NDA (II) 2019 General Ability Test · Question 71 of 148

Correct - B, this is the statement that is NOT correct. A concave (diverging) lens forms only virtual, erect and diminished images of a real object - never a real image - because it always spreads light rays apart, so they only appear to come from a point on the same side as the object. Hence 'a concave lens produces both real and virtual images' is false. A convex lens, by contrast, does produce both, depending on where the object sits relative to its focus.

• (a)A convex lens produces both real and virtual images. — This is a correct statement, so not the exception: a convex lens gives a real image when the object is beyond the focus and a virtual, magnified image when the object is within the focus.
• (c)A convex lens can produce images equal, greater and smaller than the size of the object. — Also correct: a convex lens can give images larger than, equal to, or smaller than the object as the object distance changes (magnified within 2F, equal at 2F, diminished beyond 2F).
• (d)A concave lens always produces images smaller than the size of the object. — Correct as well: a concave lens always forms a diminished, virtual, erect image of a real object, so this statement is true and not the one being asked for.

A convex (converging) lens can form real images, when the object is beyond its focal point, or a virtual and magnified image, when the object lies inside the focus. A concave (diverging) lens always forms a virtual, erect and diminished image of a real object, whatever the object's position, because it never brings rays to a real focus.

The question asks for the FALSE statement. Recall the fixed rule that a concave lens (like a convex mirror) can only make virtual, diminished images of real objects. Any claim that a concave lens can form a real image is wrong.

• A convex lens forms a real image when the object is beyond the focus and a virtual, magnified image when it is within the focus.
• A concave lens always forms a virtual, erect and diminished image of a real object.
• A concave lens has a negative focal length and negative power in dioptres.
• Only a converging element can throw a real, screen-catchable image of a real object.

• Believing a concave lens can throw a real image - it cannot for a real object.
• Confusing a concave lens (diverging) with a concave mirror (converging).

Asked by picking the incorrect statement about lens behaviour, or by stating the nature of the image a given lens forms.

• practice — not a real PYQ

  For a real object, a concave lens always forms an image that is

  • (a)real and inverted
  • (b)virtual, erect and diminished
  • (c)real and magnified
  • (d)virtual and magnified

  Answer(b) virtual, erect and diminished - the fixed behaviour of a diverging lens.

• practice — not a real PYQ

  Which of the following can form a real image of a real object?

  • (a)concave lens
  • (b)convex lens
  • (c)plane glass slab
  • (d)none of these

  Answer(b) convex lens - a converging lens forms a real image when the object is beyond its focus.