Lights & Optics

Printable Notes

For centuries we understood how to use light to satisfy our needs, however we couldn’t explain how light works. Humans have been trying to understand light for last 2500 years.

Time to draw a timeline!

500 bc – Empedocles

Greek philosopher Empedocles believed Aphrodite made the human eye and lit a fire inside. The fire provided light that shone out from our eyes, when the light hit an object it would be illuminated. (But how come we can’t see in the dark?)

300 Bc – Euclid

Greek philosopher Euclid theorized that light travels in straight lines and it can be reflected.

150 AD - Ptolemy 

Ptolemy was a greek scientist, wrote that light “bends” as it passes from air to glass, and that light travels faster through air than in fluids.

1000 AD – IBN Al Haytham

• Al Haytham was an Arabian scientist who first accurately described how vision worked.

• Light bounces off objects and then travels to the eyes

1666 Ad – Isaac Newton

British scientist shined a light through a prism, he demonstrated that white light is actually a mixture of different colours of light.

1676 Ad – Ole Romer

Danish scientist was the first to find a reasonably accurate measurement of the speed of light. Measured by watching the moon Io eclipse over Jupiter, noted the amount of time it took to eclipse Jupiter was greater when Earth was farther away from Jupiter.

1920 AD - Albert A. Michelson

Michelson was a American scientist who Refined Romer’s experiment by using the tops of two mountains. Found that light travels 299 798 km/s (pretty darn close) actual speed of light in a vacuum is roughly 300 000 km/s.

5 basic Properties of Light    

1. Light travels in straight lines.

2. Light can be reflected.

3. Light bends by travelling in different mediums

4. Light is a form of energy (radiation)

5. Light is fast (300 000 km/s)

Brightness!

Why do lights appear brighter the closer you stand to it?

Luminous:

Describes objects that produce light (ex. the sun, fire, lightbulbs)

Non-luminous:

Describes objects that do not produce light but may reflect it; most objects on earth are non-luminous (ex. wood, rock, tinfoil)

LIGHT Intensity:

The quantity of light shining (measured in lumens)

• The further the light source the fewer the amount of light rays reach your eyes (appears dimmer)

• The closer the light source the greater amount of light rays reach your eyes (appears brighter)

CHECKPOINT 1

PRODUCING VISIBLE LIGHT

NATURAL LIGHT SOURCES:

• Bioluminescence (photophore)

• Sunlight

• Lightning

• Anything hot (burning log, lava, heated metal)

ARTIFICIAL LIGHT SOURCES:

• Incandescent light

• Fluorescent light

• Phosphorescent light

• Light Emitting Diode

Draw A mind Map!

Bioluminescence

Bioluminescence:

The production and emission of light by a living organism.

Light emission is the result of a chemical reaction converts chemical energy to light energy, this occurs in the photophores (specialized cells) of bioluminescent animals

Artificial Light Sources

INCANDESCENT LIGHT:

Thin metal called a filament is surrounded by glass, electrical energy flowing through the filament heats it to become white hot.

• Invented by Thomas Edison (Apparently)… contested by Joseph Swan (UK), or Lewis Latimer (US)

• First light ball filament was actually a piece of bamboo

FLUORESCENT LIGHT:

A glass tube coating in phosphor is filled with mercury gas, electricity is passed through the mercury gas, which releases UV rays and excites the phosphor to produce white light.

• Neon lights work on the same principle however they lack the phosphor powder.

PHOSPHORESCENT LIGHT:

Slightly different from fluorescence UV light hits phospor and stores up energy Emits light after uv light has stopped and glows in dark

Light Emitting Diode (LED):

Is a semiconductor (A material that kind of conducts electricity) that emits light when an electric current is passed through it

Which artificial light is the best ?

Many factors come into play:

• Cost

• Convenience

• Appearance

• Durability

Light Interactions

When light strikes an object it can interact with it in different manners, light has 3 options when interacting with objects:

1. Light is absorbed

2. Light is reflected

3. Light is refracted

Case 1 : Light is Absorbed

Recall that light is a form of energy, when light strikes an object some of that energy may be absorbed by the object and converted into heat energy.

Absorption of light by a material depends on how much light is let through the material:

Transparent:

Describes materials that allow light to pass through with little or no reflection; (Ex. glass)

Translucent:

Describes materials that allow some light to pass through; (Ex. frosted window)

Opaque:

Describes materials that do not allow light to pass through; (Ex. wood, books )

Materials that are opaque will absorb some of the light’s energy while reflecting the rest. Materials that are darker absorb more energy than objects that are lighter. Materials that are transparent absorb less of the light’s energy while refracting the rest.

Case 2 : Light is Reflected

Euclid’s Law of Reflection:

Angle of incidence of a wave hitting a surface equals the angle of reflection.  All surfaces obeys this law.

Angle of Incidence =  Angle of Reflection

Types of reflection

Specular (mirror-like) Reflection:

Reflection that happens when parallel rays hit a smooth surface; all rays reflect at the same angle

• Your eyes must be in line with the reflected rays to get a clear image.

Diffuse Reflection:

Reflection that happens when parallel rays hit a rough surface; all rays reflect at different angles, the light scatters.

•  Object is not clearly being reflected however the eye can see it from many different positions.

Playing a trick on our eyes

Mirrors (and lenses) can use light to fool our eyes, just like a hall of mirrors we can be tricked by how we perceive objects.

Real vs Virtual Images

Real Image:

Formed when light rays from an object converge after reflecting off a mirror, or refracting through a medium.

• Ex. Magnifying glass, concave mirrors

• Real images always appear upside down

Virtual Image:

Formed when light rays from an object diverge after reflecting off a mirror, or refracting through a medium. Our brain incorrectly perceives the light as having converged ‘behind’ the medium.

• Ex. Concave lense, Convex mirror

• Virtual images always appear right side up

Reflectors aka Mirrors

Plane Mirror:

A reflective surface that is flat

Concave Mirror:

• Reflecting surface that is curved inward like the inside of a spoon

• Light rays reflected from it converge (come together).

• Ex) reflecting telescope or car headlights

Convex Mirrors:

• Reflecting surface that bulges out like the back of a spoon

• Light rays reflected from it diverge (spread out).

• Ex) side mirrors

CHECKPOINT 2

Reflection Terms

Rays Diagrams:

Diagram used to represent how light travels; each ray has a straight arrow to show the direction of travel

These terms can be drawn rather than written down

Incident Ray:

A ray approaching a surface

Reflected Ray:

A ray that leaves or reflects a surface at the point of incidence

Point of incidence:

Where an incident ray strikes the surface

Normal:

An imaginary line drawn perpendicular to the surface (90º angle to the surface) directed outward from point of incidence  

Angle of Incidence:

Angle between the incident ray and the normal.

Angle of Reflection:

Angle between the reflected ray and the normal.

Ray Diagrams for Convex & Concave mirrors

Remember SAP-CE!

1. Setup - draw the mirror, normal, focal point, and of eye

2. Arrow - draw arrow pointing up, it can be placed behind, on, or in front of the focal point

3. Parallel Ray - draw a light ray from the top of the arrow parallel to normal, it reflects through the focal point.

4. Center Ray - draw a light ray from the top of the arrow towards the center of the mirror, make sure it reflects at same angle!

5. Extend Rays - extend light rays until they converge. Draw an arrow from normal to convergence point (don’t forget tail on normal, head on point). If they never converge then its a blur!

Is it real or virtual?

Point of convergance is on the ‘eye’ side - the image is considered real

Point of convergance is on the non ‘eye’ side - the image is considered virtual

ray diagrams of yoda’s reflection

Use SAP - CE to draw Yoda’s reflection. The real Yoda is represented by the green arrow, while the reflected Yoda is represented by the red arrow.

There are four different cases:

1. Concave mirror - When Yoda is placed behind the focal point he reflects upside down and smaller.

2. Concave mirror - When Yoda is placed on the focal point he reflects as a blur.

3. Concave mirror - When Yoda is placed in front of the focal point he reflects right side up and larger.

special case : convex mirror

4. Convex mirror - There is no focal point! Yoda is reflected right side up and smaller no matter the position of Yoda and the mirror.

Case : 3 light is Refracted

Human eyes assume the light rays move through the medium in a straight line so we incorrectly identify where an object is in the other substance.

Our brain gets tricked, much like when you accidentally walk into a mirror in a hall-of-mirrors. Your eyes perceive the light incorrectly!

Refraction:

Bending of light as it travels from one material to another material.

Refraction occurs because the speed of light changes in different substances!

1. In space speed of light travels at 300 000 km/s

2. There are no particles in space, what happens when light strikes particles? It slows down : Bends towards the normal.

3. As a result… The denser the new medium = the more the light slows down and refracts.

LAw of Refraction:

• Light goes from air to water (more dense) = light bends towards normal

• Light goes from water to air (less dense) = light bends away from normal

Refractors aka lenses

Concave Lens: 

• Piece of transparent material that is thinner in the middle than at the edges

• Light rays passing through it diverge (spread out).

Convex Lens:

• Piece of transparent material that is thicker in the middle than at the edges

• Light rays passing through it converge (come together). 

Focal Point:

Point at which light rays meet, or appear to meet, after being reflected by a mirror, or refracted by a lens.

Ray Diagrams for Convex & Concave lenses

Remember SAP-CE…again!

1. Setup - draw the lens, normal, focal point, and of eye

2. Arrow - draw arrow pointing up, it can be placed behind, on, or in front of the focal point

3. Parallel Ray - draw a light ray from the top of the arrow parallel to normal, it refracts through the focal point on the opposite side of the lens

4. Center Ray - draw a light ray from the top of the arrow straight through the center of the lens and out the opposite side

5. Extend Rays - extend light rays until they converge. Draw an arrow from normal to convergence point (don’t forget tail on normal, head on point). If they never converge then its a blur!

Is it real or virtual?

Point of convergance is on the ‘eye’ side - the image is considered real

Point of convergance is on the non ‘eye’ side - the image is considered virtual

Lens ray diagram

Use SAP - CE to draw the storm trooper’s reflection. The storm trooper is represented by the green arrow, while the refracted storm trooper is represented by the red arrow.

There are four different cases:

1. When the storm trooper is placed between the focal point and the mirror it refracts right side up and larger.

2. When the storm trooper is placed on the focal point it refracts as a blur.

3. When the storm trooper is placed behind the focal point it refracts upside down and smaller.

Special case- Concave lens

4. Concave lenses will always make the object appear up right and smaller.

CHECKPOINT 3

Ray Diagram Assignment

Law of Reflection Lab

Lab Rubric