The photoelectric effect is obtained when you shine a light on a material, resulting in the emission of electrons.
The kinetic energy of the electrons depends on the frequency of the light:
K = h(f - f₀)
where:
K = kinetic energy
h = Planck constant
f = light frequency
f₀ = threshold frequency
Rearranging the formula in the form y = m·x + b, we get:
K = hf - hf₀
where:
K = dependent variable
f = indipendent variable
h = slope
hf₀ = y-intercept
Every material has its own threshold frequency, therefore, what stays constant for all the materials is h = Planck constant (see picture attached).
Hence, the correct answer is C) the slope.
