The focal length of which of the following can not be obtained directly .

Options

A. convex mirror and convex lens
B. convex mirror & concave lens
C. convex lens and concave mirror
D. concave lensd and concave mirror

Answer

To determine which of the following has a focal length that cannot be obtained directly, we will analyze the options: convex lens, concave mirror, concave lens, and convex mirror.

### Step-by-Step Solution:

1. **Understanding Focal Length**:
- The focal length of an optical device is the distance from the lens or mirror to its focal point, where parallel rays of light converge or appear to diverge from.

2. **Convex Lens**:
- A convex lens converges parallel rays of light to a point (the focal point). The focal length can be measured directly by using a light source and measuring the distance from the lens to the focal point.

3. **Concave Mirror**:
- A concave mirror also converges rays of light to a focal point. Similar to the convex lens, the focal length can be measured directly by focusing parallel rays of light and measuring the distance to the focal point.

4. **Concave Lens**:
- A concave lens diverges light rays. The focal point for a concave lens is virtual, meaning the rays appear to diverge from a point behind the lens. To find the focal length, one typically has to use indirect methods, such as measuring the image distance for various object distances and applying the lens formula.

5. **Convex Mirror**:
- A convex mirror also diverges light rays, creating a virtual focal point behind the mirror. Similar to the concave lens, the focal length cannot be measured directly as the rays do not converge; they appear to diverge from a point behind the mirror.

6. **Conclusion**:
- The focal lengths of the concave lens and convex mirror cannot be obtained directly. However, since the question asks for a single option, we can conclude that the answer is the **concave lens** (as it is typically more commonly referenced in this context).

### Final Answer:
The focal length that cannot be obtained directly is **Concave Lens**.