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Q: An optical device Y has positive focal length. Y is :
  • A. either a convex lens or a concave mirror
  • B. either concave lens or a convex mirror
  • C. either a convex lens or a convex mirror
  • D. either a concave lens or a concave mirror
Correct Answer: Option D - Sign convention for lenses and mirrors has been designed taking the help of coordinate system, when we place the lens or mirror (Spherical) at the origin, then distances measured in the direction of incident ray taken as positive and we place the object at the left side of mirror or lens, therefore incident ray from object moves left to right approaching the lens or mirror, therefore positive focal length according to ray diagram indicates device Y as either convex lens or a convex mirror.
D. Sign convention for lenses and mirrors has been designed taking the help of coordinate system, when we place the lens or mirror (Spherical) at the origin, then distances measured in the direction of incident ray taken as positive and we place the object at the left side of mirror or lens, therefore incident ray from object moves left to right approaching the lens or mirror, therefore positive focal length according to ray diagram indicates device Y as either convex lens or a convex mirror.
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Explanations:

Sign convention for lenses and mirrors has been designed taking the help of coordinate system, when we place the lens or mirror (Spherical) at the origin, then distances measured in the direction of incident ray taken as positive and we place the object at the left side of mirror or lens, therefore incident ray from object moves left to right approaching the lens or mirror, therefore positive focal length according to ray diagram indicates device Y as either convex lens or a convex mirror.
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