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Optics looks at the properties and behaviour of light!

Chapter 4: Wave Model of Light

Past Theories

Pythagoras – believed that light consisted of beams made up of tiny particles that carried

information about an object to the eye so we could see it

Galileo – believed to be the first person to try to determine the speed of light. He and an

assistant stood on two hilltops about 1 km apart with lanterns. Galileo uncovered his

lantern first and his assistant was suppose to uncover his lantern when he saw Galileo’s

light. This did not work well and Galileo could not calculate the speed of light!

Michelson – He is the first person to accurately carry out experiments to measure the speed of

light. He used a strong light source, an 8 sided rotating mirror and another large mirror

about 35 km away. Using the distance the light travelled and the speed at which his

mirrored wheel was spinning he was able to calculate the speed of light.

Scientific knowledge of light has led to the development of early technologies such as:

Microscope – created by the Janssen’s who experimented with lenses and tubes. By

moving the tubes in and out they made small objects appear larger

Telescope – created by Galileo who made his own lenses to magnify objects in space

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Light Specifics

Light – a form of energy that can be detected by the human eye

Speed of light – The speed of light is 300 000 000 m/s or 3 x 108 m/s

* Compare speed of light with speed of sound:

- Speed of sound at sea level is about 330 m/s (1200 km/h) compared to the speed of light

at 300 000 000 m/s (1 000 000 000 km/h)

- light travels extremely fast – so fast that we cannot notice the time required for light to

travel normal distances around us

- The light from a distant lightening strike reaches us almost instantly but the sound from

the strike (the thunder) takes longer to reach us! The longer it takes for you to

hear the thunder after seeing the lightening, the further away the lightening is!

Properties of light waves

Light waves have the same features as ocean waves:

Relationship between frequency and wavelength:

High frequency waves have short wavelengths while low frequency waves have long

wavelengths

The red has the longest wavelength but the least refraction

The violent has the shortest wavelength but the most refraction

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The light that we see is called visible light. It is only one small part of the larger electromagnetic

spectrum. The physical make up of our eyes allow us only to see visible light in the form of the

colors below!

Visible Light Spectrum

Visible light – a form of energy that can be detected with our eyes.

Roy G Biv to remember colors!

Red has the smallest refraction, orange refracts a little more, yellow a little more and so on ...

violet has the largest refraction!

A prism refracts light and disperses it, separating its colors. Different colors of light are carried

by light waves that have different wavelengths. An object appears blue in sunlight because only

the blue color is reflected. The other colors are absorbed because of the properties of their

wavelengths.

Other examples of light dispersion to separate colors occur in sun catchers, rainbows in the sky

or when you use a sprinkler on the lawn.

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Electromagnetic radiation – movement of electric and magnetic energy through space

Electromagnetic radiation is always present but we do not realize it because its wavelengths are

too short or too long for our eyes to see! We can only see the visible light portion of the

electromagnetic spectrum!

The electromagnetic spectrum has 7 types of electromagnetic radiation which can be categorized

in order by the size of their wavelengths, their frequencies and their energy:

Everyday Uses:

1. Radio waves – used in telecommunications (phone, radios, radar, satellite communication)

2. Microwaves – cooking food, Wireless LAN, Bluetooth devices

3. Infrared – motion sensors, night vision devices, infrared cameras detect heat loss in homes

4. Visible light – Everything we see, microscopes, CD players, fax machines, photocopiers

5. Ultraviolet – sun tanning, black lights, glow in the dark objects, fluorescent lamps

6. X-rays – x-rays, radiation treatment for cancer, airport security scanners

7. Gamma rays – Gamma radiation sterilizes hospital equipment, used to kill some cancer cells

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Positive and Negative effects of Electromagnetic Radiation

* Higher energy radiation, such as X-rays and Gamma rays, is more harmful and dangerous.

Electromagnetic Radiation Positive Effects Negative Effects

Radio Waves Improved telecommunication Uncertain of long term

exposure effects

Microwaves Quick cooking of food May decrease nutritional value

of foods when used in heating

Infrared Improved night vision Long term exposure can have

irreversible effects on eyesight

Ultraviolet Used to treat jaundice in

babies

Skin cancer

X-rays Medical detection Over exposure can lead to

cancer

Gamma rays Radiation therapy for cancer May kill other exposed cells

To remember the electromagnetic spectrum visit the electromagnetic song at:

http://www.youtube.com/watch?v=bjOGNVH3D4Y

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Chapter 5: Laws of Reflection

Properties of visible light

1. Rectilinear propagation – light travels in a straight line, like when we make shadows

2. Reflection – specular reflection using mirrors and diffuse reflection using dust

3. Refraction – bending or changing direction of wave as it passes from one material to another

such as a popsicle stick appearing bent in a glass of water

4. Dispersion – formation of a rainbow as light separates into its different colors

5. Travels through a vacuum – does not require a medium such as the light from stars that

reaches earth by travelling through space

6. Travels through transparent, translucent and opaque materials (in varying amounts)

Transparent – can see through it (glass, air, water)

Translucent – cannot see through (frosted or stained glass)

Opaque – light cannot pass through and so we cannot see through it (doors, wood)

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Laws of Reflection in Mirrors

Ray diagram – uses straight lines to show the path of light rays

Incident light ray – the incoming light ray

Reflected light ray – the ray that bounces off the surface of the barrier (surface, mirror etc)

Normal – the imaginary line that is perpendicular to the barrier

Angle of incidence – the angle formed by the incident ray and the normal (i)

Angle of reflection – the angle formed by the reflected ray and the normal (r)

Two types of reflection:

Specular reflection – reflection from a mirror like surface which produces an image of the

Surroundings

Diffuse reflection – reflection from a rough surface that does not produce a clear image

but instead allows you to see what is on the surface

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Examples of specular versus diffuse reflection:

Matte versus Glossy Paint: Matte paints have a higher proportion of diffuse reflection

resulting in lower luster. Gloss paints have a greater proportion of specular

reflection resulting in a shinier appearance.

Unglazed versus Glazed ceramics: Unglazed has higher proportion of diffuse reflection;

glazed has greater proportion of specular reflection

Matte versus Glossy photographs: same effect as matte versus glossy paint!

Types of Mirrors

Plane mirror – flat, smooth mirrors like bathroom mirrors

Concave mirror – has a reflecting surface that curve inwards like the inside of a metal spoon

Convex mirror – has a reflecting surface that curves outward like the safety mirror on the front of

a school bus

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Ray Diagrams for Mirrors

Key Backgroud Knowledge:

Law of Reflection – observations on all types of surfaces show that the angle of incidence is

alway equal to the angle of reflection

Object – the initial object facing the mirror (if you look at a mirror, you are the object)

Image – the appearance of the object that was facing the mirror (the image of yourself in the

mirror is not you, just a likeness of you)

Real image – happens when reflected or refracted rays meet and the image appears to be in front

of the mirror. It is often distorted and you need a screen to see it clearly

Virtual image – the reflected rays do not meet, but their extended rays meet at the object. The

image appears to be behind the mirror

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Object size – size of the original object

Image size – size of the reflected/refracted image

Object distance – distance between the object and the mirror

Image distance – distance between the image and the mirror

Upright – same drection as the original object

Inverted – upside down from original object

Prinipal axis – a straight line that is perpindicular to the centre of a mirror or lens

Vertex – the point where the principal axis meets the mirror

Focal point – the point where converging light ray meet or diverging light rays diverge

(converging means come together and diverging mean to spread out as per the diagram below)

Focal length – distance from the lens (vertex) to the focal point

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Drawing Ray Diagrams for Plane Mirrors

SPOT Characteristic Plane Mirror

S = size (sizes of object and image) Image size = Object size

P = position (object distance or image distance) Image distance = Object distance

O = Orientation (upright or inverted) Upright, flipped in plane mirrors

T = Type (real or virtual) Virtual

Drawing Ray Diagrams for Concave Mirrors

Concave mirrors are more complicated because the characteristics depend on location of object:

SPOT Characteristic Object between focal

point and mirror

Object between focal

point and 2x focal

point

Object beyond the

2x focal point

S = size (sizes of

object and image)

Image is larger than

object

Image is larger than

object

Image is smaller than

object

P = position (object

distance and image

distance)

Image distance is

larger than object

distance

Image distance is

larger than object

distance

Image distance is

smaller than object

distance

O = Orientation

(upright or inverted)

Upright Inverted Inverted

T = Type (real or

virtual)

Virtual Real Real

In curved mirrors:

- incident rays travelling parallel to the principal axis are reflected through the focal point

- incident rays going throug the focal point are reflected parallel to the principal axis

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Object between mirror and focal point

* Any ray that is drawn beyond the mirror is an extended ray and should be a dotted line, not a

solid line.

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Object between the focal point and 2x the focal point

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Object beyond the 2F point

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Drawing Ray Diagrams for Convex Mirrors

SPOT Characteristic Convex Mirror

S = size (sizes of object and image) Image is smaller than object

P = position (object distance or image distance) Image distance is smaller than object distance

O = Orientation (upright or inverted) Upright

T = Type (real or virtual) Virtual

* The focal point for convex ray diagrams is behind the mirror! Any lines drawn beyond the

mirror are dotted, not solid.

* rays travelling parallel to the principal axis will reflect so that there extended rays go through

the focal poing

* rays travelling through the focal point will cause the reflected ray to be parallel to the principal

axis

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Chapter 6: Lenses Refract Light to Form Images

(The first part of this section is found in chapter 5 but fits better here with chapter 6)

Refraction

Refraction – the bending of light rays when they travel from one medium to another (ie: from air

to water)

The human brain does not recognize that light rays become bent or refracted as they travel from

water to air and so the apparent position of an object is different from its actual position.

As the light rays travel from one medium to another their speed changes.

- speed will decrease as it travels from one medium to another that has a greater density

(slows down as it goes from air to water). This will result in the ray bending

toward the normal

- speed will increase if it travels from one medium to another with a lesser density. This

will result in the ray bending away from the normal.

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Lenses

Lens – a curved piece of transparent material such as glass or plastic that refracts light in a

predicatable way (like camera lenses or contact lenses).

There are 2 types: concave and convex:

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Converging vs Diverging

Uses of Concave and Convex Lenses:

Concave Convex

Eye glasses (fix near-sightedness) Magnifying glasses

Eye Glasses (fix far-sightedness)

How do lenses fix your vision?

The type of lens you need depends on your vision problem – does your eye lens converge light

rays to a point in front of your retina or behind it??

If your eye refracts light too much then you are nearsighted and you will need to use concave

lenses in your glasses.

If your eye does not refract enough light then you are far-sighted and you will need to use a

convex lens in your glasses.

Usually, convex lenses in glasses make someone’s eyes look lager where concave lenses make

someone’s eyes and face look smaller!