(a) Define Photoelectric effect ? Mention its one practical application in daily life.
(b) Electrons are emitted with zero velocity from a mental surface when it is exposed to radiation of wavelength 6800 Å. Calculate thresholds freqency `(V_o)` and work function `(W_o)` of the mental.

Answer

### Step-by-Step Solution

#### (a) Define Photoelectric Effect and Mention One Practical Application

1. **Definition of Photoelectric Effect**:
- The photoelectric effect is a phenomenon where electrons are emitted from a material (usually a metal) when it absorbs electromagnetic radiation, such as light. When light of sufficient energy strikes the surface of the metal, electrons absorb this energy and are ejected from the surface.

2. **Practical Application**:
- One practical application of the photoelectric effect in daily life is in solar panels. In solar panels, sunlight strikes the surface, causing electrons to be emitted. This movement of electrons generates an electric current, which can be harnessed for power.

#### (b) Calculation of Threshold Frequency `(V_o)` and Work Function `(W_o)`

1. **Given Data**:
- Wavelength (λ) = 6800 Å = \(6800 \times 10^{-10}\) m

2. **Calculate the Energy (E)**:
- The energy of the emitted electrons can be calculated using the formula:
\[ E = \frac{hc}{\lambda} \]
- Where:
- \(h\) (Planck's constant) = \(6.626 \times 10^{-34} \, \text{J s}\)
- \(c\) (speed of light) = \(3 \times 10^8 \, \text{m/s}\)
- Substituting the values:
\[ E = \frac{(6.626 \times 10^{-34} \, \text{J s}) \times (3 \times 10^8 \, \text{m/s})}{6800 \times 10^{-10} \, \text{m}} \]
- Calculating:
\[ E \approx 2.923 \times 10^{-19} \, \text{J} \]

3. **Determine the Work Function (W_o)**:
- Since electrons are emitted with zero velocity, the work function \(W_o\) is equal to the energy calculated:
\[ W_o = E = 2.923 \times 10^{-19} \, \text{J} \]

4. **Calculate the Threshold Frequency (V_o)**:
- The threshold frequency can be calculated using the formula:
\[ W_o = h V_o \]
- Rearranging gives:
\[ V_o = \frac{W_o}{h} \]
- Substituting the values:
\[ V_o = \frac{2.923 \times 10^{-19} \, \text{J}}{6.626 \times 10^{-34} \, \text{J s}} \approx 4.414 \times 10^{14} \, \text{s}^{-1} \]

### Final Answers:
- **Photoelectric Effect**: A phenomenon where electrons are emitted from a material when it absorbs electromagnetic radiation.
- **Practical Application**: Solar panels.
- **Threshold Frequency (V_o)**: \(4.414 \times 10^{14} \, \text{s}^{-1}\)
- **Work Function (W_o)**: \(2.923 \times 10^{-19} \, \text{J}\)