For the photoelectric effect to occur efficiently, the metal used as the cathode should have which of the following properties?

Introduction / Context:
The photoelectric effect is the emission of electrons from a metal surface when it is exposed to electromagnetic radiation of sufficient frequency, such as ultraviolet light. This effect provided crucial evidence for the particle nature of light and the concept of photons. The ease with which electrons are emitted depends strongly on the nature of the metal used as the cathode. This question asks what property the metal should have to make photoemission easier.

Given Data / Assumptions:

• We are considering metals used as cathodes in photoelectric experiments or devices such as photo cells.
• Important properties include melting point, work function, and electrical resistance.
• Work function is the minimum energy needed to remove an electron from the metal surface.
• We assume light of fixed frequency is incident on the metal and we want efficient electron emission.

Concept / Approach:
According to Einstein photoelectric equation, the maximum kinetic energy of emitted electrons is given by K max = h * f minus phi, where h is Planck constant, f is the frequency of incident light, and phi is the work function of the metal. The work function represents the minimum photon energy required to liberate an electron from the surface. A lower work function means electrons can be emitted with lower frequency or lower energy light. Therefore, to obtain photoelectric emission easily, the cathode metal should have a low work function. Melting point and resistance may be important for other design reasons but are not the primary factor governing photoelectric emission threshold.

Step-by-Step Solution:
Step 1: Recall that work function phi is the energy needed to remove an electron from the metal surface to a point just outside the metal. Step 2: The condition for photoelectric emission is that the photon energy h * f must be greater than or equal to the work function phi. Step 3: If phi is small, then even light of moderate frequency can satisfy h * f greater than or equal to phi and cause emission. Step 4: If phi is large, much higher frequency light is required, which may not be available or practical in an experiment or device. Step 5: Therefore, to ensure that the photoelectric effect occurs readily, one chooses metals with low work function as cathode materials.

Verification / Alternative check:
Common photoelectric materials include alkali metals such as cesium and potassium, which have relatively low work functions and readily emit electrons under visible or near ultraviolet light. In contrast, metals with high work function require ultraviolet or even higher energy photons to show significant photoelectric effect. Device designers pick low work function materials to build sensitive photo detectors and light meters. This practical engineering choice confirms the theoretical reasoning that low work function is desirable.

Why Other Options Are Wrong:
High melting point: While useful for high temperature applications, it does not directly determine the threshold for photoelectric emission. Low melting point: This could make the device unstable at moderate temperatures and again does not control electron emission threshold. Low electrical resistance: Good electrical conductivity is helpful, but even highly conductive metals may have high work functions and thus be poor photoemitters.

Common Pitfalls:
A frequent confusion is to associate photoelectric sensitivity solely with good conductivity. While the metal must conduct, the crucial quantity is the work function. Another mistake is to think that any strong light will cause emission from any metal, ignoring the threshold frequency requirement. Remember that for efficient photoelectric emission, the cathode metal should combine adequate conductivity with a low work function so that incident photons can easily liberate electrons.

Final Answer:
For the photoelectric effect to occur easily, the cathode metal should have a low work function.