3.2.10Waves

Correcting Sight — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 24 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

0%read

Lesson summary

3.2.10 Correcting Sight — IGCSE PHYSICS notes: Explain short sight and long sight and how diverging and converging lenses correct them.Cornea and lens — together they form a real, inverted image on the retina, and the cornea does most of the bendingDistant objects — ciliary muscl…

Near objects
Normal eye
Description
Correction
Glasses and contact lenses
Laser surgery

Part 1 of 8How the eye focuses

Cornea and lens

Cornea and lens together they form a real, inverted image on the retina, and the cornea does most of the bending

Distant objects

Distant objects ciliary muscles relax and the lens is thin with a long focal length

Near objects

Near objects the muscles contract and the lens is thicker with a shorter focal length

Normal eye

Normal eye focuses from about 25 cm up to infinity

Part 2 of 8Short sight

Description

Description eyes are more curved than normal or the eyeball is too long, with a wider lens of larger focusing power

Symptom

Symptom near objects are seen clearly but distant objects are blurred

Focus

Focus light is brought to a focus in front of the retina

Correction

Correction a diverging (concave) lens
It makes the rays diverge before they reach the eye
The image then forms on the retina

Part 3 of 8Long sight

Description

Description eyes are less curved than normal or the eyeball is too short, with a narrower lens of smaller focusing power

Symptom

Symptom distant objects are seen clearly but near objects are blurred

Focus

Focus light is brought to a focus behind the retina

Correction

Correction a converging (convex) lens
It makes the rays converge before they reach the eye
The image then forms on the retina

Part 4 of 8Comparing the defects

Defect	Cannot see	Focus falls	Corrective lens
Short sight	Far objects	In front of the retina	Diverging
Long sight	Near objects	Behind the retina	Converging

Part 5 of 8Glasses, contact lenses and surgery

Glasses, contact lenses and surgery

Glasses and contact lenses

Glasses and contact lenses do the same job, and contact lenses sit on the cornea

Laser surgery

Laser surgery reshapes the cornea so it focuses correctly

Lens power

Lens power measured in dioptres, equal to 1 ÷ f with f in metres
Converging lenses have positive power
Diverging lenses have negative power

Part 6 of 8Drawing the correction

Short sight

Short sight parallel rays meet before the retina and a diverging lens in front of the eye moves the focus back onto the retina

Long sight

Long sight rays focus behind the retina and a converging lens moves the focus forward onto the retina

Part 7 of 8Related vision facts

Ageing

Ageing the eye lens becomes less flexible, so the near point moves further away and reading glasses are needed

Astigmatism

Astigmatism uneven corneal curvature, corrected by non-spherical lenses

Pupil

Pupil the iris makes it smaller in bright light and larger in dim light

Part 8 of 8More from the specification

1Vision diagnostic imaging using optical fibres

Vision diagnostic imaging using optical fibres optometric instruments utilize high-resolution optical fibres to inspect retina surfaces and correct refractive sight defects.

2Refraction and Corrective Lenses

Converging and diverging lenses refract light at each air-glass boundary based on refractive index, altering focal distance to form clear images on the retina.

3Real vs Virtual Images

A converging lens forms a real, inverted image on a screen when the object is beyond the focal length,
but produces a magnified virtual image when used as a magnifying glass.

4Total Internal Reflection

Light passing from a medium of higher refractive index to lower refractive index undergoes total internal reflection when the angle of incidence exceeds the critical angle.

5Law of Regular Reflection

When light strikes a smooth reflective boundary, the reflected ray obeys the law of reflection where the angle of incidence equals the angle of reflection.

6Law of reflection

Law of reflection when a light ray meets a reflecting boundary, angle of incidence equals angle of reflection.

7Refraction across boundaries

Refraction across boundaries light refracts towards the normal when passing into a medium with higher refractive index.

8Virtual images in correcting lenses

Virtual images in correcting lenses diverging lenses form virtual images positioned closer than the eye's uncorrected far point.

9Plane mirror

Flat mirror.

10Optically dense

Having a high refractive index

11Reflection

Light bouncing off a surface.

12Refraction

Change in direction of light when it changes speed between media.

13Boundary

Surface between two media at which waves are partially reflected

14Refractive index

Ratio of the speed of light in a vacuum to its speed in a material

15Critical angle

Incline angle at which sliding just begins.

16Virtual image

Image formed where rays only appear to meet, not projectable on a screen

17Optical fibre

Thin transparent strand that guides light by TIR

18Principal focus

Point where parallel rays meet or appear to come from

Key terms

Term	Meaning
Short sight	Defect in which distant objects are blurred because the image forms in front of the retina
Long sight	Defect in which near objects are blurred because the image forms behind the retina
Retina	Light-sensitive layer at the back of the eye where the image forms
Cornea	Front surface of the eye that does most of the bending of light
Dioptre	Unit of lens power, 1 ÷ f with f in metres

Avoid these mistakes

Mixing up the lenses — short sight needs a diverging lens and long sight a converging lens.

Saying short-sighted people cannot see near things — they cannot see far things.

Drawing the focus on the retina without the lens — the uncorrected focus is in front of or behind it.

Saying the corrective lens changes the eyeball — it changes the path of rays before they enter the eye.

Last updated: 15 October 2026