When both the object and its image move together in the same direction relative to the observer, then there is

Analysis of the Question

This question tests your understanding of parallax, a fundamental concept in optics and laboratory physics (specifically when using optical benches or mirrors). You are asked to identify what happens when an object and its image move in unison relative to the observer.

Concept Explanation

Parallax is the apparent displacement or difference in the position of an object viewed along two different lines of sight. In physics experiments involving mirrors or lenses:

• Parallax exists when the object and the image do not move together. If you move your head from side to side and the image appears to "shift" relative to the object, there is parallax.
• No parallax occurs when the object and the image are at the same position in space. Because they are at the same point, they move together as a single unit when you shift your viewpoint. Therefore, if they move together, it indicates that the image is perfectly coincident with the object.

Step-by-Step Solution

1. Understand the condition: The question states that the object and its image move "together in the same direction."
2. Relate to optics: When an observer shifts their position (moving their head), if the object and image are at the same distance from the observer, they will appear to stay locked together.
3. Conclusion: This "locking" effect is the definition of the absence of parallax. If there were parallax, the image would appear to slide across the object as you move your head. Since they move together, the parallax has been eliminated.

Identifying the Answer

Based on the principles of optics, when the object and image move together, it confirms that there is no parallax error. This is the standard method used in laboratories to locate the exact position of an image.

• Option A (no parallax error): Correct. This is the condition where the object and image are coincident.
• Option B (blurred image): Incorrect. A blurred image is usually due to poor focusing, not the relative movement of the object and image.
• Option C (mechanical error): Incorrect. This refers to equipment faults, not an optical phenomenon.
• Option D (parallax error): Incorrect. This would occur if the object and image moved independently of each other.

Exam Tip

Think of "parallax" as "the sliding effect." If you are doing an experiment with a pin and a mirror, keep moving your head until the pin and its image stop "sliding" past each other. Once they move as one, you have successfully removed the parallax error!