an object is placed at following distances in front of concave mirror of focal length 20 cm a 10cm b 20cm c 30 d 40cm
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Answer:
Solution :
In this case the focal length of concave mirror is 10 cm. This means that the focus (F) of this concave mirror is at a distanace of 10 cm from the mirror and its centre of curvature ( C) is at a distance of 2×10=20cm form the mirror.
(i) A diminished real image is formed by a concave morror when the object is beyond C. Here C is at 20 cm. So, the diminished real image will be formed when the object is at a distance greater then 20 cm, which in this problem is 25cm. So, the position of object for a diminished real image is 25 cm.
(ii) A magnified real image is formed by a concave mirror when the object is between F and C. Here F is at 10 cm and C is at 20 cm. So, the magnified real image will be formed when the object is at a distance btween 10 cm and 20 cm, which in this problem is 15 cm. So, the position of object for a magnified real image is 15 cm.
(iii) A magnified virtual image is formed by a concave mirror when the object is within focus (F) at a distance less than focal length or less than 10 cm, which in this problem is 8 cm. So, the position of object for ma magnified virtual image is 8 cm.
(iv) An image of the same size as object is formed by a concave mirror when the object is at centre of curvature ( C). Here C is at 20 cm. So, the image of same as the object will be formed when the object is at 20 cm form the concave mirror. Thus, the position of object for an image of same size as the object is 20 cm.
Please note that we have given so many details in answering the above question just to make you understand clearly. There is, no need to give so many details in the examination. In the examination, the answer to the above question can just be written as :
(i) 25 cm (ii) 15 cm (iii) 8 cm (iv) 20 cm