The relative luminosity of wavelength 600 nm is 0.6. Find the radiant flux of 600 nm needed to produce the same brightness sensation as produced by 120 W of radiant flux at 555 nm.

Explanation

To solve the problem, we need to find the radiant flux of 600 nm that produces the same brightness sensation as 120 W of radiant flux at 555 nm.

### Step-by-Step Solution:

1. **Understand the relationship between luminous flux and radiant flux**:
- The luminous flux (in lumens) produced by a source is related to its radiant flux (in watts) and the relative luminosity of the wavelength.
- For 555 nm, 1 watt of radiant flux corresponds to 685 lumens.

2. **Calculate the luminous flux for 120 W at 555 nm**:
- Luminous flux at 555 nm = Radiant flux × Luminous efficacy
- Luminous flux for 120 W at 555 nm = 120 W × 685 lumens/W = 82,200 lumens.

3. **Set up the equation for luminous flux at 600 nm**:
- Let the radiant flux at 600 nm be \( P \).
- The luminous flux at 600 nm can be expressed as:
\[ \text{Luminous flux at 600 nm} = P \times 685 \times 0.6 \]
- Here, 0.6 is the relative luminosity for 600 nm.

4. **Equate the luminous fluxes**:
- Since the brightness sensation should be the same, we can equate the luminous fluxes:
\[ 82,200 \text{ lumens} = P \times 685 \times 0.6 \]

5. **Solve for \( P \)**:
- Rearranging the equation gives:
\[ P \times 0.6 \times 685 = 82,200 \]
- Dividing both sides by \( 0.6 \times 685 \):
\[ P = \frac{82,200}{0.6 \times 685} \]

6. **Calculate \( P \)**:
- First, calculate \( 0.6 \times 685 = 411 \).
- Then, calculate \( P = \frac{82,200}{411} \approx 200 \) watts.

### Final Answer:
The radiant flux of 600 nm needed to produce the same brightness sensation as produced by 120 W of radiant flux at 555 nm is approximately **200 W**.