3.2.10Waves
Correcting Sight — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 24 of 35 1 quiz
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
Section 1 of 10How 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
Section 2 of 10Short 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
Section 3 of 10Long 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
Section 4 of 10Comparing 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 |
Section 5 of 10Glasses, 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
Section 6 of 10Drawing 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
Section 7 of 10Related 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
Section 8 of 10More 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
Section 9 of 10Key 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 |
Section 10 of 10Exam technique
1
Mixing up the lenses — short sight needs a diverging lens and long sight a converging lens.
2
Saying short-sighted people cannot see near things — they cannot see far things.
3
Drawing the focus on the retina without the lens — the uncorrected focus is in front of or behind it.
4
Saying the corrective lens changes the eyeball — it changes the path of rays before they enter the eye.
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