Light Waves
5. Colour and Optical Instruments
Learning outcomes
- I can explain how white light is dispersed into a spectrum.
- I can distinguish between reflection and absorption of light.
- I can describe how the human eye forms images.
- I can explain how microscopes and telescopes use lenses.
- I can identify applications of optical instruments.
Colour and Optical Instruments
White light may appear colourless, but it contains a range of visible wavelengths. When these wavelengths are separated, they form the familiar visible spectrum from red through violet.
The behaviour of light also explains why objects appear different colours. Materials can reflect, absorb, or transmit different wavelengths of visible light.
These same principles of reflection, refraction, image formation, and colour are used in the human eye and in optical instruments such as:
- microscopes
- telescopes
- cameras
- binoculars
- magnifying glasses
- spectacles
White Light
White light is a mixture of many wavelengths of visible light.
The visible spectrum is commonly described using the sequence:
red → orange → yellow → green → blue → violet
These colours do not represent completely separate sections. The visible spectrum is continuous, so there are many wavelengths and colours between them.
Different wavelengths are perceived by our visual system as different colours.
The Visible Spectrum
Visible light occupies only a small part of the electromagnetic spectrum.
Its wavelengths are approximately:
380–750 nm
with the exact limits depending on how human visual sensitivity is defined.
Across the visible spectrum:
red → longer wavelength, lower frequency
violet → shorter wavelength, higher frequency
A nanometre is:
1 nm = 10⁻⁹ m
Dispersion
Dispersion is the separation of light into its component wavelengths.
A glass prism can disperse white light into a visible spectrum.
This happens because the refractive index of glass is slightly different for different wavelengths of visible light.
As a result, different wavelengths are refracted by different amounts.
Why a Prism Separates Colours
When white light enters a prism:
- The light passes from air into glass.
- Its speed decreases.
- The light refracts.
- Different wavelengths refract by slightly different amounts.
- The light reaches the second surface of the prism.
- It refracts again as it leaves the glass.
- The different wavelengths emerge travelling in different directions.
The result is a visible spectrum.
In typical glass:
violet light is refracted more strongly than red light.
Refractive Index and Colour
The refractive index of a material is not exactly the same for every wavelength.
For visible light in ordinary glass:
- red light generally experiences a slightly lower refractive index
- violet light generally experiences a slightly higher refractive index
Therefore, violet light is normally slowed and bent more strongly than red light.
This wavelength dependence of refractive index produces dispersion.
Rainbows
A rainbow is a natural example of the separation of sunlight into colours.
Sunlight entering a water droplet undergoes:
- refraction
- dispersion
- internal reflection
- further refraction as it leaves
Different wavelengths emerge from the droplet at different angles.
Millions of droplets contribute light to the rainbow seen by an observer.
Recombining the Spectrum
The colours separated by a prism can be recombined.
If the different wavelengths are directed back together correctly, they can once again produce light that appears white.
This demonstrates that the prism does not create the colours.
Instead:
the colours were already present in the white light.
The prism simply separates them.
Reflection of Light
Reflection occurs when light strikes a surface and some of the light returns into the original medium.
We see most ordinary objects because they reflect some light toward our eyes.
For example, a red object illuminated by white light appears red because it reflects wavelengths that stimulate our visual system as red more strongly than many of the other visible wavelengths.
Absorption of Light
Absorption occurs when light energy is taken in by a material rather than being reflected or transmitted.
The absorbed energy is converted into other forms, commonly including thermal energy.
For example, a dark surface exposed to sunlight often becomes warmer because it absorbs a large fraction of the incoming radiation.
Reflection vs Absorption
| Reflection | Absorption |
|---|---|
| Light leaves the surface | Light energy is taken into the material |
| Helps us see objects | Can transfer energy to the material |
| May be selective by wavelength | May also be selective by wavelength |
| Important in mirrors and colour | Important in colour and heating |
Most real objects both reflect and absorb some wavelengths.
Why Objects Have Colour
The colour we perceive depends strongly on which wavelengths reach our eyes.
Suppose white light shines on a red object.
The object may:
- reflect much of the red light
- absorb much of the other visible light
The reflected light reaches the eye.
The brain interprets the resulting visual signals as:
red
White Objects
A white surface reflects a broad range of visible wavelengths relatively strongly.
When these wavelengths reach the eye together, the surface can appear white.
Therefore, a white object under white light generally:
reflects much of the visible spectrum
rather than absorbing most of it.
Black Objects
An ideal black surface would absorb all visible wavelengths and reflect none.
Real black objects are not perfect absorbers, but they typically absorb a large fraction of visible light.
Because relatively little visible light is reflected toward the eye, the object appears dark.
Absorbed radiation can contribute to heating the object.
Colour Depends on the Light Source
An object's apparent colour depends on the wavelengths available in the incoming light.
A red object under white light can reflect red wavelengths.
But suppose the object is illuminated only with blue light.
There may be little or no red light available to reflect.
The red object may therefore appear:
very dark or black
Colour is therefore not determined by the object alone.
It depends on the interaction between:
- light source
- object
- observer
Coloured Filters
A coloured filter selectively transmits some wavelengths while absorbing others.
For example, an idealized red filter:
- transmits mainly red wavelengths
- absorbs much of the other visible light
Filters are used in:
- photography
- theatre lighting
- scientific instruments
- displays
- optical experiments
Additive Colour Mixing
When coloured light is combined, the process is called additive colour mixing.
The primary colours of light are commonly taken as:
- red
- green
- blue
Combining them in different proportions can produce many perceived colours.
For example:
red + green → yellow
green + blue → cyan
blue + red → magenta
Combining suitable amounts of:
red + green + blue
can produce the perception of:
white
This principle is used in electronic displays.
Pigments Are Different from Light
Paints, inks, and pigments do not combine colours in the same way as coloured light.
Pigments work mainly by selectively:
- absorbing wavelengths
- reflecting wavelengths
Mixing pigments usually causes more wavelengths to be absorbed.
This is called subtractive colour mixing.
Therefore:
mixing coloured light and mixing paint are not the same process.
The Human Eye
The human eye is an optical system that uses refraction to form an image.
Important structures include:
- cornea
- iris
- pupil
- lens
- retina
- optic nerve
Light entering the eye is focused onto the retina, where specialized cells detect it.
The Cornea
The cornea is the transparent curved surface at the front of the eye.
It provides a large part of the eye's refractive power.
As light enters the eye through the cornea, it is refracted.
The cornea begins the process of focusing incoming light toward the retina.
The Iris and Pupil
The iris is the coloured part of the eye.
The opening in its centre is the:
pupil
The iris changes the size of the pupil and therefore helps control how much light enters the eye.
In brighter conditions:
pupil becomes smaller
In darker conditions:
pupil becomes larger
The pupil itself is an opening, not a solid black structure.
The Eye's Lens
Behind the pupil is the lens.
The lens is transparent and contributes to focusing light onto the retina.
Unlike a simple rigid glass lens, the eye's lens can change shape.
This allows the eye to focus on objects at different distances.
Accommodation
Accommodation is the process by which the eye changes its focusing power to view objects at different distances.
For nearby objects, the lens generally becomes:
more strongly curved
and has greater focusing power.
For distant objects, the lens becomes:
less strongly curved
and requires less focusing power.
Muscles associated with the lens control these changes.
The Retina
The retina is the light-sensitive layer at the back of the eye.
The eye's optical system forms an image on the retina.
The retinal image is:
- real
- inverted
- usually reduced
The retina contains photoreceptor cells that respond to light.
These include:
- rods
- cones
Rods and Cones
Rods are highly sensitive to low light levels and are important for vision in dim conditions.
Cones are important for:
- colour vision
- detailed vision
- brighter-light conditions
Humans normally have three main types of cone response, with different sensitivities across the visible spectrum.
The brain compares signals from these photoreceptors to produce our perception of colour.
From Retina to Brain
The retina converts information from incoming light into electrical signals.
These signals travel along the:
optic nerve
to the brain.
The brain processes this information to produce visual perception.
Therefore, seeing involves both:
optics in the eye
and:
information processing by the nervous system.
Image Formation in the Eye
A simplified sequence is:
- Light reflects from an object.
- Light enters the cornea.
- The cornea refracts the light.
- Light passes through the pupil.
- The lens provides additional focusing.
- Light converges onto the retina.
- A real, inverted image forms on the retina.
- Photoreceptors respond to the light.
- Electrical signals travel through the optic nerve.
- The brain processes the signals into visual perception.
Cameras and Eyes
A camera and an eye have several useful similarities.
| Human Eye | Camera |
|---|---|
| Cornea and lens focus light | Lens system focuses light |
| Iris controls pupil size | Aperture controls opening |
| Retina detects light | Image sensor detects light |
| Accommodation changes focus | Camera focusing system changes focus |
| Optic nerve carries signals | Electronics transmit image data |
The comparison is useful, although the biological eye is much more complex than a simple camera.
Optical Instruments
An optical instrument uses light to help us observe, measure, record, or analyze something.
Examples include:
- magnifying glasses
- cameras
- microscopes
- telescopes
- binoculars
- spectacles
- projectors
- periscopes
- endoscopes
Optical instruments control light using components such as:
- lenses
- mirrors
- prisms
- filters
- sensors
The Magnifying Glass
A magnifying glass is one of the simplest optical instruments.
It uses a:
converging lens
When the object is positioned inside the focal length, the lens produces an image that is:
- virtual
- upright
- enlarged
Magnifying glasses are useful for examining small details.
Microscopes
A microscope allows very small objects to be observed in greater detail.
A simple compound microscope uses two main lens systems:
- objective lens
- eyepiece lens
Both are generally converging lens systems.
The Objective Lens
The objective lens is positioned close to the specimen.
Its purpose is to collect light from the small object and form an enlarged intermediate image.
In a compound microscope, the objective is responsible for much of the instrument's ability to resolve fine detail.
Different objective lenses can provide different magnifications and resolving powers.
The Eyepiece
The eyepiece, or ocular lens, is the part through which the observer looks.
It acts on the intermediate image produced by the objective.
The eyepiece allows that image to be viewed at greater angular size.
The objective and eyepiece therefore work together to produce a strongly magnified view.
Total Microscope Magnification
A common simplified calculation is:
total magnification = objective magnification × eyepiece magnification
For example:
objective = 40×
eyepiece = 10×
Then:
total magnification = 40 × 10
total magnification = 400×
The specimen appears at an angular scale corresponding to a total magnification of 400 times under the stated microscope convention.
Magnification vs Resolution
Greater magnification does not automatically reveal more detail.
Magnification describes how much larger an image appears.
Resolution describes the ability to distinguish two nearby points as separate.
An image can be made larger without revealing additional detail.
Therefore, a good microscope needs both:
- useful magnification
- high resolution
Applications of Microscopes
Microscopes are used in:
- biology
- medicine
- microbiology
- materials science
- electronics
- forensic science
- environmental science
They allow scientists to investigate structures too small to observe clearly with the unaided eye.
Telescopes
A telescope is designed primarily to collect light from distant objects and allow them to be observed at greater angular size or with greater detail.
Telescopes are used to observe:
- the Moon
- planets
- stars
- nebulae
- galaxies
- other distant objects
Two important optical telescope types are:
- refracting telescopes
- reflecting telescopes
Refracting Telescopes
A refracting telescope primarily uses lenses.
A basic refracting telescope contains:
- an objective lens
- an eyepiece
The objective is a converging lens with a relatively long focal length.
Light from a very distant object arrives in nearly parallel rays.
The objective lens brings these rays toward a focus and forms an image.
The eyepiece then allows the observer to examine that image at greater angular magnification.
Telescope Objective
The telescope objective has an extremely important job:
collecting light
A larger objective can collect more light.
This makes faint astronomical objects easier to observe.
The objective also affects the telescope's ability to distinguish fine detail.
Therefore, telescope performance is not simply about magnification.
Telescope Eyepiece
The eyepiece allows the image formed by the objective to be viewed at greater angular size.
For a simple astronomical telescope, angular magnification depends on the focal lengths of the objective and eyepiece.
A simplified magnitude relationship is:
magnification = objective focal length / eyepiece focal length
For example:
objective focal length = 1000 mm
eyepiece focal length = 20 mm
magnification = 1000/20
magnification = 50×
Reflecting Telescopes
A reflecting telescope uses a curved primary mirror rather than a large objective lens to collect and focus light.
A concave primary mirror reflects incoming light toward a focus.
Additional mirrors and an eyepiece or detector then allow the image to be observed or recorded.
Reflecting telescopes are extremely important in modern astronomy.
Refracting vs Reflecting Telescopes
| Refracting Telescope | Reflecting Telescope |
|---|---|
| Uses objective lens | Uses primary mirror |
| Focuses by refraction | Focuses mainly by reflection |
| Can suffer chromatic aberration | Mirrors avoid chromatic aberration from the primary |
| Large lenses can be difficult to manufacture/support | Large mirrors can be supported from behind |
| Common in smaller telescopes | Common in large astronomical telescopes |
Both types collect electromagnetic radiation and form images of distant objects.
Chromatic Aberration
Because different wavelengths are refracted by slightly different amounts, a simple lens may focus different colours at slightly different positions.
This produces:
chromatic aberration
It may appear as coloured fringes around an image.
Optical designers reduce chromatic aberration by:
- combining different lens materials
- using multiple lens elements
- using mirrors instead of lenses in some instruments
Dispersion therefore creates both useful effects and engineering challenges.
Binoculars
Binoculars use lenses and prisms to provide magnified views of distant objects.
A typical system contains:
- objective lenses
- eyepieces
- prisms
The prisms help:
- redirect light
- make the instrument more compact
- produce a conveniently oriented image
Binoculars demonstrate how several optical components can work together.
Cameras
A camera uses a lens system to form a real image on a sensor.
The camera controls factors such as:
- focus
- amount of light
- exposure time
- focal length
Different lenses can provide different:
- fields of view
- image scales
- focusing properties
Cameras are used in everything from phones to scientific instruments.
Spectroscopes
A spectroscope separates light according to wavelength.
It may use:
- a prism
- a diffraction grating
The resulting spectrum can reveal information about a light source.
Scientists can use spectra to investigate:
- chemical composition
- temperature
- motion
- astronomical objects
This makes dispersion and spectral analysis powerful scientific tools.
Optical Instruments in Medicine
Optical instruments have many medical applications.
Examples include:
- microscopes for examining cells
- endoscopes for viewing inside the body
- corrective lenses
- retinal imaging systems
- surgical microscopes
Modern instruments may combine lenses, mirrors, optical fibres, electronic sensors, and computers.
Endoscopes
An endoscope allows doctors to view locations inside the body.
Depending on the design, light and images can be transmitted using:
- optical fibres
- miniature cameras
- lens systems
Optical fibres can guide light through flexible pathways, making them particularly useful in medical instruments.
Optical Instruments in Science
Scientists use optical instruments to investigate objects across enormous scales.
Microscopes investigate the very small.
Telescopes investigate the extremely distant.
Spectroscopes analyze the wavelengths of light.
Cameras record images.
Together, optical instruments allow scientists to collect information that human eyes alone could not obtain.
Comparing Microscopes and Telescopes
Microscopes and telescopes both use optical principles, but they solve different problems.
| Microscope | Telescope |
|---|---|
| Observes very small nearby objects | Observes distant objects |
| Objective is close to specimen | Objective receives nearly parallel rays from distant objects |
| Produces strongly magnified view | Produces increased angular view of distant objects |
| Resolution is essential | Light gathering and resolution are essential |
| Used in biology and materials science | Used heavily in astronomy |
Both may use:
- converging lenses
- objective systems
- eyepieces
- focusing mechanisms
Image Formation Across Optical Systems
The same basic physics appears repeatedly.
Eye
Light is focused onto the retina.
Camera
Light is focused onto a sensor.
Microscope
An objective forms an enlarged intermediate image that is viewed through an eyepiece.
Refracting telescope
An objective forms an image of a distant object, which is viewed through an eyepiece.
Reflecting telescope
A curved mirror collects and focuses light.
The devices differ, but all rely on predictable behaviour of light.
Choosing an Optical Instrument
Different instruments are suited to different purposes.
To view a tiny cell:
microscope
To observe a distant planet:
telescope
To permanently record an image:
camera
To correct eyesight:
spectacles or contact lenses
To examine a spectrum:
spectroscope
To examine internal parts of the body:
endoscope
The design depends on what information needs to be collected.
A Complete Colour Example
White light shines on a blue object.
White light contains many visible wavelengths.
The object absorbs much of the incoming visible light but reflects wavelengths that strongly stimulate our perception of blue.
These reflected wavelengths enter the eye.
The cornea and lens focus the light onto the retina.
Photoreceptors respond.
Signals travel through the optic nerve.
The brain processes the information.
The object is perceived as:
blue
This example connects:
light source → reflection and absorption → eye → perception
A Complete Microscope Example
A microscope has:
objective magnification = 20×
eyepiece magnification = 10×
Calculate the total magnification.
Use:
total magnification = objective × eyepiece
total magnification = 20 × 10
total magnification = 200×
If the image looks larger but no additional detail becomes visible, simply increasing magnification further may not help.
The instrument's:
resolution
must also be considered.
A Complete Telescope Example
A refracting telescope has:
objective focal length = 1200 mm
eyepiece focal length = 25 mm
Calculate the magnitude of the angular magnification.
magnification = objective focal length / eyepiece focal length
magnification = 1200/25
magnification = 48×
The telescope makes the distant object appear at a much greater angular size.
However, useful telescope performance also depends on factors such as:
- objective diameter
- optical quality
- resolution
- atmospheric conditions
Magnification alone does not determine telescope quality.
A Reliable Strategy for Colour Problems
When analyzing the colour of an object:
- Identify the colour or wavelengths in the light source.
- Determine which wavelengths the object absorbs.
- Determine which wavelengths it reflects or transmits.
- Identify which wavelengths reach the observer's eye.
- Use those wavelengths to predict the perceived colour.
Always consider both:
the object and the illumination.
A Reliable Strategy for Optical Instruments
When analyzing an optical instrument:
- Identify the purpose of the instrument.
- Identify the main optical components.
- Determine whether reflection or refraction is being used.
- Identify whether lenses are converging or diverging.
- Determine how light travels through the system.
- Identify where an image forms.
- Determine whether the image is real or virtual where relevant.
- Determine whether magnification is involved.
- Consider resolution as well as magnification.
- Connect each optical component to its function.
Common Mistakes
Thinking a prism creates colours
The wavelengths are already present in white light. The prism separates them.
Thinking every colour bends by the same amount
Different wavelengths generally experience slightly different refractive indices.
Thinking red light bends more than violet in ordinary glass
Violet is generally refracted more strongly than red.
Thinking colour belongs only to an object
Perceived colour depends on the light source, the object's optical properties, and the observer.
Thinking a red object produces red light
Most ordinary red objects reflect available red wavelengths rather than producing them.
Thinking black objects reflect all colours
Black surfaces generally absorb much of the visible light that reaches them.
Thinking white objects absorb all colours
White surfaces generally reflect a broad range of visible wavelengths.
Confusing reflection with absorption
Reflected light leaves the surface. Absorbed light transfers energy to the material.
Thinking the pupil focuses light
The pupil is an opening. The cornea and lens perform the major focusing.
Thinking the lens performs all refraction in the eye
The cornea provides a large part of the eye's refractive power.
Thinking the retinal image is upright
The optical image formed on the retina is inverted.
Thinking magnification automatically means greater detail
Resolution determines whether fine details can actually be distinguished.
Thinking telescopes are useful only because they magnify
Light-gathering ability and resolution are also extremely important.
Thinking all telescopes use lenses
Reflecting telescopes use mirrors as their main light-collecting elements.
Key Terms
White light — light containing a broad range of visible wavelengths.
Visible spectrum — range of electromagnetic wavelengths detectable by human vision.
Dispersion — separation of light according to wavelength.
Reflection — return of light from a surface or boundary.
Absorption — transfer of light energy into a material.
Transmission — passage of light through a material.
Selective reflection — reflection of some wavelengths more strongly than others.
Selective absorption — absorption of some wavelengths more strongly than others.
Additive colour mixing — production of colours by combining coloured light.
Subtractive colour mixing — production of colours through selective absorption, as with pigments.
Cornea — transparent front surface of the eye that provides substantial refraction.
Iris — coloured structure that controls pupil size.
Pupil — opening through which light enters the eye.
Lens — transparent structure that helps focus light onto the retina.
Accommodation — adjustment of the eye's focusing power for different object distances.
Retina — light-sensitive layer at the back of the eye.
Rod — photoreceptor particularly important in low-light vision.
Cone — photoreceptor important in colour and detailed vision.
Optic nerve — nerve carrying visual signals from the retina toward the brain.
Optical instrument — device that controls or detects light for observation, measurement, recording, or analysis.
Objective lens — main light-collecting or image-forming lens in instruments such as microscopes and refracting telescopes.
Eyepiece — lens system through which an observer views an optical image.
Magnification — measure of how much larger an image appears compared with an object or, in telescopes, its angular enlargement.
Resolution — ability to distinguish nearby details as separate.
Chromatic aberration — image distortion caused by different wavelengths being focused differently.
Spectroscope — instrument used to separate and analyze light according to wavelength.
Refracting telescope — telescope using lenses to collect and focus light.
Reflecting telescope — telescope using a mirror as its primary light-collecting element.
Key Takeaways
- White light contains a broad range of visible wavelengths.
- A prism can separate white light into a visible spectrum.
- This separation is called dispersion.
- Dispersion occurs because refractive index varies with wavelength.
- Violet light is generally refracted more strongly than red light in ordinary glass.
- A rainbow involves refraction, dispersion, internal reflection, and further refraction in water droplets.
- A prism separates colours that were already present in white light.
- Reflection occurs when light returns from a surface.
- Absorption occurs when light energy is taken into a material.
- The perceived colour of an object depends on which wavelengths reach the eye.
- A white surface generally reflects a broad range of visible wavelengths.
- A black surface generally absorbs a large proportion of visible light.
- An object's apparent colour can change when the illumination changes.
- Red, green, and blue are the primary colours used in additive mixing of light.
- The human eye uses the cornea and lens to focus light onto the retina.
- The iris controls pupil size and therefore helps regulate incoming light.
- The retinal image is real, inverted, and usually reduced.
- Rods and cones detect light, and the brain processes their signals to create visual perception.
- Microscopes use lens systems to produce highly magnified views of small objects.
- Microscope performance depends on resolution as well as magnification.
- Refracting telescopes use objective lenses to collect and focus light from distant objects.
- Reflecting telescopes use mirrors as their main light-collecting components.
- Telescope performance involves light gathering and resolution as well as magnification.
- Cameras, binoculars, spectacles, spectroscopes, endoscopes, microscopes, and telescopes all apply optical principles.
- Understanding reflection, refraction, dispersion, image formation, and colour allows optical instruments to be designed for specific purposes.
Check Your Understanding
- Explain what is meant by dispersion of white light.
- Why does a glass prism separate white light into different colours?
- Compare the wavelength and amount of refraction of red and violet light in a typical glass prism.
- Explain how refraction, dispersion, and internal reflection contribute to the formation of a rainbow.
- Distinguish between reflection and absorption of light.
- Explain why a red object can appear dark when illuminated only with blue light.
- Compare how idealized white and black surfaces interact with visible light.
- Describe the roles of the cornea, iris, pupil, lens, retina, and optic nerve in vision.
- What type of image is formed on the retina? Describe its type, orientation, and relative size.
- Explain how accommodation allows the eye to focus on objects at different distances.
- Describe the roles of the objective lens and eyepiece in a compound microscope.
- A microscope has a 40× objective and a 10× eyepiece. Calculate its total magnification. Explain why increasing magnification does not necessarily reveal more detail.
- Explain how a refracting telescope uses an objective lens and eyepiece to observe distant objects.
- Compare refracting and reflecting telescopes, including the main optical component responsible for collecting and focusing light.
- Choose one optical instrument from a microscope, telescope, camera, binoculars, spectroscope, or endoscope. Explain its purpose, identify its important optical components, and describe how the behaviour of light allows the instrument to work.