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Question : 23
Total: 36
(a) To construct a ray diagram we use two rays which are so chosen that it is easy to know their directions after reflection from the mirror. List two such rays and state the path of these rays after reflection in case of concave mirrors. Use these two rays and draw ray diagram to locate the image of an object placed between pole and focus of a concave mirror.
(b) A concave mirror produces three times magnified image on a screen. If the object is placed20 cm
(b) A concave mirror produces three times magnified image on a screen. If the object is placed
Solution:
(a) The following rays of light are usually used to locate the images formed by a concave mirror :
The incident ray passes through the centre of curvature : In this case, light after reflecting from the concave mirror moves back along the same path.
m = − 3 (As image is real)
u = − 20 cm
v = ?
We have,
m = −
− 3 = − (
) ⇒ v = − 60 cm
The image is located at a distance of60 cm
= − 60 cm − ( − 20 cm )
= − 40 cm
Thus, distance between the object and the screen is40 cm
The incident ray passes through the centre of curvature : In this case, light after reflecting from the concave mirror moves back along the same path.
The ray incident obliquely to the principal axis : In this case, the incident ray will be reflected back by the reflecting surface of the concave mirror obliquely, making equal angles with the principal axis.
When the object is placed between pole and focus of the concave mirror, image is formed behind the mirror which is virtual, erect and magnified.
(b) Given :
We have,
The image is located at a distance of
Thus, distance between the object and the screen is
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