Which one among the following is used in a simple microscope?

Q: What is the answer to CDS (I) 2025 GK Q29?

C. Correct — C, Converging lens. A simple microscope is nothing more than a single convex lens used as a magnifying glass. The object is placed between the lens and its principal focus, and in that position the refracted rays diverge on emerging, so they do not meet; traced backwards they appear to come from a point on the same side as the object, giving a virtual, erect and enlarged image that the eye can view directly. The shorter the focal length, the greater the magnification, which is why a strongly curved lens makes a more powerful magnifier. This is also the reason a compound microscope, which puts two convex lenses in line, gives far higher magnification than a single one — its objective forms a real magnified image which the eyepiece then magnifies again, exactly as a simple microscope would. (Provisional — pending the official UPSC key.)

(a)Diverging lens
(b)Concave mirror
(c)Converging lens
(d)Plane mirror

Answer

Why

Correct — C, Converging lens. A simple microscope is nothing more than a single convex lens used as a magnifying glass. The object is placed between the lens and its principal focus, and in that position the refracted rays diverge on emerging, so they do not meet; traced backwards they appear to come from a point on the same side as the object, giving a virtual, erect and enlarged image that the eye can view directly. The shorter the focal length, the greater the magnification, which is why a strongly curved lens makes a more powerful magnifier. This is also the reason a compound microscope, which puts two convex lenses in line, gives far higher magnification than a single one — its objective forms a real magnified image which the eyepiece then magnifies again, exactly as a simple microscope would.

Why the others are wrong

(a)Diverging lens — A concave, or diverging, lens always forms a virtual, erect and DIMINISHED image, whatever the object distance. It cannot magnify, so it cannot serve as a magnifying glass; its use is in correcting short sight.
(b)Concave mirror — A concave mirror can indeed give an erect magnified virtual image when the object lies inside its focus — that is how a shaving or dentist's mirror works — but it does so by reflection, and a microscope is a refracting instrument in which light passes through the element.
(d)Plane mirror — A plane mirror gives an image exactly the same size as the object and cannot magnify at all. Plane mirrors appear in microscopes only as accessories for directing light onto the specimen, not as the magnifying element.

Concept

A convex lens bends parallel rays towards a real focus, and where the object sits relative to that focus decides the image. Beyond twice the focal length the image is real, inverted and diminished; between the focus and twice the focal length it is real, inverted and enlarged; and inside the focus it is virtual, erect and enlarged. The last of these is the magnifying-glass configuration, and it is the whole of the simple microscope. A compound microscope stacks two such lenses so that the magnifications multiply.

The item is testing whether the words 'converging' and 'diverging' are firmly attached to 'convex' and 'concave'. Fix them by function: a converging lens is thick in the middle and brings light together, a diverging lens is thin in the middle and spreads it apart, and only the first can produce an enlarged image. The mirror options are there to catch a candidate who remembers that a concave mirror magnifies and does not stop to ask whether a microscope works by reflection or refraction. Keep the instrument list straight: simple microscope, one convex lens; compound microscope, convex objective plus convex eyepiece; refracting telescope, the same two but with a long-focus objective; reflecting telescope, a concave mirror as objective.

Key facts

A simple microscope is a single convex, that is converging, lens used as a magnifying glass.
The object is placed between the lens and its principal focus, giving a virtual, erect and magnified image.
The shorter the focal length of the lens, the greater the magnification.
A concave, diverging, lens always gives a virtual, erect and diminished image and is used to correct short sight.
In a compound microscope both the objective and the eyepiece are convex lenses, and the two magnifications multiply.

Which optical element can magnify, and by what means

Converging (convex) lens→Refracts light to a real focus. With the object inside the focus it gives a virtual, erect, enlarged image — the magnifying glass, and the whole of the simple microscope.

Diverging (concave) lens→Always a virtual, erect, DIMINISHED image. Cannot magnify at all; used to correct short sight.

Concave mirror→Can magnify, but by reflection — shaving mirrors, dentists' mirrors, headlight reflectors. A microscope is a refracting instrument.

Plane mirror→Image always the same size. In a microscope it only steers light onto the specimen.

Two elements can enlarge; only one of them does it by refraction, which is what a microscope requires.

Study next

Image formation by convex and concave lenses at each object positionThe lens formula, magnification and the power of a lens in dioptresThe compound microscope and the astronomical telescope — objective and eyepiece in eachDefects of vision — myopia, hypermetropia and presbyopia — and the lenses that correct themRefraction, refractive index and total internal reflection

Common traps

Choosing the concave mirror because it can magnify, without asking whether the instrument works by reflection or refraction.
Detaching 'converging' from 'convex' and 'diverging' from 'concave' under time pressure.
Confusing the simple microscope with the compound one; the simple instrument uses a single lens.

As an instrument-to-component question, as a statements item on the lenses used in microscopes and telescopes, or as a numerical on lens power or magnification.

Related PYQs

NDA_GAT_2018_II_Q942018Open & attempt →

Consider the following statements about a microscope and a telescope : 1. Both the eyepiece and the objective of a microscope are convex lenses. 2. The focal length of the objective of a telescope is larger than the focal length of its eyepiece. 3. The magnification of a telescope increases with the increase in focal length of its objective. 4. The magnification of a microscope increases with the increase in focal length of its objective. Which of the statements given above are correct?

(a) 1 and 3 only
(b) 1 and 4
(c) 2, 3 and 4
(d) 1, 2 and 3

Answer(d) 1, 2 and 3

The officially keyed statement of the same fact. Its first statement — that both lenses of a microscope are convex — settles the family this question belongs to, and its fourth statement is false for the reason given here, that shorter focal length, not longer, means greater magnification in a microscope.

CDS_GK_2021_II_Q12021Open & attempt →

Where should an object be placed in front of a convex lens to get a real and enlarged image of the object ?

(a) At twice the focal length
(b) At infinity
(c) Between the principal focus and twice the focal length
(d) Beyond twice the focal length

Answer(c) Between the principal focus and twice the focal length

The neighbouring position on the same lens, keyed officially. That item asks where a convex lens gives a REAL enlarged image, which is the objective's job in a compound microscope; move the object one step closer, inside the focus, and you get the virtual enlarged image of the simple microscope.

Practice

practice — not a real PYQ
In a simple microscope, the object must be placed in which one of the following positions to obtain a magnified image?
- (a)Beyond twice the focal length
- (b)At twice the focal length
- (c)Between the lens and its principal focus
- (d)At the principal focus
Answer(c) Between the lens and its principal focus — only there does a convex lens give a virtual, erect and enlarged image.
practice — not a real PYQ
A concave lens is used to correct which one of the following defects of vision?
- (a)Hypermetropia
- (b)Myopia
- (c)Presbyopia
- (d)Astigmatism
Answer(b) Myopia — a diverging lens moves the image back onto the retina in short sight; long sight is corrected with a converging lens.