Spherical aberration in a thin lens can be reduced by

Options

A. using a monochromatic light
B. usinig a boublet combination
C. using a circular annular mark over the lens
D. increasing the size of the lens

Explanation

To solve the question about how to reduce spherical aberration in a thin lens, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Spherical Aberration**:
- Spherical aberration occurs when light rays that strike the lens near its edge (marginal rays) do not converge at the same point as those that strike near the center (paraxial rays). This leads to a blurred image.

2. **Identify the Cause**:
- The main cause of spherical aberration is the shape of the lens. A spherical lens does not focus all incoming light rays to a single point, particularly those rays that enter the lens at steep angles.

3. **Blocking Marginal Rays**:
- One effective way to reduce spherical aberration is to block the marginal rays that contribute to the aberration. By doing this, only the paraxial rays, which focus more accurately, will be allowed to pass through the lens.

4. **Use of a Circular Annular Mask**:
- To block the marginal rays, a circular annular mask can be placed over the lens. This mask will have a central opening that allows only the paraxial rays to pass through while blocking the outer rays that cause spherical aberration.

5. **Result**:
- By using the circular annular mask, the remaining rays (the paraxial rays) will converge more accurately at the focus, thus reducing the spherical aberration and improving the quality of the image produced by the lens.

### Final Answer:
Spherical aberration in a thin lens can be reduced by using a circular annular mask to block the marginal rays.

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