5.1. Light http://www.ck12.org
FIGURE 5.5
Low frequency light (red) is unable to
cause ejection of electrons from the metal
surface. At or above the threshold fre-
quency (green), electrons are ejected.
Incoming light that has the same inten-
sity but a higher frequency (blue) causes
the ejection of the same number of elec-
trons, but the ejected electrons move at a
greater speed.FIGURE 5.6
Photoelectric cells convert light energy into electrical energy, which pow-
ers devices like this calculator.Step 1: List the known quantities and plan the problem.
Known
- frequency (ν) = 5.75× 1014 Hz
- Planck’s constant (h) = 6.626× 10 −^34 J•s
Unknown
- energy (E)
Apply the equationE=hνto solve for the energy.
Step 2: Calculate.
E= ( 6. 626 × 10 −^34 J·s)×( 5. 75 × 1014 Hz) = 3. 81 × 10 −^19 JStep 3: Think about your result.
While the resulting energy may seem very small, this is for only one photon of light. The sources of light that we
encounter on a daily basis emit huge quantities of photons. Recall that one hertz is equal to a reciprocal second, so
the units agree in this equation.