Experiment No.1
Determination of Planck’s Constant
Diagram:
Objective: To determine Planck’s constant using a Photocell
Apparatus: Photocell, Mercury lamp & different colour filters.
Theory: The photoelectric equations corresponding to colour filters of frequency 𝑓1 and 𝑓2 are
h𝑓 1 = h𝑓0 – eV1 …………………………. (1)
h𝑓2 = h𝑓 0 – eV2……………………………… (2)
where 𝑓 0 is the threshold frequency of the photosensitive material used in the photocell, e is
the charge of an electron, V1 & V2 are the stopping potentials corresponding to the colour filters
of frequency 𝑓1 & 𝑓 2. Combining the above two equations, Planck’s constant is given by
𝑒 (𝑉1 − 𝑉2)
ℎ = (𝑓1− 𝑓2)
If V be the voltage applied in the photocell & I be the photo current produced in the circuit,
then the negative voltage required to produce zero photo current is the stopping potential.
Observation Table: I -V characteristics (For at least four different frequencies)
Frequency𝑓 1 =
Frequency 𝑓2 =
Voltage Photocurrent Voltage Photocurrent
(V)
(I)
(V)
(I)
Volt
μA
Volt
μA
Frequency 𝑓 3 =
Voltage
(V)
Volt
Photocurrent
(I)
μA
Frequency 𝑓 4 =
Voltage Photocurrent
(V)
(I)
Volt
μA
Calculation
Frequency of colour filter
𝑓 (Hz)
Stopping potential
V (volt)
Planck’s constant h
( J-sec)
Mean h
(J-sec)
Result:
Calculated h: ________
Percentage Error
________
Standard value of h: ________