SPI Physics · Light and Optics
The Eye, Dispersion and Instruments
Defects of the eye are corrected with the right lens, a prism splits white light into seven colours because each bends by a different amount, and the air's shifting layers make stars twinkle.
Why this matters
Six questions in the bank, five EASY and one MODERATE: the lens for astigmatism and for long sight, the name for a prism splitting white light, the colour glass bends least, why a twinkling star looks brighter, and the instrument for distant objects. All are recall, so the three tables on this page are the whole job.
Concept 1 of 3: Defects of vision and the lens that corrects each
A healthy eye changes the shape of its lens to bring any object from 25 cm to far away into focus on the retina. When the image lands in front of the retina or behind it, a lens in the spectacles moves it back.
Definition
- The eye lens forms a real, inverted image on the retina. The ciliary muscles change the lens's focal length to focus near or far (accommodation).
- A normal eye sees clearly from its near point, 25 cm, to its far point at infinity.
- Image in front of the retina: the eye needs a diverging (concave, negative power) lens. Image behind it: a converging (convex, positive power) lens.
| Defect | What goes wrong | Corrected with |
|---|---|---|
| Myopia (short sight) | Far objects blur; the image forms in front of the retina | Concave lens |
| Hypermetropia (long sight) | Near objects blur; the image forms behind the retina | Convex lens |
| Presbyopia | Old age: the ciliary muscles weaken, so near and often far vision both suffer | Bifocal lens |
| Astigmatism | The cornea is curved unequally, so lines in one direction blur | Cylindrical lens Not convex or concave: a cylindrical lens. |
| Cataract | The eye lens turns cloudy | Surgery to replace the lens |
Short sight takes a concave lens, long sight a convex one: the opposite of what the names might suggest.
Practice this conceptself-check · 4 quick reps
The same idea in a practice question:
Practice questionEasy
Example 1 · Light and Optics · The Eye, Dispersion and Instruments
The defect of astigmatism can be rectified using ____ :-
Concave lens for long sight
Long sight (hypermetropia) puts the image behind the retina and needs a converging, convex lens. The concave lens is for short sight.
Bifocal for astigmatism
A bifocal lens is for presbyopia. Astigmatism comes from an unevenly curved cornea and needs a cylindrical lens.
Concept 2 of 3: Dispersion, the spectrum and atmospheric refraction
White light is a mix of colours. Glass slows each colour by a slightly different amount, so a prism bends them by different angles and they fan out. The moving air above us bends starlight in the same way, but by an amount that keeps changing.
Definition
- Dispersion is the splitting of white light into its colours. The band of colours is the spectrum: VIBGYOR (violet, indigo, blue, green, yellow, orange, red).
- Red has the longest wavelength, bends least and has the lowest refractive index in glass. Violet bends most.
- Atmospheric refraction: air's temperature and density keep changing, so starlight is bent by varying amounts. More light bent towards the eye makes the star look brighter, less makes it dimmer: it twinkles.
| Event | Cause |
|---|---|
| A prism forms a spectrum | Dispersion: each colour refracts by a different amount |
| A rainbow | Dispersion and reflection inside raindrops; seen with the Sun behind you |
| Stars twinkle | Atmospheric refraction that keeps changing |
| Planets do not twinkle | They are close, so they look like discs, and the changes average out |
| The Sun is seen about 2 minutes before it rises | Atmospheric refraction bends its light over the horizon |
| The sky is blue | Air scatters blue light much more than red |
| Danger signals are red | Red is scattered least, so it is seen from far away |
Splitting is dispersion; bending is refraction; spreading by tiny particles is scattering.
Practice this conceptself-check · 4 quick reps
The same idea in a practice question:
Practice questionEasy
Example 2 · Light and Optics · The Eye, Dispersion and Instruments
Splitting of white light into seven colours on passing through a glass prism is called ____ :-
Calling the splitting 'refraction'
Each colour is refracted, but the splitting of white light into colours has its own name: dispersion.
Violet bends least
Red bends least and has the smallest refractive index in glass; violet bends most.
Planets twinkle too
Planets are near enough to look like small discs, so the changes from many points cancel and they shine steadily.
Concept 3 of 3: Optical instruments and what each is for
Each instrument stretches the eye in one direction: farther away, smaller, around a corner, or into the colours of light. Match the instrument to its job.
Definition
Two pairs are swapped most often:
- Telescope for things far away; microscope for tiny things close by.
- Spectroscope splits light into a spectrum; periscope looks over an obstacle.
| Instrument | Used for |
|---|---|
| Telescope | Distant objects such as stars and planets |
| Binoculars | Distant objects, with both eyes |
| Microscope | Very small objects close by, such as cells |
| Periscope | Seeing over an obstacle, using two plane mirrors |
| Kaleidoscope | Repeating patterns from plane mirrors |
| Spectroscope | Splitting light into its spectrum to study it |
| Chronometer | Measuring time very accurately Not an optical instrument at all. |
The suffix tells the job: tele means far, micro means small, peri means around.
Practice this conceptself-check · 3 quick reps
The same idea in a practice question:
Practice questionEasy
Example 3 · Light and Optics · The Eye, Dispersion and Instruments
Distant objects can be seen with the help of ?
Telescope and microscope swapped
A telescope is for far objects and a microscope for tiny near ones. 'Tele' means far, as in telephone.
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 (3)
Defects of vision and the lens that corrects each5 rows
| Defect | What goes wrong | Corrected with |
|---|---|---|
| Myopia (short sight) | Far objects blur; the image forms in front of the retina | Concave lens |
| Hypermetropia (long sight) | Near objects blur; the image forms behind the retina | Convex lens |
| Presbyopia | Old age: the ciliary muscles weaken, so near and often far vision both suffer | Bifocal lens |
| Astigmatism | The cornea is curved unequally, so lines in one direction blur | Cylindrical lens Not convex or concave: a cylindrical lens. |
| Cataract | The eye lens turns cloudy | Surgery to replace the lens |
Short sight takes a concave lens, long sight a convex one: the opposite of what the names might suggest.
Dispersion, the spectrum and atmospheric refraction7 rows
| Event | Cause |
|---|---|
| A prism forms a spectrum | Dispersion: each colour refracts by a different amount |
| A rainbow | Dispersion and reflection inside raindrops; seen with the Sun behind you |
| Stars twinkle | Atmospheric refraction that keeps changing |
| Planets do not twinkle | They are close, so they look like discs, and the changes average out |
| The Sun is seen about 2 minutes before it rises | Atmospheric refraction bends its light over the horizon |
| The sky is blue | Air scatters blue light much more than red |
| Danger signals are red | Red is scattered least, so it is seen from far away |
Splitting is dispersion; bending is refraction; spreading by tiny particles is scattering.
Optical instruments and what each is for7 rows
| Instrument | Used for |
|---|---|
| Telescope | Distant objects such as stars and planets |
| Binoculars | Distant objects, with both eyes |
| Microscope | Very small objects close by, such as cells |
| Periscope | Seeing over an obstacle, using two plane mirrors |
| Kaleidoscope | Repeating patterns from plane mirrors |
| Spectroscope | Splitting light into its spectrum to study it |
| Chronometer | Measuring time very accurately Not an optical instrument at all. |
The suffix tells the job: tele means far, micro means small, peri means around.
Watch out for (6)
- Concave lens for long sight→ Defects of vision and the lens that corrects each
- Bifocal for astigmatism→ Defects of vision and the lens that corrects each
- Calling the splitting 'refraction'→ Dispersion, the spectrum and atmospheric refraction
- Violet bends least→ Dispersion, the spectrum and atmospheric refraction
- Planets twinkle too→ Dispersion, the spectrum and atmospheric refraction
- Telescope and microscope swapped→ Optical instruments and what each is for
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