Unit c light and optical systems
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Unit C: Light and Optical Systems. Topic 1: What is Light? . Light : the form of energy that we can see Natural l ight source : a non-human-made source of light, such as the Sun or fire

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Unit c light and optical systems

Unit C: Light and Optical Systems

Topic 1 what is light

Topic 1: What is Light?

  • Light: the form of energy that we can see

  • Natural light source: a non-human-made source of light, such as the Sun or fire

  • Light radiates(spreads out) from the Sun in all directions in a form of energy transfer called radiation (the transfer of radiant energy such as light).

  • A VERY small percentage of the Sun’s energy actually reaches the Earth

Unit c light and optical systems

  • Our lives are totally dependant on the energy from the Sun. As sunlight is not always available, people have developed artificial light sources (ex. light bulb)

  • Chemicals can also produce light (ex. batteries, matches)

  • Nuclear fission also provides us with energy on a large scale

The first basic principle of light

The First Basic Principle of Light

  • Light is a form of energy

  • When light is absorbed by a surface, it can be transformed into several different forms of energy

    Light-> Thermal Energy, Electrical Energy,

    Chemical Energy

    Ex. Trees, solar panels, a black sweater

The first basic principle of light1

The First Basic Principle of Light

  • The brightness or intensity of light indicates how much energy a surface will receive

  • A surface can absorb more energy if the brightness of the light intensifies

Sources of light

Sources of Light

  • Artificial sources of light have enabled us to be productive after the Sun is gone during the day

    • Torches, candles, lamps, light bulbs, etc. are artificial sources of light that help us to see at night

  • Observatories are located far from urban areas to avoid light pollution from large cities at night

Sources of light1

Sources of Light

  • Communities are attempting to conserve the light energy they use

    • New street lights are more energy efficient and are being designed to direct light toward the ground to avoid illuminating the sky

    • In the headlights of cars, yellow sodium vapor lights are being used in newer cars as they are more energy efficient

Incandescent sources

Incandescent Sources

  • Incandescent source: an object that is heated to such a high temperature that it emits visible light

  • Incandescence: the emission of visible light by a hot object

    • Examples: candle flames, light bulbs

  • In a light bulb, the filament is heated a follows:

    Electrical Energy -> Thermal Energy -> Visible Light Energy

Incandescent sources1

Incandescent Sources

  • 95% of the energy given off by incandescent light bulbs is released as heat

  • The filament of a light bulb is usually made of the chemical element tungsten (W)

Fluorescent sources

Fluorescent Sources

  • Fluorescence: the process in which high-energy, invisible UV light is absorbed by the particles of an object, which then emits some of this energy as visible light, causing the object to glow

    UV Light Energy -> Energy Absorbed -> Visible Light

    by Particles Energy

  • Fluorescent source: a source that produces light when exposed to light of a particular wavelength

Fluorescent sources1

Fluorescent Sources

  • In fluorescent tubes, an electric current causes mercury vapors in the tube to give off UV radiation. A phosphor coating on the inside of the tube absorbs the UV energy and glows

    • Figure 3.6 page 181

Fluorescent sources2

Fluorescent Sources

  • Disadvantages of fluorescent tubes

    • Expensive to manufacture

    • Difficult to dispose of

    • Phosphor and mercury are toxic

  • Advantages of fluorescent tubes

    • Very little thermal energy is produced when lit

    • More energy efficient

Phosphorescent sources

Phosphorescent Sources

  • Similar to a fluorescent light source

  • Light energy is absorbed by particles that can store the light and later release it as visible light

  • Phosphorescence: the persistent emission of light following exposure to and removal of a source of radiation

  • Phosphorescent source: a substance that gives off visible light released after the light energy has been absorbed by particles and stored

Phosphorescent sources1

Phosphorescent Sources

  • Examples include TV and computer screens, surfaces that glow in the dark (watches, dials, TV remote)

    Differences between Fluorescent and Phosphorescent Sources

    • Particles in a fluorescent source release their light energy immediately

    • Phosphorescent particles take longer to release their light energy and glow after the light source has been removed

Chemiluminescent sources

Chemiluminescent Sources

  • Chemiluminescence: the emission of light resulting from a chemical reaction and not involving heat

    Chemical Energy -> Visible Light Energy

  • Chemiluminescent source: a chemical reaction that produces particles that give off visible light energy

    • Example: glow sticks

Bioluminescent sources

Bioluminescent Sources

  • Bioluminescence: the emission of light produced by chemical reactions inside the bodies of living creatures

  • Bioluminescent source: an organism that relies on chemical reactions inside its body to produce light

The cost of lighting

The Cost of Lighting

  • Electrical energy costs about 8¢ per kilowatt hour (kWh)

    • 8¢/kWh

  • A kilowatt hour (kWh) is approximately 1000 watts (W) of electrical power operating for one hour

  • We can calculate the cost of lighting by multiplying the power of the bulb (kW or W) by the time (h)



How much will it cost to leave a 60W light bulb on for 10 hours if the electrical energy cost is 8¢/kWh?

  • Convert W to kW by ÷ by 1000

  • Calculate the number of kWh by multiplying the power (in kW) by the number of hours

  • Calculate the cost of leaving the light on for 10h by multiplying the number of kWh by the cost per kWh

    • Cost = 4.8¢

The ray model of light

The Ray Model of Light

  • Ray Model: a model of light based on the observation that light travels in a straight line

  • Ray: straight line that represents the path of a beam of light

  • You can use this model to predict where shadows will form and how big they will be

The ray model of light1

The Ray Model of Light

  • When light travels through clear substances, the rays continue along in a straight path

    • Clear substances are transparent

  • When light travels through non-clear objects (ex. a piece of paper), the object scatters the light (bends the light rays) and you cannot see images from this light

    • These object are translucent

  • When light is completely blocked from a material, the material is called opaque

    • Figure 3.11A, 3.11B. 3.12 page 185

Unit c light and optical systems

Topic 1 Review p. 187 #1-6

Topic 2 reflection

Topic 2: Reflection

  • Reflection: the process in which light strikes a surface and bounces back off that surface

    • Ex. Light reflecting off a mirror allowing you to see an image of yourself



  • The difference between seeing your own image (mirror reflection) or seeing a solid object (words on a page) depends on the surface from which light reflects

  • A smooth surface is better at reflecting an image, while a rough surface tends to scatter the light in all




  • To analyze the process of reflection:

    • Imagine the surface is flat

    • The ray of light from a light source that strikes the surface is called the incident ray

    • The ray that bounces off that surface is called the reflected ray

Unit c light and optical systems

  • To describe these rays:

    • Draw a line perpendicular to the reflecting surface (90°) at the point where the incident ray strikes the surface

      • This is called the normal line

    • The angle between the incident ray and normal line is the angle of incidence (i)

    • The angle between the normal line and the reflected ray is the angle of reflection (r)

      • Figure 3.15



  • Inferring the Law of Reflection Lab, p. 190-191

  • Plane mirror: a mirror with a flat surface

    • Shows the image exactly as it is

      Forming an Image

  • Angle of reflection should EQUAL the angle of incidence

    • This is the same on ALL types of surfaces, with NO exceptions!

Law of reflection

Law of Reflection

  • The angle of reflection equals the angle of incidence

    • Ex. If the angle of incidence (i) is 60°, then the angle of reflection (r) will be 60°

  • The incident ray, the normal line, and the reflected ray also lie in the same plane (an imaginary flat surface)

Law of reflection1

Law of Reflection

How do reflected rays form an image that you can see in a mirror?

  • Light shines on an object

  • Light reflects off the object in ALL directions

  • All of the object’srays that hit the mirror reflect off the mirror according to the law of reflection

  • The rays that reach your eyes appear to be coming from a point BEHIND the mirror

  • This happens for EVERY point on the object

  • Your brain interprets the pattern of light (travelling in straight lines) that reaches your eye as an image of the object behind the mirror

Law of reflection2

Law of Reflection

  • An image in the mirror is EXACTLY the same size as the object and appears to be the same distance from the mirror as the object

    • This is true only for plane (flat) mirrors

Curved mirrors

Curved Mirrors

  • Convex Mirrors: mirrors that bulge outward

  • Form images that appear much smaller and farther away than the object

  • Can reflect light from a large area

    • Ex. Security mirrors in stores, passenger mirrors in cars

  • Cannot focus an image on a screen, but you can see an image in the mirror (virtual image)

Curved mirrors1

Curved Mirrors

  • Concave Mirrors: mirrors that are caved in

  • Can focus an image onto a screen

  • Magnifies the image in the mirror

    • Ex. Make-up mirrors

Rough surfaces

Rough Surfaces

  • When light strikes a smooth surface, the lines reflect according to the law of reflection

  • When light strikes a rough surface, the normallines go in many different directions. Each light ray that strikes the surface will reflect according to the law of reflection. However, since the normal lines point in different directions, the reflected rays will go in different directions. This results in the reflected rays appearing scattered.

Rough surfaces1

Rough Surfaces

  • These seemingly scattered light rays reflect off the paper (for example) in all directions.

  • The light that reaches your eyes appears white because there is no pattern on the paper.

  • IF there is print on the paper, the ink absorbs light. Nothing that strikes the print is reflected, so your eyes see it as black.

Using reflections

Using Reflections

  • Reflectors on bicycles use the law of reflection to reflect light from other vehicles

  • Pool players use this law to improve their game

    • The pool ball will travel in a straight line until it hits something (bank shot)

Topic 2 review

Topic 2 Review

  • Page 199 #1-4

Topic 3 refraction

Topic 3: Refraction

REFRACTION: the bending of light when it travels from one medium to another

  • Light bends because it changes speed when it moves between materials that have different densities



  • Light travels slowly in a dense material

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

Around a bend with light

Around a Bend with Light

  • When light moves from a less dense medium to a more dense medium (ex. air to water), the light will bend TOWARDthe normal

  • When light moves from a more dense medium to a less dense medium (ex. water to air), the light will BEND AWAY from the normal

    • The degree to which the light bends depends on the material the light travels through

Around a bend with light1

Around a Bend with Light

  • Angle of Refraction (R): the new direction of the light following refraction

  • When the angle of incidence (i) increases, the angle of refraction (R) also increases

  • BUT doubling the angle of incidence DOES NOT double the angle of refraction

Around a bend with light2

Around a Bend with Light

  • Refraction can also occur when light travels through air at different temperatures

    • WARM air is less dense than COLD air

  • Light bends as it travels through

    different densities of air

  • This can result in a mirage

    • The air close to the ground is hotter and less dense than the air higher up. As a result, light from the sky directed at the ground is bent upward as it enters the less dense air

Is that all there is to light

Is that all there is to light?

Topic 4 lenses and vision

Topic 4: Lenses and Vision

Types of Lenses

  • Lens: a curved piece of transparent material such as glass or plastic

  • Light refracts as it passes through a lens, causing the light rays to bend

Types of lenses

Types of Lenses

  • A double 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 passing through the flatter areas

  • This causes the rays of light to spread out (DIVERGE) after passing

    through the lens

Types of lenses1

Types of Lenses

  • A double convex lens is thicker in the middle than around the edges

  • This causes the refracting light rays to come together (CONVERGE)

Lenses and images

Lenses and Images

  • An image forms where light rays from an object converge

    • The lens directs light from the left portion of the object to the right portion of the image

    • Light from the top of the object is directed to the bottom of the image

  • An image formed from a double convex lens is sometimes upside-down

    • Example: Film projectors are double convex lenses and the film must be placed upside-down to see the image properly on the screen

Eye spy

Eye Spy

  • The lens in a human eye is a convex lens

    • This lens takes light rays from objects and focuses them (brings them back to the point) through refraction

    • This allows us to see objects

  • In the eye, 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

Unit c light and optical systems

Disorders of the Eye

  • Near-sighted: people who have trouble seeing distant objects (eyes are too long)

  • Far-sighted: people who have trouble seeing objects close to them (eyes are too short)

    • Knowing how light behaves can help eye specialists correct vision problems

    • Figure 3.32B and 3.32C

Comparing the eye and the camera

Comparing the Eye and the Camera


  • You can see objects when light from an object focuses on your retina


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

    • Figure 3.33A

Putting it in focus

Putting it in Focus

  • In a camera, if an object moves closer to the film, the lens must move farther from the film to keep the image in focus

  • In the eye, your ciliary muscles change the shape of the lens to focus an image

  • If an object moves closer to you, your eye muscles make the lens bulge in the middle, keeping the object in focus

    • This is called accommodation

Putting it in focus1

Putting it in Focus

  • Near point: the shortest distance at which an object is in focus in the eye

    • Adult: 25cm

    • Babies: 7cm

  • Far point: the longest distance at which an object is in focus in the eye

    • Infinity (ex. stars)

Bringing in the light

Bringing in the Light


  • If light is too dim, the camera’s diaphragm (controls the aperture or opening of a lens) and shutter (limits the passage of light) can be adjusted to allow the correct amount of light to reach the film


  • The iris (colored portion) functions like the diaphragm of a camera. If the light is dim, the iris increases the size of the eye’s opening (pupil) to let in more light

Bringing in the light1

Bringing in the Light

  • This natural adjustment of the size of the pupil is called the iris adjustment

  • This happens very rapidly as you move from dark environments (pupils enlarge) to bright environments (pupils shrink)

Seeing the image

Seeing the Image


  • The image is focused onto light-sensitive film

  • Light energy causes a chemical change in the film which records the image

    Human Eye

  • The retina senses the light, produce electrical impulses that travel to the brain through the optic nerve

  • The point where the optic nerve enters the retina does not have any light-sensing cells (blind spot)

  • Layers of tissue and liquid (humors) keep the eye rigid and refract light that enters the eye

Unit c light and optical systems

  • Topic 4 Review p. 220 # 1-4

Topic 5 extending human vision

Topic 5: Extending Human Vision


  • Help us to see distant objects more clearly

  • Refracting telescope: a telescope with a convex lens (objective lens) to collect and focus light from a distant object,

    and an eyepiece lens to magnify

    the image



  • Reflecting telescope: a telescope with a concave mirror (primary or objective mirror) to collect rays of light from a distant object

    • It forms a real image which is then magnified by the eyepiece lens

  • The lens in a refracting telescope

    and the mirror in a reflecting

    telescope both collect light



  • Reflecting and refracting telescopes both need a large collector (lens or mirror) to gather as much light as possible from the distant object

  • For the greatest magnification, the telescope needs to have as large a distance as possible between the object being viewed and its image

    • The farther the image is from the lens, the greater the magnification

    • The farther the image is from the mirror, the greater the magnification



  • Binoculars are two reflecting telescopes mounting side by side

  • The telescopes are shortened by placing glass blocks (prisms) inside. These prisms serve as plane mirrors

  • Light in binoculars is reflected back

    and forth inside a short tube,

    rather than travelling down a

    long tube of a telescope

Microscopes telescopes and scientific knowledge

Microscopes, Telescopes, and Scientific Knowledge

  • Magnifying glasses are used for magnifications up to 10X the actual size

  • Compound light microscopes have an objective lens (4X, 10X, 40X) that forms a real image of the object, then an

    eyepiece (ocular) lens (10X)

    magnifies the image further

Unit c light and optical systems

Topic 5 Review p. 226 #1-4

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