This means that the frequency of the EM radiation determines if an electron leaves and what speed it does after leaving (the velocity can only happen if hf ≥ f ).
The intensity (brightness) of the incident EM radiation affects only the number of electrons that leave.
The photoelectric effect is often seen as the inverse of X-ray production because in the photoelectric effect EM radiation causes electrons to leave, whereas in X-ray production electrons cause EM radiation to leave.
To demonstrate the photoelectric effect
1. Charge an electroscope by induction. 2. Set up three scenarios as shown in Fig. 30.12.
positively or negatively charged zinc plate
positively charged zinc plate
NOTE
An electroscope is an instrument used to measure relative magnitude of static charge present.
negatively charged zinc plate
red laser light
ultraviolet light
ultraviolet light
a gold leaf FIG 30.12
3. In Fig. 30.12a, the zinc plate, whether positive or negative, will be unchanged by red light, showing red is of too low a frequency to work.
4. In Fig. 30.12b, the UV light will not affect a positive plate as it does not have an excess of electrons to allow them to leave.
5. In Fig. 30.12c, UV light causes electrons to leave a negatively charged plate because of the photoelectric effect. This causes the gold leaf to fall, as it loses charge. Any light with greater than or equal frequency to UV will have this effect on negatively charged zinc.
