Study the given ray diagrams and select the correct statement from the following:
(a) Device X is a concave mirror and device Y is a convex lens, whose focal lengths are 20 cm and 25 cm respectively
(b) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 10 cm and 25 cm respectively
(c) Device X is a concave lens and device Y is a convex mirror, whose focal lengths are 20 cm and 25 cm respectively
(d) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively
Answer
For X
- Because the screen is on the other side of the object, thus it is a lens
- Since the image is obtained on a screen, it is a real image. Hence, it is a convex lens.
- Since he focuses the sun’s image , the object placed at infinity.
- And, for a convex lens, object at infinity forms an image at 2F .
- Hence, the 2 × Focal length = 20 cm, and focal length is 10 cm
- Thus, X is a convex lens of focal length 10 cm
For Y
- Because the screen is on the same side of the object which means it cannot be a lens. Thus, it is a mirror
- Since the image is obtained on a screen, it is a real image. Hence, it is a concave mirror.
- Since he focuses the sun’s image , the object placed at infinity.
- And, for a concave mirror, object at infinity forms an image at the focus .
- Hence, the focal length of the concave mirror is 25 cm .
- Thus, Y is a concave mirror of focal length 25 cm
So, the correct answer is (b)