Question:medium

An object is placed at 60.0 cm in front of a concave mirror of focal length 30 cm. At what distance from the mirror should a screen be placed to obtain a sharp image?

Show Hint

Notice that the object is placed at \(60\text{ cm}\), which is exactly twice the focal length (\(2f = 2 \times 30 = 60\text{ cm}\)).
When an object is placed at the center of curvature (\(C\)), its real image is also formed at the center of curvature (\(C\)).
This allows you to identify the answer instantly without doing calculations.
  • 60.0 cm in front of the mirror
  • 30.0 cm back of the mirror
  • 90.0 cm in front of the mirror
  • 90.0 cm back of the mirror
Show Solution

The Correct Option is A

Solution and Explanation

Step 1: Compare the object distance with the focal length.
The focal length is $f = 30\text{ cm}$, so the centre of curvature is at $2f = 60\text{ cm}$, and the object is placed exactly at $u = -60\text{ cm}$, which is exactly at the centre of curvature.
Step 2: Recall the special property of the centre of curvature.
Whenever an object sits at the centre of curvature of a concave mirror, the reflected rays retrace a symmetric path and meet again at that same point, so the image also forms right at the centre of curvature.
Step 3: Confirm with the mirror formula. \[ \frac{1}{v} = \frac{1}{f} - \frac{1}{u} = \frac{1}{-30} - \frac{1}{-60} = -\frac{2}{60}+\frac{1}{60} = -\frac{1}{60} \implies v = -60\text{ cm} \] This matches our expectation, the image is $60\text{ cm}$ in front of the mirror.
Step 4: State where to place the screen.
Since the image is real, it can be caught on a screen placed at that very spot.
\[ \boxed{60.0\text{ cm in front of the mirror}} \]
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