SNC2P

Light and Geometric Optics

• Optics is the science dealing with light and vision.

• Optical means ‘relating to the eye’

Light

• Light is radiant energy that travels (radiates) outward in all directions from the object giving off the light.

• Light energy is the form of radiant energy that can be detected by the unaided human eye.

Sources of Light

• All objects that we see are sources of light, even if they do not produce light themselves.

• Objects that produce light are said to be luminous.

• Sun, burning candle, light bulb, flash of lightning.

Non-Luminous Objects

• Most objects do not produce light on their own. Objects are able to be seen because light reflects (bounces off) them to our eyes.

• Coloured liquids and stained glass windows allow light to pass through them and this allows us to see the objects.

• Objects that we see because light reflects from them or passes through them are called non-luminous sources of light.

• Non-luminous objects are sometimes referred to as illuminated objects.

Luminous Sources of Light

• Luminous Sources of Light can be both natural and artificial.

• Natural sources of light are those that produce light without human intervention.

• Sun, Northern Lights, glow of red-hot lava

• Artificial sources of light are those that are produced and controlled by humans.

• Car headlights, neon signs, flashlights, televisions

• Some sources of light, such as burning wood, may be classified as both natural and artificial.

How Light is Produced

• Light is produced when other forms of energy (heat or chemical) are converted into light energy.

• The four most common ways in which luminous objects produce light are:

• Incandescence• Passing an electric current through a gas• Fluorescence and Phosphorescence• Light Emitting Diode (LED)

Incandescence

• When steel is heated to temperatures over 2000 degrees Celsius, it gives off white light.

• A substance that gives off light because it has been heated to a high temperature is called incandescent.

• Some substances are heated to incandescence when they are rubbed (friction).

• Substances can be heated to incandescence by electricity.

• The light from an old household light bulb is produced by heating a fine coil of tungsten wire (filament) until the temperature rises and gives off a bright light.

• These old bulbs are called incandescent bulbs.

• Toasters and ovens also become incandescent when heated.

Did you know that…

• The space inside an incandescent light bulb is filled with nitrogen or argon.

• If the space was filled with oxygen, the gas would react with the hot filament.

• Substances may also be heated to incandescence using chemical energy.

• When wood, wax, kerosene, or oil is burned, energy is given off in the form of light and heat because of a chemical reaction.

Passing an Electric Current through a Gas

• When an electric current is passed through a gas, light is sometimes produced.

• A natural occurrence of this is lightning produced during a thunderstorm.

• Another natural example of this is the Aurora Borealis (Northern Lights).

• An artificial way of producing light by passing an electric current through a gas is Neon Lights. Other gases are used to produce different colours.

Sodium vapour lamps produce an intense light and is often used to illuminate streets and highways.

Mercury vapour lamps are very bright and are used to light hockey rinks and sports stadiums.

Fluorescence

• Some objects give off light when invisible ultraviolet light is shone on them.

• Substances that produce visible light when exposed to ultraviolet light are called fluorescent.

Stamp collectors shine UV light onto stamps to check for special fluorescent markings.

A Fluorescent Light Bulb

• A fluorescent light tube consists of a long, cylindrical tube that filled with mercury vapour.

• The inside of the tube is coated with a fluorescent powder. Electrodes are placed at either end of the tube.

• When an electric current is passed through the vapour, UV light is produced. The UV light hits the powder which then produces visible light.

• A fluorescent tube is much more efficient than a regular incandescent light bulb.

• About 20% of the electrical energy used in a fluorescent light is converted to visible light, but only 5% of the electrical energy used in an incandescent bulb is converted to visible light.

• If you place you hand near a fluorescent tube, it feels cool. If you place you hand near an incandescent bulb, it feels warm.

• When a fluorescent tube is switched off, no more UV light is produced and the coating stops giving off light.

• Some fluorescent substances though, continue to give off light for hours after the energy source has been removed. These substances are called phosphorescent.

• Luminous dials on watches, glow in the dark objects and similar items are made of phosphorescent materials.

CFL bulbs• Governments around the world recommend

that homes and businesses switch to compact fluorescent light bulbs (CFLs).

• CFL bulbs last longer than an incandescent bulb and use less energy.

• CFLs cost more than an incandescent bulb and the mercury in the CFLs can cause environmental pollution.

• All used fluorescent bulbs should be taken to a hazardous waste collection site.

• CFLs were devised to reduce the length of regular fluorescent light fixtures.

Light Emitting Diode (LED)

• The Light Emitting Diode is a very efficient light producing technology.

• An electric current passes through an LED, it emits light.

• LEDs save energy, last longer and stay cooler than Incandescent bulbs of CFL bulbs.

Other ways that light is produced

• Chemiluminescence• Bioluminescence

Chemiluminescence

• Light can be the result from the energy released in chemical reactions. The products of the chemical reaction give off visible light. This process is called chemiluminescence.

Bioluminescence

• Some living things can make themselves luminous using a chemical reaction similar to chemiluminescence. This is called bioluminescence.

• Many organisms that live deep in the ocean use bioluminescence because so little sunlight reaches far below the surface of the water.

• Fireflies, glow worms, some fish, some bacteria

Properties of Light

Light that is being transmitted through the air is reflected from water, ice, and rocks.

The Transmission, Reflection, and Absorption of Light

• Light can be affected in three different ways when it strikes matter:

• Transmitted• Reflected• Absorbed

• A substance, such as air, allows light to pass though it as it the air wasn’t even there.

• When light passes through a substance, light is transmitted through the substance.

• Any substance that transmits light is called a medium

• Other substances cause light to bounce or reflect from them.

• Mirrors and white paper reflect most of the light that strikes them.

• Some substances absorb the light that strikes them.

• A black surface absorbs most of the light energy hitting it. This light energy is then turned into heat energy which is why dark surfaces feel warm.

Transparent, Translucent, and Opaque Substances

• All substances can be classified according to how they transmit, reflect, or absorb light.

• A transparent substance is one that transmits light readily.

• Objects can be seen clearly through a transparent substance.

• Some examples of transparent substances are:

• window glass, air, clean water, and plastic food wrap.

• A translucent substance is one that transmits some of the light that hits it.

• Objects can be detected, but not clearly seen through a translucent substance.

• Frosted glass

• An opaque substance is one that does not transmit any light.

• When light strikes an opaque object, the light is reflected or absorbed.

• Rocks, wood, and metals

More Properties of Light

• Light travels in straight lines (as long as the medium in which the light is travelling remains the same). This property is called the linear motion of light.

Laser

• We see things because they reflect light into our eyes:

Homework

• Light is invisible when it travels through a vacuum or a transparent substance.

• A vacuum is a region that contains no matter at all, not even gases.

• Light can only be detected by the effects it produces.

• On a clear night, the light beam from a flashlight cannot be seen.

• On a foggy night, the light beam can be seen because the water droplets in the air reflect some of the light to you eyes.

The Light Ray Model of Light

• The Light Ray Model is one of the simplest and most successful models ever developed.

• The Light Ray Model is based on a single property of light (linear motion).

• This model has played a role in the development of many optical instruments such as telescopes and cameras.

• Because light travels in straight lines, the geometric principles related to straight lines can be used to show what happens to light when it strikes various objects.

• So many effects produced by light can be explained using this model, which makes up the branch of study called Geometric Optics.

• To use the Light Ray Model, two terms need to be defined.

• The path taken by a fine beam of light through a medium is called a light ray.

» The light ray is represented in a diagram by a single, straight line.

» The direction of the ray of light is shown by an arrow pointing in the direction of travel.

• A group of parallel light rays is called a light beam.

• A ray box is a device used to produce beams and rays of light for experiments.

Shadows

Shadows are places where light is “blocked”

Rays of light

Shadow

• A shadow is a dark region that forms behind an object. The object is being illuminated brightly on the other side of the shadow.

• Shadows are formed when some or all of the light falling on an object is absorbed or reflected.

• Transparent and colourless substances do not produce shadows.

• Translucent objects may transmit some light, but they also absorb or reflect the striking light.

• Opaque objects do not transmit any light so all of the light is absorbed or reflected. Very dark shadows form behind opaque objects.

•This figure is of a ray box and two screens. It shows a group of light rays (light beam) travelling from the box to the screens. •Although a light beam is actually made up of a large number of rays, only a few are drawn to represent the beam.

•This diagram is the same as the previous diagram showing only a few light rays. •This is an even more simplified version showing only the outer limits of the beam.

Umbra and Penumbra

• A very dark shadow, where there is no light is called an umbra.

• Umbra have sharply defined edges.

• There is a lighter shadow with edges that are not as sharp as an umbra, called a penumbra.

Light Ray Model Explains the Formation of Shadows

• Consider the shadow produced by a point light source. The light rays are given off in all directions from the light source.

• Some of the light rays strike an opaque object and are absorbed, forming an umbra behind the object.

• The other light rays continue to travel in straight lines from the light source as they were not stopped by an opaque object.

Notice that to define a shadow, it is only necessary to draw the light rays travelling from the point source to the outside edges of the opaque object . The shape of the umbra is the same as the opaque object.

The size of the umbra depends on the distance between the light source and the opaque object. If the object is far from the light, a small shadow will be produced. When the object is closer to the point source, the shadow will be large.

• A different kind of shadow is formed if a large light source is used. The light is no longer coming from one single point.

• The larger light sources are called extended light sources.

• When an extended light source is used, the resulting shadow does not have sharply defined edges like the shadow of a point source.

• Instead, a lighter area of a shadow is formed (penumbra) because light from the extended source overlaps at the outside edges of the opaque object

Penumbra

Penumbra

Umbra

Reflection of Light

Regular Reflection of Light.

• The reflection of light from smooth, shiny surfaces, such as mirrors, is called regular reflection.

• Light that travels towards a mirror is called the incident light.

• Light that is reflected from a mirror is called the reflected light.

• The study of regular reflection requires measuring the angles at which the incident light strikes and the reflected line leaves the mirror.

• The use of a normal is used to measure the angles from all mirror surfaces. A normal is drawn at right angles to the reflecting surface.

• The angle of incidence is the angle between the incident ray and the normal.

• The angle of reflection is the angle between the normal and the reflection ray.

Two Laws of Reflection

• The Light Ray Model can be used to explain how light reflects from a smooth, flat surface.

• The First Law of Reflection states that the angle of incidence equals the angle of reflection.

• The Second Law of Reflection states that the incident ray, the refection ray and the normal are all on the same plane.

Clear vs. Diffuse Reflection

Smooth, shiny surfaces have a clear reflection:

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

Diffuse Reflection

• Most objects that reflect light have rough, dull surfaces. What happens when parallel rays of light strike a rough surface?

• Because the surface is not smooth, the light rays are scattered in many directions. This is called diffuse reflection.

• The laws of reflection still apply to rough surfaces!

Plane Mirrors

• Most objects that you can see are non-luminous (you can see them because they reflect light to your eyes).

• Most non-luminous objects have rough surfaces and will reflect light in a manner that reveals their shape, colour, and texture.

• Some non-luminous objects reflect light is such a way that an image is formed. These are called mirrors.

• Polished metal and calm water are also mirrors.

• A flat reflecting surface is called a plane.• Plane means two dimensional surface, just like

in math!

Characteristics of Images(used to study and compare images)

Characteristic Possible Description

Size •Smaller than the object viewed•Larger than the object viewed•Same size as the object viewed

Attitude •Upright (right side up)•Inverted (upside down)

Location •Appears closer to the plane than the object•Appears further to the plane than the object•Appears to be on the opposite side of the plane than the object

Type •Real image (can be placed onto a screen)•Virtual image (can only be seen by looking through an optical device)

The image of an object reflected by a plane mirror appears to be at the same distance behind the mirror’s surface as the object is itself in front.

Drawing Images formed by a Plane Mirror

1. Draw a line representing the mirror and shade one side of it. The shade represents the reflective coating of the mirror. Draw the object in front of the mirror.

2. Draw the normals from the mirror surface to significant points, such as the top and bottom of the object. The number of other points depends on the object.

3. Measure the length of each normal drawn from the mirror to the object. Extend the normal an equal length behind the mirror surface. Complete the image by connecting the ends of the normals behind the mirror.

Application of a Plane Mirror

• A Periscope!• It is an instrument in which plane mirrors are used to

fold light so that the image of an object can be brought down to a lower level.

• It is used for observing enemy movements from trenches without any danger of being seen. Sailors on submarines use periscopes to see things above the water level.

Curved Mirrors

• Curved Mirrors have as many different uses as plane mirrors.

• Curved mirrors for this class are spherical mirrors because they have the same curvature as a sphere.

• There are two basic curved mirrors.• Convex mirrors bulge outwards in the middle.• Concave mirrors bulge inwards in the middle.

To help identify the two spherical mirrors, think of the word “cave” in concave. The surface of a concave mirror is curved in like the opening into cave!

Curved Mirror Terminology

Center and Radius of Curvature

• The center of the sphere from which the curved mirrors are from is called the center of curvature (C).

• The distance from the center of the curvature to the surface of the mirror is called the radius of curvature.

Vertex and Principle Axis

• The center of the mirror is called the Vertex (V).

• The line that passes through the center of curvature and the vertex is called the principal axis (P).

V

Vertex

• Any other line that passes from the center of curvature (c) to the surface of the mirror is called a secondary axis.

• Secondary axis meet the surface of the mirror at 90 degree angles.

• The radius of the circle is an example os a secondary axis.

• The secondary axis and the principle axis are normals to the surface of the mirror.

Secondary axis, also known as a radius

Focal Point

• The point that is half way between the C and the V is called the Focal Point (F).

• The length between the C and F is called the focal length.

Concave Mirrors

When parallel lines shine along the P. Axis of a concave mirror, reflecting rays meet at the Focal point (F).

Concave Mirror

Two Rules of Reflection for Concave Mirrors

• Light always reflects according to the law of reflection, regardless of whether the reflection occurs off a flat surface or a curved surface.

• Using reflection laws enables us to determine the image location for an object.

• The image location is where all reflected light appears to diverge from. To determine this location we need to know how light reflects off a mirror.

• Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection.

• Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection.

Step by Step Method for Drawing Ray Diagrams (Concave)

• Pick a point on the top of the object and draw two incident rays traveling towards the mirror.

• Using a straight edge, accurately draw one ray so that it passes exactly through the focal point on the way to the mirror.

• Draw the second ray such that it travels exactly parallel to the principal axis. Place arrowheads upon the rays to indicate their direction of travel.

• Once these incident rays strike the mirror, reflect them according to the two rules of reflection for concave mirrors.

• The ray that passes through the focal point on the way to the mirror will reflect and travel parallel to the principal axis. Use a straight edge to accurately draw its path.

• The ray which traveled parallel to the principal axis on the way to the mirror will reflect and travel through the focal point. Place arrowheads upon the rays to indicate their direction of travel. Extend the rays past their point of intersection.

• Mark the image of the top of the object• The image point of the top of the object is the point

where the two reflected rays intersect.

Image of Object Beyond C

• When the object is located beyond C, the image will always be located somewhere in between C and F.

• The image will be an inverted image. That is to say, if the object is right-side up, then the image is upside down.

• The image is reduced in size• The image is a real image. Light rays actually

converge at the image location. • If a sheet of paper was placed at the image location,

the actual replica of the object would appear projected upon the sheet of paper.

What if the object is right at C?

• When the object is located at C, the image will also be located C.

• The image will be inverted (a right-side-up object results in an upside-down image).

• The image dimensions are equal to the object dimensions.

•Finally, the image is a real image. Light rays actually converge at the image location.

•The image of the object could be projected upon a sheet of paper

What if the object is between C and F?

• When the object is located in front of C, the image will be located beyond the C.

• The image will be inverted (a right-side-up object results in an upside-down image).

• The image dimensions are larger than the object dimensions.

•The image is a real image. Light rays actually converge at the image location.

•The image of the object could be projected upon a sheet of paper.

What if the image is between F and the mirror?

• When the object is located beyond the focal point, the image will always be located somewhere on the opposite side of the mirror.

• The image will be an upright image. If the object is right-side up, then the image will also be right-side up.

• In this case, the image is magnified

• The image location can only be found by extending the reflected rays backwards beyond the mirror.

• The point of their intersection is the virtual image location. It would appear to any observer as though light from the object were diverging from this location.

What is the image is right on F?

• When the object is located at the focal point, no image is formed.

• Light rays from the same point on the object will reflect off the mirror. They will neither converge nor diverge.

• After reflecting, the light rays are traveling parallel to each other and do not result in the formation of an image

Convex Mirrors

• When parallel lines shine along a principle axis on a convex mirror, the reflected rays diverge from one another.

• If you extend the reflected rays to behind the mirror, they would meet up at a point called the virtual focal point.

Two rules of Reflection for Convex Mirrors

• Any incident ray traveling parallel to the principal axis on the way to a convex mirror will reflect so that its extension will pass through the focal point.

• Any incident ray traveling towards a convex mirror such that its extension passes through the focal point will reflect and travel parallel to the principal axis.

Step by Step Method for Drawing for Ray Diagrams (Convex)

• Pick a point on the top of the object and draw two incident rays traveling towards the mirror.

• Using a ruler, draw one ray so that it travels towards the focal point on the opposite side of the mirror

• Draw the second ray so that it travels exactly parallel to the principal axis. Place arrowheads upon the rays to indicate their direction of travel.

• Once these incident rays strike the mirror, reflect them according to the two rules of reflection for convex mirrors.

• The ray that travels towards the focal point will reflect and travel parallel to the principal axis. Use a straight edge to accurately draw its path.

• The ray that traveled parallel to the principal axis on the way to the mirror will reflect and travel in the direction of the focal point. Align a straight edge with the point of incidence and the focal point, and draw the second reflected ray.

• Locate and mark the image of the top of the object.

• The top of the image is the point where the two reflected rays intersect. Since the two reflected rays are diverging, they must be extended behind the mirror in order to intersect.

• Using a straight edge, extend each of the rays using dashed lines. Draw the extensions until they intersect.

• The point of intersection is the top of the image.

• All images produced by convex mirrors produce images with the following characteristics:

• located behind the convex mirror • a virtual image • an upright image

Refraction

• Reflection occurs when light rays bounce off object.

• Using the laws of reflection, the direction in which reflected light travels can be predicted and the location of an image can be predicted.

• But what happens when light moves from air into a different medium?

As you look through the side of the glass at the portion of the pencil located above the water's surface, light travels directly from the pencil to your eye. Since this light does not change medium, it will not refract.

As you look at the portion of the pencil which was submerged in the water, light travels from water to air. This light ray changes medium and subsequently undergoes refraction. As a result, the image of the pencil appears to be broken. The portion of the pencil which is submerged in water also appears to be wider than the portion of the pencil which is not submerged.

• Refraction is the bending of light when it travels from one medium to another.

• Light bends because it changes speed when it moves between materials with different densities.

• Light travels more slowly in thicker/denser material.

• The bending of light makes an object’s image appear to be in a different position from where the object really is.

The Direction of Bending

• Refraction is the bending of the path of a light wave as it passes from one material into another material.

• The refraction occurs at the boundary and is caused by a change in the speed of the light wave upon crossing the boundary.

• The tendency of a ray of light to bend one direction or another is dependent upon whether the light wave speeds up or slows down upon crossing the boundary.

• The direction which the path of a light wave bends depends on whether the light wave is traveling from a more dense (slow) medium to a less dense (fast) medium or from a less dense medium to a more dense medium.

• As light travels through a given medium, it travels in a straight line. However, when light passes from one medium into a second medium, the light path bends. Refraction takes place. The refraction occurs only at the boundary.

• Once the light has crossed the boundary between the two media, it continues to travel in a straight line. Only now, the direction of that line is different than it was in the former medium.

On the diagram, the direction of the students is represented by two arrows known as rays. The direction of the students as they approach the boundary is represented by an incident ray (drawn in blue). And the direction of the students after they cross the boundary is represented by a refracted ray (drawn in red).

• Since the students change direction (refract), the incident ray and the refracted ray do not point in the same direction.

• See the perpendicular line drawn to the boundary at the point where the incident ray strikes the boundary?

• A line drawn perpendicular to the boundary at the point of incidence is known as a normal line.

• Observe that the refracted ray lies closer to the normal line than the incident ray does. In such an instance as this, we would say that the path of the students has bent towards the normal.

Light Traveling from a Fast to a Slow Medium

• If a ray of light passes across the boundary from a fast material into a slow material, then the light ray will bend towards the normal line.

• FST = Fast to Slow, Towards Normal• If a ray of light passes across the boundary

from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal line.

• Now suppose that the each individual student in the train of students speeds up once they cross the masking tape.

• The first student to reach the boundary will speed up and pull ahead of the other students. When the second student reaches the boundary, he/she will also speed up and pull ahead of the other students.

• This continues for each student, causing the line of students to now be traveling in a direction further from the normal.

The refracted ray (in red) is further away from the normal then the incident ray (in blue). In such an instance , the path of the students has bent away from the normal.

Light Traveling from a Slow to a Fast Medium

• If a ray of light passes across the boundary from a slow material into a fast material, then the light ray will bend away from the normal line.

• SFA = Slow to Fast, Away From Normal• If a ray of light passes across the boundary

from a material in which it travels slow into a material in which travels faster, then the light ray will bend away from the normal line.

Light and Colour

Colour

• Colour is the property of light that allows us to visually distinguish between two objects that have identical shape, structure, and size.

White Light

• The sun is the most important source of light. The light given off by the sun is called white light.

• In the 17th century, Sir Isaac Newton (an English scientist) conducted a famous experiment.

• He placed a prism in front of a beam of white light and a band of colours emerged. Each band refracted at a different angle, producing a rainbow effect.

• Newton realized that colour was not added to light, but already present in white light.

• Next, Newton passed the band of colours through a reverse prism and this time only the white beam appeared.

• He proposed that white light is the result of mixing together all of the different colours of light.

The Spectrum

• When white light is refracted into different colours, the resulting pattern is called a spectrum.

• For sunlight, the colours range from red through orange, yellow, green, blue, indigo, and violet. (ROY G BIV)

• The colours are always the same and always in the same order.

• The separation of white light into its colours is called dispersion

• The spectral colours can be mixed together (recombined) into white light by using a prism, lens, or concave mirror.

• When light strikes an object, the light may be reflected off the object, absorbed by the object, or transmitted through the object.

• If light is shone at a pair of blue jeans, the jeans absorbs all of the colours except blue. Only the blue light is reflected or transmitted.

• The colour seen when light strikes an opaque object depends on which colours are reflected and which ones are absorbed.

• The white paper used to write on is reflecting all colours so all that is seen is white.

• The black ink is absorbing all of the colours so black is seen.

Additive Primary Colours• The three colours red, green, and blue are called

the additive primary colours. • They are called additive because adding all three

together in proper amounts, will produce white light.

• The three primary colours can also be combined in differing intensities to produce any of the spectral colours, as well as colours not in the spectrum.

• No other combination of coloured lights can produce red, green, or blue.

Additive Primary Colour Additive Primary Colour Additive Secondary Colour

Red Green Yellow

Red Blue Magenta

Blue Green Cyan

The light of two additive primary colours will produce a secondary colour.

• Television screens use additive primary colours. The screen contains many groups of three tiny phosphor dots.

• Each dot glows with a different colour when it receives energy – one dot glows red, another green, and the third glows re

Additive Primary Colour Complimentary Colour Light Transmitted

Red Cyan (Blue and Green) White Light

Green Magenta (Red and Blue) White Light

Blue Yellow (Red and Green) White Light

•It is possible to obtain white light by adding two coloured lights together•Any two colours that combine to produce white light are called complementary colours.

How We See Colour• The retina of the human eye contains two types

of cells that respond to light.• Some cells look like tiny cylinders, called rods, and they

detect the presence of light.• The other cells are cones because they look like cones.

These cells respond to colour.» Some of the cones respond to green, some respond to red,

and the third type of cone responds to blue.

• Signals from the rods and all three type of cones travel along the optic nerve to the brain. The brain interprets the shape and colour of the objects seen.

Colour Blindness

• Humans share the phenomenon of seeing in full colour with such organisms as goldfish, bees, and apes.

• Some people’s eyes have defective cone cells so they have difficulty detecting some colours. This condition is known as colour blindness.

• The term colour blindness is a bit misleading since most people can actually see some colour.

Normal VisionA person who confuses red and green

Subtractive Primary Colours

• Cyan, Magenta, and Yellow are subtractive primary colours because some portion of white light has been removed in order to produce it.

• Cyan is made up of green and blue light. It is missing the red.

• If cyan light and red light are shone onto a screen, white light will be reflected.

• Cyan and red are complimentary colours because together they form white light.

Subtractive Primary ColoursLight Colour Includes Missing

Cyan Blue and Green(B + G)

Red(-R)

Magenta Red and Blue(R + B)

Green(-G)

Yellow Red and Green(R + G)

Blue (-B)

Using Colour Filters

• Colour filters subtract colour from the light they transmit. This theory is called the subtractive colour theory.

• When white light strikes a red filter, all of the other colours are absorbed (subtracted) and only the red colour is transmitted.

• Only blue light passes through a blue filter• Only green light passes through a green filter

• Each subtractive colour filter absorbs one of the primary colours.

• Yellow light is made up of green and red.• A yellow filter will let green and red pass through, but

not blue.

Primary Colour Filter Absorbed colour Transmitted Colours

Magenta (made up of red and blue)

green Red and blue allowed through

Yellow (made up of red and green)

blue Red and green allowed through

Cyan (made up of green and blue)

red Blue and green allowed through

Subtractive Primary Colour Filter

Absorbed Colour Transmitted colour

Magenta (made up of red and blue)

+Yellow (made up of red and green)

Green absorbed

Blue absorbed

Red transmitted through

Magenta (made up of red and blue)

+Cyan (made up of blue and green)

Green absorbed

Red absorbed

Blue transmitted through

Yellow (made up of red and green)

+Cyan (made up of blue and green)

Blue absorbed

Red absorbed

Green transmitted through

An Example

• If you place a magenta filter in front of a beam of white light and then place a cyan filter in front of the light transmitted, What light will shin on to a screen.

• White light is made up of red, green, and blue light. • The magenta filter will take out the green, leaving the

blue and red light.• The cyan filter will take out the red, leaving only the

blue.

Try this one!

• If you have a beam of white light (red, green, blue) and place a yellow filter in front. What colour(s) will transmit?

• Minus the blue, so green and red still pass through

• Then place a red filter in front of the transmitted beam. What colour(s) will transmit onto a screen?

• Minus the green, so red will still pass through

Black

• Black is not a colour. Instead it is the absence of coloured light.

• When a combination of filters is used, black can be the result. All of the other colours have been absorbed and none are being transmitted.

Applications of Colour Filters

• Skiers and boarders where amber-coloured glasses/goggles while on the hill.

• The yellow in the glasses will prevent the blue in the bright sunlight from reflecting off the snow and striking the eyes.

• This prevents a condition called snow blindness, a temporary blindness caused by the glare light reflected by snow.

Lenses

• Knowing how curved mirrors reflect light can help you understand how lenses affect light rays.

• A lens is a curved piece of transparent material. Light refracts as it passes through a lens, causing the light rays to bend.

Basic Lens Shapes

•A converging lens is a lens that is thickest in the middle and causes incident parallel rays to converge at a single point.

•A diverging lens is a lens that is thinnest in the middle and causes incident parallel rays to spread apart after refraction.

Diverging (Concave) Lenses

• A concave lens is thinner and flatter in the middle than around the edges.

• Light passing through the thicker, more curved areas of the lens will bend more than light that passes through the flatter, thinner area in the middle.

• Rays of light are spread out (diverged) after passing through the lens.

Optical Center

Secondary Principal Focus (F’)

Converging (Convex) Lenses

• A convex lens is thinker in the middle than around the edges.

• This thicker middle causes the refracting light rays to come together (converge)

Optical Center

Secondary Principal Focus (F’)

Three Rules for a Converging Lens

1. A ray parallel to the principal axis is refracted through the principal focus.

2. A ray through the secondary principal focus is refracted parallel to the principal axis.

3. A ray through the optical center continues straight through without being refracted.

Using Ray Diagrams with Converging Lenses

• Draw the ray parallel to the principal axis. Draw the refracted ray so that it passes through the principal focus.

• Draw a ray from the top of the object through the middle of the lens. This ray is undeviated. Where the rays meet, that is where the image is.

• You can also draw the incident ray passing through F’ on the way to the lens. The result will be a parallel ray to the principal axis.

Locating Images – Object beyond 2F’

•Image is •Smaller•Inverted•Between F and 2F•Real

Locating Images – Object at 2F’

•Image is •Same size•Inverted• At 2F•Real

Locating Images – Object between 2F’ and F’

•Image is •Larger•Inverted•Beyond 2F•Real

Locating Images – Object AT F’

•Image is •No clear image due to emergent rays being parallel

Locating Images – Between F’ and Lens

•Image is •Larger•Upright•Behind the lens•Virtual

Three Rules for a Diverging Lens

1. A ray parallel to the principal axis is refracted as if it had come through the principal focus.

2. A ray that appears to pass through the secondary principal focus is refracted parallel to the principal axis.

3. A ray through the optical center continues straight through without being refracted.

•A diverging lens always produces the same image no matter where the object is.

•Smaller•Upright•On the same side as the object•Virtual

The Eye

The Human Eye

• The human eye is the optical instrument that helps most of us learn about the external world.

How We See Colour• The retina of the human eye contains two types

of cells that respond to light.• Some cells look like tiny cylinders, called rods, and they

detect the presence of light.• The other cells are cones because they look like cones.

These cells respond to colour.» Some of the cones respond to green, some respond to red,

and the third type of cone responds to blue.

• Signals from the rods and all three type of cones travel along the optic nerve to the brain. The brain interprets the shape and colour of the objects seen.

• Most people believe that we see with our eyes. In reality, the eye acts as a light gathering instrument. We actually see with our brain!

• The cornea-lens combination acts like a converging lens and produces smaller, real, inverted images on the retina.

• Impulses from the retina travel through the optic nerve to the brain. The brain takes the inverted image and flips it so that we see the image as it appears.

• This is how the lens in a normal human eye focuses light rays onto the retina.

• Light refracts through the lens onto a light-sensitive area at the back of the eye called the retina.

• The image you see is formed on the retina.

Myopia – Nearsightedness

•For some people, the eye has a longer shape.

•This means the imaged forms in front of the retina. These people are near sighted (have trouble seeing far objects).

Hyperopia – Farsightedness

•For some people, the eye has a shorter length so the image has not formed by the time it reaches the retina. The image forms behind the retina.

•These people are far sighted and have trouble seeing things close up.

Presbyopia

• Presbyopia is an age-related loss of flexibility of the lens inside the eye.

• With the onset of presbyopia, you'll find you need to hold books, magazines, newspapers, menus and other reading materials farther away in order to see the print clearly.

• Knowledge of how light behaves when it travels through lenses helps eye specialists correct vision problems.

• A convex lens in placed in front of the far sighted eye to help bend the light rays to form an image on the retina.

Eyes and Cameras

• There are many similarities between the human eye and a camera.

• You know that you see objects when light is reflected to your eye, refracted through your lens, and focussed on your retina.

• In a camera, the lens refracts the light and the film senses the light.