Question

A student uses a lens of focal length 40cm and another of -20cm. Write the nature of each lens

Answer 1

1)The lens of 40cm focal length is convex lense.[ () ]

2) The lense with -20cm focal length is concave lense.[ )( ]

Answer 2

The focal length of 40cm is a Convex Lens and the focal length of -20cm is a Concave Lens.

The Focal length of 40 cm is a Convex Lens.

Nature of Convex Lens :

• The focal length of any convex lens is always positive in value
• The shape of a convex lens is such that it is thinner at the edges and thicker bulking at the middle
• It is a converging lens as if the incident rays are parallel, they are converged to intersect at the focus on the other side.
• The image formed in a convex lens is Real and inverted in most cases except the last where the object is between the focus and the lens where the image formed is Virtual and Erect.

The Focal length of -20 cm is a Concave Lens.

Nature of Convex Lens :

• The focal length of any concave lens is always negative in value
• The shape of a concave lens is such that it is thicker and wide at the edges and slimmer at the middle
• It is a diverging lens as if the incident rays are parallel, they are diverged out as if they are virtually intersecting at the focus on the side of the object.
• The image formed in a convex lens is Virtual and Erect in all the cases of object position.

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