kinetic energy of the photoelectrons from a given metal is directly proportional to the frequency of incident light.
(v) There is no time lag between the incidence of light and the ejection of photoelectrons.

a) The number of photons incident normally per unit area per unit time is determined by the intensity of radiations.
b) (i) Red light, because the energy of red light is less than that of blue light
(hv)_R < (hv)_B
(ii) Blue light, because the energy of blue light is greater than that of red light
(hv)_B > (hv)_R

i) The momentum of photon of frequency v is given by,
\(p=\frac { hv }{ c } =\frac { h }{ \lambda } \)
ii) The wavelength of a photon in terms of its momentum is,
\(\lambda =\frac { h }{ p } \)
iii) According to de Broglie, the above equation is completely a general one and this is applicable to material particles as well. Therefore, for a particle of mass m traveling with speed u, the wavelength is given by,
\(\lambda =\frac { h }{ mv } =\frac { h }{ p } \)
iv) This wavelength of the matter waves is known as de Broglie wavelength. This equation relates the wave character (the wavelength \(\lambda\)) and the particle character (the momentum p) through Planck's constant.