A convex lens is said to have a real focus and a concave lens a Virtual Focus.Why?

Academic Physics NCERT Class 7

Convex lenses bend parallel light rays to form a point at a given distance, called the focal length.

Concave Lenses bend parallel light rays in the opposite Direction, so they never form a point. By extrapolating the lines, you can calculate a virtual focal point in the opposite direction.

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Updated on: 10-Oct-2022

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What is a lens? Distinguish between a convex lens and concave lens. Which of the two is a converging lens : convex lens of concave lens?
You are provided with a convex mirror, a concave mirror, a convex lens and a concave lens. You can get an inverted image from$(a)$. both concave lens and convex lens.$(b)$. both concave mirror and convex mirror.$(c)$. both concave mirror and convex lens.$(d)$. both convex mirror and concave lens.
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You are provided with a concave mirror, a convex mirror, a concave lens and a convex lens. To obtain an enlarged image of an object you can use either$(a)$. concave mirror or convex mirror.$(b)$. concave mirror or convex lens.$(c)$. concave mirror or concave lens.$(d)$. concave lens or convex lens.
How would a pencil look like if you saw it through (a) a concave lens, and (b) a convex lens? (Assume the pencil is close to the lens). Is the image real or virtual?
(a) With the help of a labelled diagram, explain how a concave lens perges a beam of parallel light rays. Mark the principal axis, optical centre, principal focus and focal length of the concave lens on the diagram.(b) State whether concave lens has a real focus or a virtual focus.(c) List some things that concave lens and concave mirror have in common.
A spherical mirror and a thin spherical lens have each a focal length of -15 cm. Themirror and the lens are likely to be(a) both concave.(b) both convex.(c) the mirror is concave and the lens is convex.(d) the mirror is convex, but the lens is concave.
A spherical mirror and a spherical lens each have a focal length of, –15 cm. The mirror and the lens are likely to be:(a) both concave.(b) both convex.(c) the mirror is concave but the lens is convex.(d) the mirror is convex but the lens is concave.
The magnification produced by a spherical mirror and a spherical lens is + 0.8.(a) The mirror and lens are both convex (b) The mirror and lens are both concave(c) The mirror is concave but the lens is convex (d) The mirror is convex but the lens is concave
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State two differences between a convex and a concave lens.
When sunlight is concentrated on a piece of paper by a spherical mirror or lens, then a hole can be burnt in it. For doing this, the paper must be placed at he focus of:(a) either a convex mirror or convex lens (b) either a concave mirror or concave lens(c) either a concave mirror or convex lens (d) either a convex mirror or concave lens
A spherical mirror and a spherical lens each have a focal length of -10 cm. The mirror and the lens are likely to beboth concaveboth convexthe mirror is concave and the lens is convexthe mirror is convex and the lens is concave
(a) Explain with the help of a diagram, why the concave lens is also called a perging lens.(b) Define the principal focus of a concave lens.
Fill in the following blanks with suitable words:(a) A convex lens _________ rays of light, whereas a concave lens _________ rays of light.(b) Lenses refract light to form images: a _________ lens can form both real and virtual images, but a perging lens forms only _________ images.

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