The focal length of a lens is same for all the colours in visible spectrum.

Answer

### Step-by-Step Solution:

1. **Understanding Focal Length**:
- The focal length of a lens is the distance from the lens to the point where parallel rays of light converge (or appear to diverge from) after passing through the lens.

2. **Visible Spectrum**:
- The visible spectrum consists of light of different colors, which correspond to different wavelengths. These colors include red, orange, yellow, green, blue, indigo, and violet.

3. **Refractive Index**:
- The focal length of a lens is influenced by the refractive index of the material from which the lens is made. The refractive index determines how much light bends when it enters the lens.

4. **Focal Length and Color**:
- It is a common misconception that the focal length of a lens varies with different colors of light. However, in a thin lens made of a uniform material, the refractive index does not change significantly across the visible spectrum.

5. **Conclusion**:
- Therefore, the statement that "the focal length of a lens is the same for all the colors in the visible spectrum" is true. The focal length remains constant for all colors because the refractive index of the lens material does not vary significantly with the different wavelengths of visible light.

### Final Answer:
The focal length of a lens is the same for all colors in the visible spectrum because the refractive index of the lens material does not change significantly with different wavelengths of light.

---