Sign convention for mirrors and lenses used in optics. This topic is useful for Class 10 and 12th. The questions are asked on this topic in JEE or other competitive exams.
What is sign convention?
In the case of spherical mirrors or lenses, all distances
such as focal length (f), the distance of the object(u), and the distance of image(v) are
used with a sign (+) or (-). This is called sign convention in optics.
These signs are used on the basis of following the New
Cartesian Sign Convention.
1. The
object is always placed on the left side of the mirror or lens. It means light
from the object falls on the mirror or lens from the left-hand side.
2. All
distances are measured from the pole of the mirror or the optical centre of the lens.
3. Distances
towards the left side from the mirror or lenses along with x- axis is taken negative
(-ve) and towards the right side from the mirror or lenses are taken positive (+ve).
4. The height measured upward and perpendicular (above x-axis) is taken positive(+ve) and the height measured downward and perpendicular (below the x-axis) is taken negative (-ve)
Distances measured in the case of spherical mirrors
1.
Distance of object (u) is always negative
(-ve).
2.
Focal length of a convex mirror is positive
(+ve) and the focal length of a concave mirror is negative (-ve).
3.
Image distance (v) in the concave mirror is negative
(-ve) for the real images and positive (+ve) for the virtual image.
4. Image distance in a convex mirror is always positive (+ve).
Distances measured in the case of spherical lenses
1. Distance
of object (u) is always negative (-ve).
2. Focal
length of a convex lens is positive (+ve) and the focal length of a concave lens is
negative (-ve).
3. Image
distance for real and inverted image is positive(+ve) and for virtual and erect
image is negative (-ve).
4. Image
distance (v) in the concave mirror is negative (-ve) for real images and positive (+ve)
for the virtual image.
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