A small bulb is placed at the focal point of a converging lens. When the bulb is switched on, the lens produces:

Options

A. a convergent beam of light
B. a divergent beam of light
C. a parallel beam of light
D. a patch of coloured light

Explanation

To solve the question, we need to analyze the behavior of light when it passes through a converging lens (also known as a convex lens) with a bulb placed at its focal point. Here’s the step-by-step solution:

### Step 1: Understand the Setup
- A small bulb is placed at the focal point of a converging lens.
- The converging lens has a focal point where parallel rays of light converge after passing through the lens.

**Hint:** Recall the definition of the focal point in relation to a convex lens.

### Step 2: Analyze Light Rays from the Bulb
- When the bulb is switched on, it emits light in all directions.
- The light rays that travel towards the lens from the bulb will be considered.

**Hint:** Think about how light behaves when it originates from a point source.

### Step 3: Determine the Path of Light Rays
- Light rays that are emitted from the bulb and directed towards the lens will be treated as diverging rays.
- Rays that are emitted parallel to the principal axis will pass through the focal point after refraction.

**Hint:** Remember the principle of refraction through a lens: parallel rays converge at the focal point.

### Step 4: Identify the Resulting Beam of Light
- Since the bulb is at the focal point, the rays that come from the bulb and pass through the lens will emerge as parallel rays.
- This is because the lens takes the diverging rays from the focal point and converts them into parallel rays.

**Hint:** Consider the lens formula and how it relates to the object distance and image distance.

### Step 5: Conclusion
- Therefore, when the bulb is switched on, the lens produces a parallel beam of light.

**Final Answer:** The lens produces a parallel beam of light.