Achromatism in a surveyor's telescope is:
Correct Answer: B. A desirable property
📚 Detailed Explanation: Achromatism Is a Desirable Property in a Surveyor's Telescope
Why B (Achromatism is desirable in a surveyor's telescope) is correct: Achromatism means freedom from chromatic aberration — where different wavelengths of light focus at slightly different distances, causing colour fringes around the image. In a surveying telescope, an achromatic objective lens produces a sharp, colour-free image of the staff, which is essential for accurate readings.
Desirable Optical Properties of a Surveyor's Telescope
| Property | Definition | Why Important in Surveying |
|---|---|---|
| Achromatism | Freedom from chromatic aberration; all wavelengths of light focus at the same point | Produces a sharp, colour-fringe-free image; essential for precise staff reading (cross hairs must be sharp) |
| Magnification | Telescope magnifies the distant staff image | Allows accurate reading at long distances; typically 20–30× in levels |
| Field of view | The angular range visible through the eyepiece | Wider field makes it easier to find and follow the staff |
| Freedom from spherical aberration | All rays from a point on the axis converge at the same focal point | Ensures a sharp, undistorted image across the full field |
| Brightness (light gathering) | Amount of light transmitted through the lens system | Allows use in low-light conditions; depends on objective diameter |
How Achromatism Is Achieved: The objective lens in a surveying telescope is an achromatic doublet — a combination of a crown glass convex lens and a flint glass concave lens. Their opposite dispersions cancel each other, bringing two wavelengths (usually red and blue) to the same focus, virtually eliminating colour fringes.
- Achromatism = freedom from chromatic aberration = all colours focus at same point.
- Achieved with an achromatic doublet (crown + flint glass) objective lens.
- Result: sharp, colour-free image → accurate staff reading → essential in surveying.