A Short-Sighted Eye Needs a Lens That Weakens the Light
Name the parts of the eye and what each does, see how accommodation moves the focus between the near and far points, identify myopia, hypermetropia and presbyopia from their symptoms, and calculate the power of the lens each one needs.
Why does the eye change its lens instead of moving it?
A camera focuses by moving its lens closer to or further from the sensor. The eye cannot do that: the retina is at a fixed distance from the lens, built into the wall of the eyeball, and nothing inside can slide.
So the eye does the only other thing available. It changes the shape of the lens, and therefore its focal length. Muscles around the lens squeeze it thicker to focus something near, and relax to let it thin out for something far away.
Looked at through the lens formula of the previous chapter, the eye is solving with held constant and changing constantly — so must change to compensate. Every defect of vision in this chapter is a failure of that adjustment, either because the range of is wrong or because itself is the wrong length.
And every correction is the same idea: place another lens in front of the eye so that the combination produces the focal length the eye cannot reach on its own — which is where the power-addition rule of the last chapter earns its keep.
This page covers the first part of the CBSE Class 10 Science chapter on the human eye: the parts and their functions, the power of accommodation, the three common defects of vision, and the calculation of the corrective lens.
So the eye does the only other thing available. It changes the shape of the lens, and therefore its focal length. Muscles around the lens squeeze it thicker to focus something near, and relax to let it thin out for something far away.
Looked at through the lens formula of the previous chapter, the eye is solving with held constant and changing constantly — so must change to compensate. Every defect of vision in this chapter is a failure of that adjustment, either because the range of is wrong or because itself is the wrong length.
And every correction is the same idea: place another lens in front of the eye so that the combination produces the focal length the eye cannot reach on its own — which is where the power-addition rule of the last chapter earns its keep.
This page covers the first part of the CBSE Class 10 Science chapter on the human eye: the parts and their functions, the power of accommodation, the three common defects of vision, and the calculation of the corrective lens.
What does each part of the eye do?
Light is bent mostly at the front, adjusted by the lens, and detected at the back.
- Cornea — the transparent bulge at the front. Most of the bending of light entering the eye happens here, not at the lens, because the change from air to cornea is the largest change of medium in the path
- Iris — the coloured muscular ring behind the cornea. It controls the size of the pupil
- Pupil — the dark opening in the middle of the iris. It regulates how much light enters the eye
- Crystalline lens — provides the fine adjustment of focus, so that a sharp image forms on the retina
- Ciliary muscles — hold the lens and change its curvature, and therefore its focal length
- Retina — the light-sensitive screen at the back, containing enormous numbers of rod and cone cells. The image is formed here
- Optic nerve — carries the signals from the retina to the brain
Two features of the retinal image that questions ask about.
- The image on the retina is real and inverted, exactly as a convex lens must produce for a distant object
- The brain interprets it as erect, which is why the world does not look upside down. The correction happens in the brain, not in the eye
How the iris earns its place in the list. Walk from bright sunlight into a dim room and for a moment you can see almost nothing; after a few seconds the room becomes visible. The iris has widened the pupil to admit more light. Step back outside and the reverse happens, faster. The iris is protecting the retina from too much light and helping it in too little.
Why the cornea does most of the work and the lens gets the credit. The cornea's bending is fixed, so it cannot focus on anything in particular; the lens's bending is adjustable, so it decides what is in focus. The cornea supplies the power and the lens supplies the control — and that division explains why a corrective lens in front of the eye works at all, since it simply adds to the fixed part.
One structure worth knowing that carries no receptors. At the point where the optic nerve leaves the retina there are no rod or cone cells, so no image can be detected there. That region is the blind spot, and you do not notice it because the brain fills in from the other eye and from the surroundings.
- Cornea — the transparent bulge at the front. Most of the bending of light entering the eye happens here, not at the lens, because the change from air to cornea is the largest change of medium in the path
- Iris — the coloured muscular ring behind the cornea. It controls the size of the pupil
- Pupil — the dark opening in the middle of the iris. It regulates how much light enters the eye
- Crystalline lens — provides the fine adjustment of focus, so that a sharp image forms on the retina
- Ciliary muscles — hold the lens and change its curvature, and therefore its focal length
- Retina — the light-sensitive screen at the back, containing enormous numbers of rod and cone cells. The image is formed here
- Optic nerve — carries the signals from the retina to the brain
Two features of the retinal image that questions ask about.
- The image on the retina is real and inverted, exactly as a convex lens must produce for a distant object
- The brain interprets it as erect, which is why the world does not look upside down. The correction happens in the brain, not in the eye
How the iris earns its place in the list. Walk from bright sunlight into a dim room and for a moment you can see almost nothing; after a few seconds the room becomes visible. The iris has widened the pupil to admit more light. Step back outside and the reverse happens, faster. The iris is protecting the retina from too much light and helping it in too little.
Why the cornea does most of the work and the lens gets the credit. The cornea's bending is fixed, so it cannot focus on anything in particular; the lens's bending is adjustable, so it decides what is in focus. The cornea supplies the power and the lens supplies the control — and that division explains why a corrective lens in front of the eye works at all, since it simply adds to the fixed part.
One structure worth knowing that carries no receptors. At the point where the optic nerve leaves the retina there are no rod or cone cells, so no image can be detected there. That region is the blind spot, and you do not notice it because the brain fills in from the other eye and from the surroundings.
What is accommodation, and what are the near and far points?
Accommodation is the eye's ability to change the focal length of its lens so that objects at different distances are focused on the same retina.
How the adjustment is made.
- To see a distant object: the ciliary muscles relax, the lens becomes thin, its focal length increases, and the weakly converging light from far away is brought exactly to the retina
- To see a near object: the ciliary muscles contract, the lens becomes thicker, its focal length decreases, and the strongly diverging light from close by is still brought to the retina
Thicker lens, shorter focal length, more power — which is the same relation as in the lens chapter, now performed by muscle.
The two limits of the range.
- The far point is the greatest distance at which an object can be seen clearly. For a normal eye it is infinity
- The near point, also called the least distance of distinct vision, is the closest distance at which an object can be seen clearly. For a normal adult eye it is about ** cm
So a normal eye focuses everything from cm to infinity, and that whole span is covered by the lens changing shape.
Why there is a near point at all. The lens cannot be squeezed beyond a certain thickness, so its focal length cannot be reduced below a certain value. Bring an object closer than the near point and the eye has run out of adjustment — the image falls behind the retina and looks blurred.
Try it and you can find your own near point. Bring a page of print slowly towards your eyes until the letters stop being sharp. That distance is your near point**, and for most people it is close to cm — which is why cm is used as the standard value in every calculation in this chapter.
Why the ciliary muscles ache after long close work. Reading holds the lens at its thickest, which means the ciliary muscles are contracted continuously. Looking at something distant relaxes them, which is the physical basis of the advice to look away from close work at intervals.
One boundary case worth naming. A person whose near point is further away than cm has lost part of the range, and a person whose far point is closer than infinity has lost the other part. Those two losses are exactly the two defects of the next section, and describing them as a shrunken range rather than as diseases makes both the cause and the correction obvious.
How the adjustment is made.
- To see a distant object: the ciliary muscles relax, the lens becomes thin, its focal length increases, and the weakly converging light from far away is brought exactly to the retina
- To see a near object: the ciliary muscles contract, the lens becomes thicker, its focal length decreases, and the strongly diverging light from close by is still brought to the retina
Thicker lens, shorter focal length, more power — which is the same relation as in the lens chapter, now performed by muscle.
The two limits of the range.
- The far point is the greatest distance at which an object can be seen clearly. For a normal eye it is infinity
- The near point, also called the least distance of distinct vision, is the closest distance at which an object can be seen clearly. For a normal adult eye it is about ** cm
So a normal eye focuses everything from cm to infinity, and that whole span is covered by the lens changing shape.
Why there is a near point at all. The lens cannot be squeezed beyond a certain thickness, so its focal length cannot be reduced below a certain value. Bring an object closer than the near point and the eye has run out of adjustment — the image falls behind the retina and looks blurred.
Try it and you can find your own near point. Bring a page of print slowly towards your eyes until the letters stop being sharp. That distance is your near point**, and for most people it is close to cm — which is why cm is used as the standard value in every calculation in this chapter.
Why the ciliary muscles ache after long close work. Reading holds the lens at its thickest, which means the ciliary muscles are contracted continuously. Looking at something distant relaxes them, which is the physical basis of the advice to look away from close work at intervals.
One boundary case worth naming. A person whose near point is further away than cm has lost part of the range, and a person whose far point is closer than infinity has lost the other part. Those two losses are exactly the two defects of the next section, and describing them as a shrunken range rather than as diseases makes both the cause and the correction obvious.
How do you identify a defect of vision and choose the lens?
Ask which end of the range has been lost. If distant objects are blurred the eye is too strongly converging; if near ones are blurred it is too weakly converging.
Myopia, or short-sightedness. The person can see near objects clearly but not distant ones.
- What is happening: the image of a distant object forms in front of the retina
- Why: either the lens has too much curvature, giving too much converging power, or the eyeball is too long
- The far point has come closer than infinity
- Correction: a concave lens, of suitable negative power, which diverges the light a little before it enters the eye so that the image falls back onto the retina
Hypermetropia, or long-sightedness. The person can see distant objects clearly but not near ones.
- What is happening: the image of a near object forms behind the retina
- Why: either the lens has too little converging power, or the eyeball is too short
- The near point has moved further away than cm
- Correction: a convex lens, of suitable positive power, adding the converging power the eye lacks
Presbyopia. With advancing age the ciliary muscles weaken and the lens loses its flexibility, so the power of accommodation falls. The near point recedes, and reading becomes difficult even for someone who never needed spectacles.
And it often occurs together with myopia, so the person cannot see clearly either far or near. The correction is a bifocal lens — the upper part concave, for distant vision, and the lower part convex, for reading. That arrangement matches where you look for each task: up for the road, down for the page.
Cataract is a different kind of problem: the crystalline lens becomes cloudy and milky, so it scatters light instead of focusing it. No spectacle lens can correct it, because the fault is the loss of transparency rather than the wrong power, and the treatment is surgical replacement of the lens.
Worked identification. Name the defect and the lens in each case.
- A student who cannot read the blackboard but reads a book comfortably — myopia, concave lens
- A person who holds a newspaper at arm's length to read it but sees a distant signboard clearly — hypermetropia, convex lens
- An older person who needs one pair of spectacles for driving and another for reading — presbyopia, bifocal lens
- A person whose vision has become foggy and who sees a halo around lights — cataract, requiring surgery rather than a lens
The trap this section exists to prevent. Myopia takes a concave lens and hypermetropia a convex one, and the two are swapped more often than any other pair in the chapter. Fix them by the cause: myopia is too much converging power, so it needs a diverging lens to take some away. Match the correction to the excess, not to the symptom.
Myopia, or short-sightedness. The person can see near objects clearly but not distant ones.
- What is happening: the image of a distant object forms in front of the retina
- Why: either the lens has too much curvature, giving too much converging power, or the eyeball is too long
- The far point has come closer than infinity
- Correction: a concave lens, of suitable negative power, which diverges the light a little before it enters the eye so that the image falls back onto the retina
Hypermetropia, or long-sightedness. The person can see distant objects clearly but not near ones.
- What is happening: the image of a near object forms behind the retina
- Why: either the lens has too little converging power, or the eyeball is too short
- The near point has moved further away than cm
- Correction: a convex lens, of suitable positive power, adding the converging power the eye lacks
Presbyopia. With advancing age the ciliary muscles weaken and the lens loses its flexibility, so the power of accommodation falls. The near point recedes, and reading becomes difficult even for someone who never needed spectacles.
And it often occurs together with myopia, so the person cannot see clearly either far or near. The correction is a bifocal lens — the upper part concave, for distant vision, and the lower part convex, for reading. That arrangement matches where you look for each task: up for the road, down for the page.
Cataract is a different kind of problem: the crystalline lens becomes cloudy and milky, so it scatters light instead of focusing it. No spectacle lens can correct it, because the fault is the loss of transparency rather than the wrong power, and the treatment is surgical replacement of the lens.
Worked identification. Name the defect and the lens in each case.
- A student who cannot read the blackboard but reads a book comfortably — myopia, concave lens
- A person who holds a newspaper at arm's length to read it but sees a distant signboard clearly — hypermetropia, convex lens
- An older person who needs one pair of spectacles for driving and another for reading — presbyopia, bifocal lens
- A person whose vision has become foggy and who sees a halo around lights — cataract, requiring surgery rather than a lens
The trap this section exists to prevent. Myopia takes a concave lens and hypermetropia a convex one, and the two are swapped more often than any other pair in the chapter. Fix them by the cause: myopia is too much converging power, so it needs a diverging lens to take some away. Match the correction to the excess, not to the symptom.
Formula
How do you calculate the focal length and power of the corrective lens?
Use the lens formula with the eye's defective point as the image and the point you want to see as the object.
For myopia the object is at infinity, because that is what the person cannot see, and the image must be brought to the defective far point, where the eye can still focus.
Worked example 1. A person with myopia has a far point of m. Find the focal length and power of the correcting lens.
The lens must take an object at infinity and place its image at the far point, m in front of the eye:
The negative sign confirms a concave lens, exactly as the previous section required.
Worked example 2. A myopic person has a far point of cm. Find the power needed.
Notice the pattern: for myopia, is just the far point distance made negative, and . The nearer the far point, the stronger the lens needed.
**For hypermetropia the object is at the normal near point, cm, because that is where the person wants to be able to read, and the image must be formed at the defective near point, where the eye can focus.
Worked example 3.** A person with hypermetropia has a near point of m. Find the focal length and power of the correcting lens.
The positive sign confirms a convex lens.
Worked example 4. A hypermetropic person has a near point of cm. Find the power required.
**Both and are negative in the hypermetropia calculation, because the object and the image are both on the same side as the object — in front of the lens. The image is virtual, which is exactly what a convex lens gives when the object is inside its focus, and it is the case that lets the eye focus what it otherwise could not.
The three things to get right every time.
- Convert every distance to metres before computing the power
- Myopia: object at infinity, image at the far point, answer negative
- Hypermetropia**: object at cm, image at the defective near point, answer positive
And one check that catches a wrong set-up. If a myopia calculation gives a positive power, or a hypermetropia calculation a negative one, the object and image have been swapped. The sign of the answer must agree with the type of lens the defect needs, and that agreement is the best available verification.
For myopia the object is at infinity, because that is what the person cannot see, and the image must be brought to the defective far point, where the eye can still focus.
Worked example 1. A person with myopia has a far point of m. Find the focal length and power of the correcting lens.
The lens must take an object at infinity and place its image at the far point, m in front of the eye:
The negative sign confirms a concave lens, exactly as the previous section required.
Worked example 2. A myopic person has a far point of cm. Find the power needed.
Notice the pattern: for myopia, is just the far point distance made negative, and . The nearer the far point, the stronger the lens needed.
**For hypermetropia the object is at the normal near point, cm, because that is where the person wants to be able to read, and the image must be formed at the defective near point, where the eye can focus.
Worked example 3.** A person with hypermetropia has a near point of m. Find the focal length and power of the correcting lens.
The positive sign confirms a convex lens.
Worked example 4. A hypermetropic person has a near point of cm. Find the power required.
**Both and are negative in the hypermetropia calculation, because the object and the image are both on the same side as the object — in front of the lens. The image is virtual, which is exactly what a convex lens gives when the object is inside its focus, and it is the case that lets the eye focus what it otherwise could not.
The three things to get right every time.
- Convert every distance to metres before computing the power
- Myopia: object at infinity, image at the far point, answer negative
- Hypermetropia**: object at cm, image at the defective near point, answer positive
And one check that catches a wrong set-up. If a myopia calculation gives a positive power, or a hypermetropia calculation a negative one, the object and image have been swapped. The sign of the answer must agree with the type of lens the defect needs, and that agreement is the best available verification.
Exam tip
What layout keeps an eye-defect numerical safe?
Write which point is the object and which is the image, with signs, before touching the formula. In this chapter the set-up is the difficult part and the arithmetic is easy.
- For myopia: object at infinity, so ; image at the defective far point, negative. Answer must be negative
- For hypermetropia: object at the **normal near point m; image at the defective near point, also negative. Answer must be positive
- Convert to metres first.** A near point of cm is m, and using gives a power a hundred times too small
- State the type of lens in words as well as giving the sign: *concave lens of power D*
- Name the cause when asked: too much curvature or too long an eyeball for myopia; too little power or too short an eyeball for hypermetropia
- Say that the retinal image is real and inverted, and that the brain interprets it as erect
- Describe accommodation with the muscles: ciliary muscles contract, lens thickens, focal length decreases
- For presbyopia, name the bifocal arrangement — concave above, convex below
The misconception to name. The near point of a normal eye is cm, and that is the object distance in a hypermetropia problem, not the image distance. The image goes at the person's own defective near point, because that is where their eye can still focus. Swapping the two gives a negative power for a defect that needs a convex lens — which is why the sign check at the end is worth the ten seconds.
- For myopia: object at infinity, so ; image at the defective far point, negative. Answer must be negative
- For hypermetropia: object at the **normal near point m; image at the defective near point, also negative. Answer must be positive
- Convert to metres first.** A near point of cm is m, and using gives a power a hundred times too small
- State the type of lens in words as well as giving the sign: *concave lens of power D*
- Name the cause when asked: too much curvature or too long an eyeball for myopia; too little power or too short an eyeball for hypermetropia
- Say that the retinal image is real and inverted, and that the brain interprets it as erect
- Describe accommodation with the muscles: ciliary muscles contract, lens thickens, focal length decreases
- For presbyopia, name the bifocal arrangement — concave above, convex below
The misconception to name. The near point of a normal eye is cm, and that is the object distance in a hypermetropia problem, not the image distance. The image goes at the person's own defective near point, because that is where their eye can still focus. Swapping the two gives a negative power for a defect that needs a convex lens — which is why the sign check at the end is worth the ten seconds.
Did you know
Why can you see nothing for a moment when you enter a dark hall?
Walk from bright sunlight into a darkened hall and for several seconds you can see almost nothing. Stand still and the seats gradually become visible. Walk back out and the reverse happens, but much faster.
Two separate things are adjusting, at two very different speeds.
The iris reacts first. In bright light it keeps the pupil small, admitting only a little light and protecting the retina; in the dark it widens the pupil to let in as much as possible. That takes a second or two, and it is a reflex — the same kind of involuntary response as the ones in the coordination chapter.
The retina takes far longer. Its light-sensitive cells have been working in bright light, and they need time to become sensitive enough to respond to very little. That slow adaptation is why standing still for half a minute reveals much more than the first glance did, long after the pupil has finished widening.
And the reverse is quick and uncomfortable. Coming out into sunlight, the pupil must close fast to protect the retina, and while it does you squint. Protection has to be faster than sensitisation, which is why the two directions feel so different.
None of this is accommodation. The lens is not involved at all — the hall is neither nearer nor further than the street. Adapting to brightness and focusing at a distance are two separate systems, controlled by two different muscles, and a question about dim light wants the iris and the retina while a question about a blurred page wants the ciliary muscles and the lens.
One more observation you can make. Close one eye and look at a distant scene, then cover a small part of it with your thumb. Move your thumb slowly and there is one position where a small object beside it vanishes entirely — you have found your blind spot, the point where the optic nerve leaves the retina and no receptors exist. You never notice it in ordinary life because the other eye covers the gap and the brain fills in the rest — which is the clearest everyday evidence that what you see is assembled by the brain and not simply received.
Two separate things are adjusting, at two very different speeds.
The iris reacts first. In bright light it keeps the pupil small, admitting only a little light and protecting the retina; in the dark it widens the pupil to let in as much as possible. That takes a second or two, and it is a reflex — the same kind of involuntary response as the ones in the coordination chapter.
The retina takes far longer. Its light-sensitive cells have been working in bright light, and they need time to become sensitive enough to respond to very little. That slow adaptation is why standing still for half a minute reveals much more than the first glance did, long after the pupil has finished widening.
And the reverse is quick and uncomfortable. Coming out into sunlight, the pupil must close fast to protect the retina, and while it does you squint. Protection has to be faster than sensitisation, which is why the two directions feel so different.
None of this is accommodation. The lens is not involved at all — the hall is neither nearer nor further than the street. Adapting to brightness and focusing at a distance are two separate systems, controlled by two different muscles, and a question about dim light wants the iris and the retina while a question about a blurred page wants the ciliary muscles and the lens.
One more observation you can make. Close one eye and look at a distant scene, then cover a small part of it with your thumb. Move your thumb slowly and there is one position where a small object beside it vanishes entirely — you have found your blind spot, the point where the optic nerve leaves the retina and no receptors exist. You never notice it in ordinary life because the other eye covers the gap and the brain fills in the rest — which is the clearest everyday evidence that what you see is assembled by the brain and not simply received.
Exam relevance
How are eye defects examined in JEE and NEET?
This is foundation work that appears in both a physics and a biology form later, and the numericals are examined directly.
Where the physics leads. Class 11 Ray Optics and Optical Instruments treats the eye as an optical instrument, with the same defects and the same corrective-lens calculation, and adds the derivation of the magnifying power of a simple and compound microscope relative to the near point of cm. **The value cm is used as a standard in every one of those formulas, so it is worth knowing why it is that number and not another.
Where the calculation leads. The corrective-lens problem in JEE Main is the same problem with more steps — sometimes with a contact lens, where the distances are measured from the eye rather than from a spectacle a little in front of it. The set-up is identical: the defective point is the image and the desired point is the object.
Where the biology leads. NEET covers the eye's structure in Class 11 Neural Control and Coordination, naming the three layers, the rods and cones, the fovea and the blind spot, and the photochemistry of vision. The parts you learn here with their functions are the same list, and the match-the-column questions draw on it.
Where accommodation leads. The ciliary muscle and the change of lens curvature are examined in both subjects, and the loss of accommodation with age is the standard example of a physiological limit. The physics explains why the near point exists and the biology explains why it recedes.
Question types to expect. At this level: label the diagram, define accommodation, identify the defect, calculate the power. In competitive papers: corrective-lens numericals, magnifying-power problems that use the near point, and assertion-reason items on which lens corrects which defect.
The single trap that costs marks. Choosing a convex lens for myopia. Myopia already has too much converging power, so it needs a diverging — concave — lens, and the sign of the calculated power confirms it. In a multiple-choice paper the wrong lens with the right magnitude is always among the options.
A second trap. Using the defective near point as the object in a hypermetropia problem. The object is at the normal near point of cm — that is what the person wants to see — and the image goes at their own near point. Reversing them gives a negative answer for a defect that needs a positive lens.
Board versus competitive emphasis. The CBSE paper marks the labelled diagram, the named defect, the stated cause and the signed power; a competitive paper marks the number and the lens type. The transferable habit is writing the object and image points with their signs first** — it costs one line and it decides the whole answer.
Where the physics leads. Class 11 Ray Optics and Optical Instruments treats the eye as an optical instrument, with the same defects and the same corrective-lens calculation, and adds the derivation of the magnifying power of a simple and compound microscope relative to the near point of cm. **The value cm is used as a standard in every one of those formulas, so it is worth knowing why it is that number and not another.
Where the calculation leads. The corrective-lens problem in JEE Main is the same problem with more steps — sometimes with a contact lens, where the distances are measured from the eye rather than from a spectacle a little in front of it. The set-up is identical: the defective point is the image and the desired point is the object.
Where the biology leads. NEET covers the eye's structure in Class 11 Neural Control and Coordination, naming the three layers, the rods and cones, the fovea and the blind spot, and the photochemistry of vision. The parts you learn here with their functions are the same list, and the match-the-column questions draw on it.
Where accommodation leads. The ciliary muscle and the change of lens curvature are examined in both subjects, and the loss of accommodation with age is the standard example of a physiological limit. The physics explains why the near point exists and the biology explains why it recedes.
Question types to expect. At this level: label the diagram, define accommodation, identify the defect, calculate the power. In competitive papers: corrective-lens numericals, magnifying-power problems that use the near point, and assertion-reason items on which lens corrects which defect.
The single trap that costs marks. Choosing a convex lens for myopia. Myopia already has too much converging power, so it needs a diverging — concave — lens, and the sign of the calculated power confirms it. In a multiple-choice paper the wrong lens with the right magnitude is always among the options.
A second trap. Using the defective near point as the object in a hypermetropia problem. The object is at the normal near point of cm — that is what the person wants to see — and the image goes at their own near point. Reversing them gives a negative answer for a defect that needs a positive lens.
Board versus competitive emphasis. The CBSE paper marks the labelled diagram, the named defect, the stated cause and the signed power; a competitive paper marks the number and the lens type. The transferable habit is writing the object and image points with their signs first** — it costs one line and it decides the whole answer.
Key takeaways
What should you know about the eye before dispersion?
Seven parts, one adjustment, three defects and one calculation.
- Cornea does most of the bending; iris controls the pupil, which regulates the light entering; the lens provides fine focusing; the retina detects the image; the optic nerve carries the signals to the brain
- The retinal image is real and inverted, and the brain interprets it as erect
- Accommodation is the change of the lens's focal length by the ciliary muscles — contract for near, relax for far
- **Far point of a normal eye is infinity; near point is about cm, the least distance of distinct vision
- Myopia: distant objects blurred, image forms in front of the retina, caused by too much curvature or too long an eyeball, corrected by a concave lens
- Hypermetropia: near objects blurred, image forms behind the retina, caused by too little power or too short an eyeball, corrected by a convex lens
- Presbyopia: ciliary muscles weaken with age and the near point recedes; corrected by bifocal lenses, concave above and convex below
- Cataract is cloudiness of the lens and needs surgery, not a lens
- Myopia calculation**: object at infinity, image at the far point, so
- Hypermetropia calculation: object at m, image at the defective near point, giving a positive power
- Convert to metres before computing power, and check that the sign matches the lens the defect needs
The sharpest self-test is the pair of numericals. Take a far point of cm and a near point of cm as two separate patients, compute both powers, and check that the first comes out negative and the second positive.
- Cornea does most of the bending; iris controls the pupil, which regulates the light entering; the lens provides fine focusing; the retina detects the image; the optic nerve carries the signals to the brain
- The retinal image is real and inverted, and the brain interprets it as erect
- Accommodation is the change of the lens's focal length by the ciliary muscles — contract for near, relax for far
- **Far point of a normal eye is infinity; near point is about cm, the least distance of distinct vision
- Myopia: distant objects blurred, image forms in front of the retina, caused by too much curvature or too long an eyeball, corrected by a concave lens
- Hypermetropia: near objects blurred, image forms behind the retina, caused by too little power or too short an eyeball, corrected by a convex lens
- Presbyopia: ciliary muscles weaken with age and the near point recedes; corrected by bifocal lenses, concave above and convex below
- Cataract is cloudiness of the lens and needs surgery, not a lens
- Myopia calculation**: object at infinity, image at the far point, so
- Hypermetropia calculation: object at m, image at the defective near point, giving a positive power
- Convert to metres before computing power, and check that the sign matches the lens the defect needs
The sharpest self-test is the pair of numericals. Take a far point of cm and a near point of cm as two separate patients, compute both powers, and check that the first comes out negative and the second positive.