NECO 2003 Physics · Question 30

An object is placed 30 cm in front of a concave mirror of focal length 20cm. Calculate the distance between the object and the image.

Options

A.90cm
B.40cm
C.30cm
D.20cm
E.10cm

Answer: C. 30cm

Explanation written with AI and checked against the answer key

Step 1: Analyze the question A concave mirror has f = +20 cm (real-is-positive convention). The object distance is u = 30 cm. We need the image distance v, and then the distance between the object and the image. Step 2: Concept Mirror formula: 1/f = 1/u + 1/v Step 3: Solve 1/v = 1/f − 1/u = 1/20 − 1/30 = (3 − 2)/60 = 1/60 v = 60 cm The image is real, formed 60 cm in front of the mirror. It is inverted and magnified (magnification = v/u = 2). Step 4: Distance between object and image Both the object and the image are on the same side of the mirror, at 30 cm and 60 cm respectively: Distance = 60 − 30 = 30 cm Step 5: Answer The distance between the object and the image is 30 cm, which is option C. Tip: Read the question carefully. Many students stop at v = 60 cm, which isn't even an option here (A is 90 cm, which would be the sum 30 + 60, a common trap). Since the object and the real image are on the same side of a concave mirror, you subtract the distances, not add them.

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