A man cannot see clearly the objects beyond a distance of 20 cm from his eyes. To see distant objects clearly he must use which kind of lenses and of what focal length

Options

A. 100 cm convex
B. 100 cm concave
C. 20 cm convex
D. 20 cm concave

Answer

To solve the problem, we need to determine the type of lens required for a man who cannot see clearly beyond a distance of 20 cm. This condition indicates that he is suffering from myopia (short-sightedness).

### Step-by-Step Solution:

1. **Understanding Myopia**:
- Myopia is a condition where distant objects appear blurry because the eye focuses images in front of the retina. This occurs when the eyeball is too long or the cornea has too much curvature.

**Hint**: Recall that myopia causes images to form before the retina.

2. **Identifying the Required Correction**:
- To correct myopia, we need to use a lens that diverges light rays, allowing them to focus further back on the retina. This is typically achieved with a concave lens.

**Hint**: Think about how lenses manipulate light. Concave lenses spread out light rays.

3. **Determining the Focal Length**:
- The man can see objects clearly only up to 20 cm. Therefore, to see distant objects clearly, the lens must create an image at the retina when the object is at infinity. The focal length (f) of the lens required can be calculated using the lens formula.
- For a concave lens, the focal length is negative. The focal length required to correct his vision can be taken as the distance at which he can see clearly, which is 20 cm.

**Hint**: Remember that for concave lenses, the focal length is negative.

4. **Conclusion**:
- Therefore, the required lens is a concave lens with a focal length of -20 cm.

**Final Answer**: The man must use a concave lens of focal length -20 cm to see distant objects clearly.