A beam of parallel rays is brought to focus by a planoconvex lens. A thin Concave lens of the same focal length is joined to the first lens. The effect of this is

Options

A. The focus shifts to infinity
B. The focal point shifts towards the lens by a small distance
C. The focal point shifts away from the lens by a small distance
D. The focus remains undisturbed

Answer

To solve the problem, we need to analyze the effect of combining a plano-convex lens and a concave lens, both having the same magnitude of focal length but opposite signs.

### Step-by-Step Solution:

1. **Identify the Focal Lengths**:
- Let the focal length of the plano-convex lens (Lens 1) be \( f_1 \) (positive value).
- Let the focal length of the concave lens (Lens 2) be \( f_2 \) (negative value).
- Since they have the same magnitude, we can express this as:
\[ f_1 = +f \quad \text{and} \quad f_2 = -f \]

2. **Use the Lens Formula**:
- The combined focal length \( F \) of two lenses in contact is given by the formula:
\[ \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} \]

3. **Substituting the Focal Lengths**:
- Substitute the values of \( f_1 \) and \( f_2 \):
\[ \frac{1}{F} = \frac{1}{f} + \frac{1}{-f} \]
- This simplifies to:
\[ \frac{1}{F} = \frac{1}{f} - \frac{1}{f} = 0 \]

4. **Determine the Combined Focal Length**:
- If \( \frac{1}{F} = 0 \), then:
\[ F = \infty \]
- This means that the combined system of the two lenses will not converge the rays to a finite point but will instead cause them to diverge.

5. **Conclusion**:
- The effect of joining a plano-convex lens and a concave lens of the same focal length (but opposite signs) is that the effective focal length of the system becomes infinite.

### Final Answer:
The effect of joining a plano-convex lens and a thin concave lens of the same focal length is that the combined focal length tends to infinity.