CDS Physics · Light and Optics
The Eye and Optical Instruments
How the eye focuses, the common defects of vision and the lenses that correct them, and which lenses make up a microscope and a telescope.
Why this matters
Five CDS questions, all from 2023 (II) to 2025 (II), three of them in 2025. Three are about defects of vision (myopia twice, presbyopia once) and two about the lenses in a microscope or a telescope.
Concept 1 of 2: Defects of vision and their correction
The eye lens focuses light on the retina. The ciliary muscles change the lens's thickness so that both near and far objects come to focus: this is accommodation. If the eye-ball is the wrong length, or the lens bends light too much or too little, the image falls in front of or behind the retina and looks blurred. A spectacle lens of the right kind moves the image back onto the retina.
Definition
The normal eye and its defects:
- Normal eye: near point about 25 cm (least distance of distinct vision), far point at infinity.
- Myopia (near-sightedness): near objects clear, distant objects blurred. The image of a far object forms in front of the retina. Causes: excessive curvature of the eye lens (too short a focal length) or an elongated eye-ball. Corrected with a concave (diverging) lens.
- Hypermetropia (far-sightedness): distant objects clear, near ones blurred. Image forms behind the retina. Causes: focal length of the eye lens too long, or an eye-ball too short. Corrected with a convex (converging) lens.
- Presbyopia: comes with ageing, as the ciliary muscles weaken and the lens stiffens, so near vision fails. Often both near and far vision need help, so bifocal (concave above, convex below) or progressive lenses are used.
- Cataract: the eye lens becomes cloudy; treated by surgery. Colour blindness is a separate, inherited defect.
| Defect | Cause and correction |
|---|---|
| Myopia (near-sightedness) | Lens too curved or eye-ball too long; concave lensQ CDS 2025 (I): myopia is also called near-sightedness. |
| Hypermetropia (far-sightedness) | Focal length too long or eye-ball too short; convex lens |
| Presbyopia | Ageing weakens accommodation; bifocal or progressive lensesQ CDS 2024 (II): presbyopia comes with age and is corrected with bifocal or progressive lenses; it does not affect colour vision. |
| Cataract | Cloudy eye lens; surgery |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q1Moderate
Example 1 · Light and Optics · The Eye and Optical Instruments
A person can see near objects clearly through his eyes but cannot see distant objects clearly. The possible reason(s) for this defect could be :
1. Excessive curvature of the eye lens.
2. The eye-ball has become too small.
3. The focal length of the eye lens is too large.
4. Elongation of the eye-ball.
Select the correct answer using the code given below :
Near-sighted means it is the far objects that blur
A near-sighted (myopic) person sees near objects clearly and distant ones blurred. Far-sighted is the opposite. The name says what the person can still see.
Myopia: lens too strong or eye-ball too long
Myopia comes from too much bending (an over-curved lens, a short focal length) or an elongated eye-ball. A small eye-ball or a long focal length gives hypermetropia instead.
Concept 2 of 2: Microscopes and telescopes
A magnifying glass is just one convex lens held close to an object, inside its focal length, so it shows a large upright image. A compound microscope and an astronomical telescope each use two convex lenses: the first, the objective, makes a real image, and the second, the eyepiece, magnifies that image the way a magnifying glass would.
Definition
The lenses in each instrument:
- Simple microscope (magnifying glass): one convex lens, object inside its focal length; image virtual, erect, enlarged.
- Compound microscope: two convex lenses. The objective has a very short focal length and faces the object; the eyepiece magnifies its image.
- Astronomical telescope: two convex lenses. The objective has a long focal length, the eyepiece a short one. Magnifying power is about . The final image is inverted, which does not matter for stars.
- Galilean telescope: convex objective with a concave eyepiece, giving an erect image.
| Instrument | Lenses used |
|---|---|
| Simple microscope | One converging (convex) lensQ CDS 2025 (I): a simple microscope uses a converging lens. |
| Compound microscope | Convex objective of very short focal length, convex eyepiece |
| Astronomical telescope | Convex objective of long focal length, convex eyepiece of short focal length |
| Galilean telescope | Convex objective, concave eyepiece |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q28Easy
Example 2 · Light and Optics · The Eye and Optical Instruments
In a simple astronomical telescope, the objective and the eyepiece used respectively, are :
Telescope objective long, microscope objective short
A telescope objective has a long focal length to gather light from far away. A microscope objective has a very short one to magnify a near object. Both instruments use convex lenses for both parts, except the Galilean telescope.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Defects of vision and their correction4 rows
| Defect | Cause and correction |
|---|---|
| Myopia (near-sightedness) | Lens too curved or eye-ball too long; concave lensQ CDS 2025 (I): myopia is also called near-sightedness. |
| Hypermetropia (far-sightedness) | Focal length too long or eye-ball too short; convex lens |
| Presbyopia | Ageing weakens accommodation; bifocal or progressive lensesQ CDS 2024 (II): presbyopia comes with age and is corrected with bifocal or progressive lenses; it does not affect colour vision. |
| Cataract | Cloudy eye lens; surgery |
Microscopes and telescopes4 rows
| Instrument | Lenses used |
|---|---|
| Simple microscope | One converging (convex) lensQ CDS 2025 (I): a simple microscope uses a converging lens. |
| Compound microscope | Convex objective of very short focal length, convex eyepiece |
| Astronomical telescope | Convex objective of long focal length, convex eyepiece of short focal length |
| Galilean telescope | Convex objective, concave eyepiece |
Watch out for (3)
- Near-sighted means it is the far objects that blur→ Defects of vision and their correction
- Myopia: lens too strong or eye-ball too long→ Defects of vision and their correction
- Telescope objective long, microscope objective short→ Microscopes and telescopes
Test yourself on Light and Optics
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.