Light and Vision: History and Properties

Knowledge About Light and Vision

• Knowledge about light and vision comes from explanations, inventions, and investigations.

Challenge of Light (Ancient Times)

• Mirrors were used in ancient times in China and Greece.
• Archimedes thought light was beams of light coming from our eyes.
• Pythagoras stated light was reflected.
• Euclid stated light travels in straight lines.

1st Century

• Ptolemy found that light bends when it travels from air to glass.

Middle Ages

• Al-Haythem wrote a book to explain optics, being the first to accurately describe how vision worked.

1670

• Isaac Newton showed that white light is a mixture of different colors of light.

1676

• Ole Romer determined the speed of light.

1920's

• Albert A. Michelson was able to determine more accurately the speed of light.

Properties of Light

• Light travels in straight lines.
• Light can be reflected.
• Light can be bent.
• Light is a form of energy.

Optical Devices (1300 AD)

• Alessandra della Spina wore the 1st pair of eyeglasses.

1595

• Zacharias Jansen built the 1st microscope.

17th Century

• Antonie van Leeuwenhoek is credited with the discovery of cells using a very simple microscope.

1600

• Galileo Galilei invented the refracting telescope.
• Isaac Newton invented the reflecting telescope.

1854

• Ignatio Porro invented the prism erecting system (simple binoculars).

1985

• The 1st Endoscope was used.

1990

• The Hubble Space Telescope, named after Edwin Hubble, expanded understanding of the universe and formed the basis of the Big Bang theory.

Light Behaves Predictably

• Light behaves in predictable ways.

Light Travels in Rays and Interacts with Materials

• Light travels in rays and interacts with materials.
• Because light travels in straight lines', the ray model of light can help to explain certain properties of light.
• A ray is a straight line that represents the path of a beam of light.
• Ray diagrams can help to demonstrate brightness or intensity of light through changes in distance.
• The ray model helps to explain how shadows can be formed when an object blocks the ray of light.

Light Interacts with Materials

• Light travels in straight lines until it strikes a surface.
• The type of surface the light hits determines how the light will continue.
• If a surface is translucent, light passes through it but is diffused so that one cannot see clearly the details of whatever is on the other side (a frosted glass window is translucent).
• If a surface is transparent, light passes through it nearly or wholly undiffused, so that one can see clearly the details of whatever is on the other side (an ordinary glass window is transparent).
• A surface that permits no light to pass through it is opaque; you can see nothing through it at all (a door is opaque).
• Luminous objects give off light (they are light sources).
• Non-luminous objects do not give off light.

Types of Reflection

• Diffuse reflection occurs if light hits a rough or uneven surface, the light is scattered.
• When light hits a smooth surface, regular reflection occurs, the light reflects at an opposite angle to the angle it hits.

The Law of Reflection

• Reflection is the process in which light strikes a surface and bounces back off that surface.
• How it bounces off the surface depends on the Law of Reflection and the type of surface it hits.
• Light coming from a light source is called an incident ray and the light that bounces off the surface is called a reflected ray.
• A line that is perpendicular (90° with the surface) to the plane mirror is called the normal line.
• The angle between the incident ray and the normal line is called the angle of incidence (i).
• The angle between the reflected ray and the normal line is called the angle of reflection (r).
• The Law of reflection states that: the angle of incidence equals the angle of reflection.
• The incident ray, the normal line and the reflected ray all lie in the same plane (an imaginary flat surface).

Reflecting Light with Curved Mirrors

• An image is formed in a mirror because light reflects off all points on the object being observed in all directions.
• The rays that reach your eye appear to be coming from a point behind the mirror.
• Because your brain knows that light travels in a straight line, it interprets the pattern of light that reaches your eye as an image of an object you are looking at.
• Mirrors that cave in are called Concave mirrors.
• Concave mirrors form an image that appears to be closer than it actually is and can be useful because it can also reflect light from a large area.
• Focal point is in front of mirror.
• They are used in security devices, flashlights, telescopes, cosmetic mirrors and car headlights.
• Mirrors that bulge out are called Convex mirrors.
• Convex mirrors form images that appear much smaller and farther away than the object but they can reflect light from a large area.
• Focal point is in behind mirror.
• They are used in rear-view mirrors and side mirrors on automobiles.

Transparent Substances Refract Light

• Refraction is the process in which light is bent when it travels from one medium to another.
• Light bends because it changes speed when it moves through materials that have different densities.
• The bending of light makes the object's image appear to be in a different position than it really is.
• Light travels slower in materials that are denser, because there are more particles.
• The Law of Refraction states that when light travels from one medium, to a more dense medium, the light will be bent toward the normal, and when it exits the denser medium into a less dense medium it will bend away from the normal.
• The new direction of light is called the angle of refraction.
• Refraction can also occur when light travels through air at different temperatures, because warm air is less dense than cold air.
• The refraction of light through air is called a mirage.

What happens when light strikes a surface?

• Absorption: Energy Transformation. Occurs on rough, dark, and opaque surfaces where some light is reflected
• Reflection: Bounces off. Occurs on smooth, shiny surfaces where some light is absorbed
• Refraction: Travels through in a new direction. Occurs on different transparent medium where some light is reflected.

Lenses Refract and Focus Light

• A lens is a curved piece of transparent material (glass/plastic).
• When light rays pass through it, the light is refracted, causing the rays to bend.
• A double concave lens is thinner and flatter in the middle than the edges.
• Light passing through the thicker more curved areas of the lens will bend more than light passing through the thinner areas, causing the light to spread out or diverge.
• A double convex lens is thicker in the middle than around the edges.
• This causes the light to come together at a focal point, or converge.
• Lenses are useful optical devices.
• Eyeglasses have been made from lenses since the thirteenth century.
• A convex lens refracts the light rays from an object so they can be focused.
• Different size lenses can converge the light rays at different distances, enabling corrections to be made to focal points.
• Light from the left portion of the object is directed to the right and the light from the top is directed to the bottom which inverts the image.
• Overhead projectors and film projectors do this.
• The formation of an image with a double convex lens depends on where the object is placed and the orientation of the light source.

Light is part of the Electromagnetic Spectrum and travels in waves

• Light is part of the Electromagnetic Spectrum and travels in waves.

The Wave Model of Light

• The wave model of light pictures light traveling as a wave.
• It doesn't explain everything about how light behaves but it helps us visualize it.
• Thinking about light traveling in waves helps to explain unpredictable behavior, like when light curves around a opening.
• When light passes through a small opening, the waves spread out.
• If the wavelength is short, the waves spread out very little, whereas longer wavelengths spread out more.

Light Waves In Action

• Sunsets can be explained using the wave model of light.
• As light waves from the sun travel through Earth's atmosphere, they strike particles of different sizes, including dust and other elements. This is called scattering.
• The longer wavelengths of the reds and oranges tend to pass around these particles, whereas, the shorter wavelengths of blue and violet, strike the particles and reflect and scatter.
• At sunset, the light we see passes through about 700 kms of the Earth's atmosphere with many more particles in the atmosphere so many more blue and violet waves are reflected away. Red and orange are the vibrant colors seen at sunset.
• When light passes through a small opening, it spreads out around each side of the opening.
• Dutch scientist Christiaan Huygens (1629-1695) suggested that light travels in a wave, not as a stream of fast moving particles.
• The high points of the wave are called crests.
• The low points of the wave are called troughs.
• The distance from crest to crest is called wavelength (the distance from one complete crest and one complete trough).
• The height of the crest or the depth of the trough from rest position is called the amplitude.
• The Frequency is the rate at which the crest and the trough move up and down.
• The number of cycles in a period of time - which is usually measured in hertz, or cycles per second.
• Different colors on the electromagnetic spectrum have different wavelengths (nanometers) and different frequencies (hertz).

The Electromagnetic Spectrum

• The sun is the most abundant source of direct natural light on the Earth.
• There are other forms of energy, invisible, that are also supplied by this source.
• The tiny band of visible light that we see is only part of the entire spectrum of light energy we receive.
• Called the electromagnetic spectrum because the light waves electrical and magnetic fields vibrate as they radiate to earth.

Applications Of Electromagnetic Radiation

• Radiation is a natural part of our environment.
• Humans have always lived on earth in the presence of radiation.
• Natural radiation reaches earth from outer space and continuously radiates from the rocks, soil, and water on the earth.
• Background radiation is that which is naturally and inevitably present in our environment.
• Levels of this can vary greatly.
• People living in granite areas or on mineralized sands receive more terrestrial radiation than others, while people living or working at high altitudes receive more cosmic radiation.
• A lot of our natural exposure is due to radon, a gas, which seeps from the earth's crust and is present in the air we breathe.
• Radiation is energy traveling through space.
• Sunshine is one of the most familiar forms of radiation.
• Sunshine delivers light, heat and suntans.
• Sunshine consists of radiation in a range of wavelengths from long-wave infrared to shorter wavelength ultraviolet.
• Beyond ultraviolet are higher energy kinds of radiation which are used in medicine and which we all get in low doses from space, from the air, and from the earth.
• Collectively we can refer to these kinds of radiation as lon radiation.
• It can cause damage to matter, particularly living tissue.
• Therefore it is necessary to control our exposure.

Radio Waves

• If you could stretch the infrared wave out even further, so it became a few millimeters long, you could get radio waves.
• Radio waves are around us all the time.
• Radio waves have a longer wavelength and a lower frequency than visible light.
• Different types of radio waves have different uses.
• Signals from radio and television stations, cell phones and even distant stars pass through your body every day.

Remote Imaging Technologies

• LANDSAT a Canadian satellite that records how different parts of the light from the Sun reflect back to the satellite.
• It's most important use is for agriculture, monitoring crops for damage by disease, pests and drought.
• RADARSAT is a Canadian telecommunications satellite, which, from time to time, sweeps the ground below it with radio waves, penetrating fog, haze, clouds and rain.
• Their reflection back to the satellite gives scientists information they can use in their studies of the Earth:
  • Monitoring ice floes
  • Search possible sites for minerals, oil and natural gas
  • Monitoring a flood
  • Maximize sandbagging efforts where needed

Microwaves

• Microwaves have the shortest wavelength and the highest frequency of the all the radio waves.
• Microwaves have three characteristics that allow them to be used in cooking:
• They are reflected by metal.
• They pass through glass, paper, plastic, and similar materials.
• They are absorbed by foods.
• Microwaves are used to detect speeding cars, to send telephone, satellite and television communications, and to treat muscle soreness.
• Microwaves are also used industrially.
• The most common consumer use of microwave energy is in microwave ovens that have been regulated since 1971.

Ultraviolet Radiation

• Just beyond the violet part of the visible spectrum are wavelengths of about 200 nm., known as ultraviolet (UV) radiation.
• This radiation is very energetic and it causes tanning but it can also do irreparable damage to us.
• UV rays can damage the cornea of the eye (fogging which can lead to a slow loss of vision).
• In more recent years, more UV radiation is reaching us because the ozone layer in the atmosphere (which protects us from the damaging radiation by absorbing the UV rays) is being thinned.
• This thinning of the ozone layer is speeded-up by the use of aerosol sprays and Freon gas, which break up the ozone particles.

Infrared Radiation

• Red light has a wavelength of about 700 nanometers, but if it could be stretched out to 100 nm, it would become heat radiation, or infrared radiation.
• It would become invisible to the eyes, but you could sense it with your skin.
• Anything that is warmer than its surroundings emit infrared rays.
• Practical applications include:
  • Motion sensors
  • Burglar alarms
  • Heat lamps

X-Rays

• They are shorter wavelengths with higher frequencies.
• These waves pass through tissue (skin and muscle) and are absorbed by the bones.
• This radiation always stays in the bone and builds up over time.
• People who work as technicians taking the x-rays must protect themselves, by leaving the room where the xray is taken and also protect the patient's other areas of the body with lead vests to prevent over-exposure.

Gamma Rays

• Gamma rays have the shortest wavelength and the highest frequency of all the waves in the electromagnetic spectrum.
• Gamma rays result from nuclear reactions and can kill cells.
• This can be useful if the cells being destroyed are harmful like cancerous cells.
• The cancerous growth of cells and tissue can be radiated, using gamma rays, and is known as radiation therapy.

Producing Visible Light

• Simply stated, light is the form of energy you can see.
• This energy can be produced naturally by the sun or fire, or artificially by light-producing technologies, like batteries.
• Radiation is the wave like transfer of light from its source in all directions.
• Light is often called radiant energy.
• Light from the sun is formed by nuclear fusion.
• The first basic principle of light: 'Light is a form of energy'.
• When light reaches a surface, it can be absorbed and transformed into other types of energy.

Transformations of light

• Into electrical energy like solar cells change light into electricity
• Into thermal energy like cameras change light into thermal images
• Into chemical energy like trees convert light energy into food (chemical energy)
• The amount of energy a surface receives depends on the intensity of the light.
• The more intense the light, the more light can be absorbed.

Sources of Light

• Natural Light Sources: the Sun
• Artificial Light Sources:
  • Incandescent: heat causes a metal filament to glow, giving off visible light.
    • Transformation electrical energy turns into thermal energy, and then into visible light energy
  • Florescent: ultraviolet light is absorbed by fabric particles which then give off some energy as light, creating a glowing effect.
    • Transformation Ultraviolet light turns into energy absorbed, which turns into visible light energy/particles energy
  • Phosphorescent: light energy is stored and released later as visible light, like in paint.
  • Wood (fire): is another natural light that we have harnessed for its light energy
  • Bioluminescence: light is produced by living organisms.
  • Chemiluminescent: light energy is released by chemical reactions, like in glow sticks
    • Transformation Chemical energy turns into Visible light energy. Other sources of light energy can come from the Earth's minerals including:
  • Thermoluminescence
  • Triboluminescence

The Colors of Light

• The various colors of the visible spectrum have slightly different wavelengths and refract by a slightly different amount.
• The primary colors of the visible spectrum are red, green and blue.
• By mixing the correct intensities of the primary colours, you will observe white light.
• Secondary colours are cyan, magenta and yellow.
• The mixing of three colors of light to produce many different colors of light is called the theory of color addition.
• Television puts this theory of color addition into practice.
• By changing the brightness of the dots that make up the screen, many different colors can be produced.
• The television works by fooling the eye into seeing colors that are not really there.

Eyes and Cameras Capture Images Using the Properties of Light

• There are many similarities between the human eye and the camera.
• The parts of the human eye are: Iris Lens, Pupil, Cornea, Vitreous Humor, Retina, Macula, and Optic Nerve

How Light Gets In

• In order to adjust the amount of light that enters the eye and the camera, a special device opens and closes to let just the right amount of light in.
• In the eye, the device (or part of the eye) that controls the amount of light entering is called the iris (the colored part of the eye), which changes the size of the pupil - in much the same way as the diaphragm controls the aperture (opening) of the camera lens.
• The natural adjustment in the size of the pupils is called the iris reflex, which is extremely rapid.
• This iris reflex action automatically adjusts the pupil when you go from a darkened area to a well-lit area, or, from a well-lit area to a darkened one.

In the camera

• The diaphragm controls the aperture (opening) of the lens and the shutter limits the passage of light.

When Light Gets Inside

• In the eye, when the photoreceptor cells in the retina detect light (rods are highly sensitive to light and cones detect color), they produce small electrical impulses from the retina to the brain, by way of the optic nerve.
• The film at the back of the camera contains light sensitive chemicals, which change when light hits it and these chemicals form the image on the film.
• The parts of a camera are housed in a rigid lightproof box, whereas layers of tissue hold the different parts of the eye together.
• The eyeball contains fluids called humours which prevent the eyeball from collapsing and refract the light that enters the eye.

Focusing The Light

• In a camera, if an object moves closer to the film, the lens must move away to keep the image in focus.
• In the human eye, the lens cannot move, so the ciliary muscles change the shape of the lens (by making the lens bulge in the middle if the image comes closer to you and stretch if the object is further away).
• This is done so that the eyeball isn't stretched.
• The process of changing the shape of the lens is called accommodation.
• As people become older, the lens stiffens and loses its' ability to change shape (doesn't bulge) and many people need to wear (convex lens) reading glasses, so that the images can be focused.

Image Formation

• The lens in the human eye is a convex lens, which focuses the light rays entering your eye to a point on your retina (a light sensitive area at the back of the eye).
• The image you see is formed on the retina.
• Some people however have eyes that are too long or too short.
• If their eye is too long, the image forms in front of the retina - this is a condition called Myopic, or near-sightedness (they have trouble seeing distant objects).
• If their eye is too short, the image forms behind the retina which means this condition is called Hyperopia or farsightedness (objects that are close to them are difficult to see).

Correcting Vision Problems With Lenses

• Knowledge of how light behaves when it travels through lenses helps eye specialists correct vision problems.
• The shortest distance at which an object is in focus is called the near point of the eye.
• The longest distance is called the far point of the eye.
• On average, an has a near point of about 25 cm, whereas babies have a near point of only 7 cm.
• The far point is infinite (because you can see the stars).

Laser Eye Surgery

• Instead of wearing glasses many people are now opting to have an eye surgeon use a laser to correct a vision problem.
• The surgeon cuts a thin flap of tissue covering the eye, fold it over, then the cornea is reshaped with a laser.
• The reshaped cornea acts like a corrective lens, allowing the light to be bent so it will properly focus on the retina.
• In 1966, Theodore H. Maiman a physicist at Hughes Aircraft Company in California became the first person to use a process called light amplification by the Stimulated emission of radiation
• This is also known as laser light.
• Incandescent lights give off many different colors and therefore have many different frequencies and wavelengths.
• The waves are jumbled and crests from one wavelength might overlap the trough of another, making the waves work against each other.
• This type of light is incoherent.
• Laser light is quite different.
• It gives off a single wavelength (frequency) of coherent light.
• Lasers have many useful applications: Scanners (bar codes in retail shops are scanned to give the price).
• Digitized data are read by a laser on a compact disk (CD).
• Lasers are use by law enforcement officers to detect the speed of vehicles and can be released in pulses or in a continuous beam.
  • Powerful enough for precise cuts through metal and surgery

Night Vision Goggles

• Light is focused onto an image intensifier.
• Inside the intensifier, the light energy releases a stream of particles, which hit a phosphor-coated screen.
• These glow green and the person looking in the goggles can view a green image.

Can you find your blind spot?

• The point where the retina is attached to the optic nerve has no light sensitive cells which is known as the blind spot.

Other Eyes in the Animal Kingdom

• Camera Eyes: Eyes that have a cornea, a lens and a retina
• Vertebrates (animals with backbones) for the most part have camera eyes.
• Fish have camera eyes with a perfectly round lens, which bulges out from the pupil, practically allowing it to see in every direction
• Birds have sharper vision than humans because have five types of cones (humans have only 3) and each are sensitive to different wavelengths of light.
• Nocturnal animals: their eyes collect as possible because of their very large pupils and also have a layer called tapetum lucidum, inside their eye, which acts as a mirror, giving them many more rods than cones in making them sensitive to low levels of light.

Compound Eyes:

• Insects and crustaceans have compound eyes.
• Eye is made up of many smaller units called ommatidium which looks like a long tube with a lens on the outer surface, a focusing cone below it, and then a light sensitive cell below that.
• It is great for spotting movement, but with so many lenses it is difficult to form a single coherent image to create mosaic image like a tv screen.

Image Storage and Transmission

• Most information today is stored digitally or converted into numbers.

Stadium Images

• The stadium image is made up of people holding different colored cards.
• Each card is assigned a seat based on the graphic representation of where the colors need to be to produce the correct effect.
• This stadium image was one of many designed by college in Caltech.

Digital Images

• Just as the stadium image, a big picture made out of small colored squares, a digital image is a picture made up of smaller colored pieces called pixels (picture elements).
• Each small pixel is assigned a place and is represented by a number.
• The numbers for the arrangement can be then stored in memory of computer and accessed at a later time.

Color and Digital image

• Once the individual pixels are the correct order, each pixel is assigned a value, which corresponds to a specific color.
• The computer reads value of each pixel and makes that pixel the correct color. Digital Image Quality that depends on its resolution.

Capturing Digital Images

• Light through camera uses charge-coupled device. Light enters each grid creates a small electrical charge, which will then be converted into a digital image and stored onto different mediums. Digital images can be sent away without having to be processed.

Explore the historical development of our understanding of light and vision, from ancient theories to modern discoveries. Learn about the contributions of Archimedes, Ptolemy, Al-Haythem, Newton, and Michelson. Key properties of light, including straight-line travel, reflection, and refraction will be discussed.