Lights & Optics

Our knowledge about light and vision comes from explanations, inventions, and investigations.

Printable Notes

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Although for centuries we know how to use light to satisfy our needs, we couldn’t explain how light works. Here are some ideas about light:

Early Light Ideas

1st idea – Pythagoras (Greek) [6th BC]

  • Human produced straight beams of light from our eyes, when they hit an object they could see it. 
  • Problem – We cannot see in the dark.
 EUCLID

EUCLID

2nd idea – Euclid (Greek) 

  • Discovered that light travels in straight lines
  •  Discovered the Law of Reflection

3rd idea – Ptolemy (Greek) [1st AD] 

  • Discovered that light bends as it passes from air to glass.
  • Light travels faster through air than in fluids

 

4th idea – Al-Haytham (Arabian) [1000 AD] 

  • First to accurately describe how vision worked.
  • Light bounces off objects and then travels to the eyes, not what Pythagoras thought!

5th idea – Newton (English) 

  •  By shining a light through a prism he demonstrated that white light is actually a mixture of different colors of light.

 6th idea – Ole Romer (DANISH) [1676]

  • First 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.
  • Albert A. Michelson (USA) [1920]   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. 

4 Basic Properties of Light    

1) Light travels in straight lines.

2) Light can be reflected.

3) Light can bend by travelling in different mediums

4) Light is a form of energy.


 Optical Devices

 DSLR CAMERA CUT IN HALF

DSLR CAMERA CUT IN HALF

Optical Device:

Any device that uses light

Ex) mirrors, lenses, microscopes, and telescopes

 Microscope:

Optical devices used for viewing very small objects; has at least two lenses: the objective lens and the eyepiece lens.

  • Different kinds: compound (including dissecting & electron)
  • First invented by Hans and Zacharias Jansen in Netherlands (1595)
  • Antonie van Leeuwenhoek: created a more powerful and useful design.  First one to identify micro-organisms, named the animalcules.
 GRIFFITH OBSERVATORY TELESCOPE

GRIFFITH OBSERVATORY TELESCOPE

Telescopes:

Optical device for viewing distant objects; there are two types: reflecting and refracting telescopes.

  • Galileo did not invent the first one but created many stronger telescopes and  made many important astronomical discoveries.
  • Telescopes use mirrors and lenses (or both) to do two things:

1)      Magnify  (ex. to see a planet!)

2)      Collect Light (see faint objects of a planet)


Types of Telescopes

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(i)  Refracting Telescope -  telescope that uses two lenses, each at the end of a long tube to from an enlarged image.

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(ii) Reflecting Telescope - telescope that uses a curved mirror and a lens to form an enlarged image.

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  • Binoculars : device for viewing distant objects made up of two short refracting telescopes fitted together.

Light And Reflection Terms

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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.


Light Behaves in Predictable Ways

Rays Diagrams:

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

Intensity:

Brightness of light; amount of light arriving per unit area at a place.

  • A light meter measures the intensity of light
  • Further the light source the lower the intensity = fewer rays reach your eyes.
  • Explains how shadows are places where light rays are blocked.
  • If light hits an object, it can’t go any father

Light Interacts with Materials

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

Non-luminous:

Describes objects that do not produce light but may reflect it; most objects on earth are non-luminous Ex) wood.

Luminous:

Describes objects that produce light; Ex) sun & fire


Types of Reflection

 THE MOUNTAIN IS PERFECTLY REFLECTED IN THE LAKE - THIS IS CALLED SPECULAR REFLECTION

THE MOUNTAIN IS PERFECTLY REFLECTED IN THE LAKE - THIS IS CALLED SPECULAR REFLECTION

Specular/Regular 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.

 THE OVERALL SHAPE AND COLOUR OF THE MOUNTAIN IS REFLECTED YET SOMEWHAT BLURRY - THIS IS CALLED DIFFUSE REFLECTION

THE OVERALL SHAPE AND COLOUR OF THE MOUNTAIN IS REFLECTED YET SOMEWHAT BLURRY - THIS IS CALLED DIFFUSE REFLECTION

Diffuse Reflection:

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

 Object can be seen from any position.


The Law of Reflection

Incident (incoming) Rays:

Ray of light that arrives at a mirror or other substance.

The shinier and smoother the surface the better the reflection.

Plane Mirrors:

Flat mirror; reflecting surface that has no curvature.

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



Reflecting Light with Curved Mirrors

Convex Mirrors:

Reflecting surface that bulges out like the back of a spoon; light rays reflected from it diverge (spread out).

  • The image appears like it is originating from a smaller point behind the mirror; useful for side mirrors.

Concave Mirror:

Reflecting surface that is curved inward like the inside of a bowl or a spoon; light rays reflected from it converge (come together).

  • Useful to collect light and direct it to a single point.
  • Ex) reflecting telescope or car headlights

Focal Point:

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

Concave Mirror Images

Image from a concave mirror depends on the object’s distance from the focal point.

       Position of Object                                           Image

(I)  Object far from the focal point                            Upside-down image

                                                                                    Smaller Size

(II)  Object closer to the focal point                          Upside-down image

                                                                                    Larger Size

(III)  Object on the focal point                                   No Image

(IV)  Object between the focal point & mirror          Upright image

                                                                                    Enlarged


Drawing Rays 4 Steps: There is a trick to this!

  • You need to understand that light travels in straight lines
  • You need to know that one light ray (incident and reflected) must always travel through the focal point!

1. Take the top of the object & draw an incident ray going parallel to the normal and one going through the focal point

2. Once the rays hit the mirror, the opposite is true; the incident rays turn into reflected rays.  The incident ray that was parallel must now go through the focal point, and the incident ray that went through the focal point must go parallel.

3. Mark the spot the reflected rays intercept

4. This point of interception is always the top of the object.


    Transparent Substances Refract Light

    When a light ray strikes a boundary where two different substances meet (interface) at an angle, it will change direction.

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

     

    Screen Shot 2018-01-17 at 8.16.48 AM.png

    Refraction:

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

    • Refraction occurs because the speed of light changes in different substances.
    •  In space (vacuum) = Light travels at 300 000 km/s

     

     WATER REFRACTS THE SHAPR OF THIS PENCIL

    WATER REFRACTS THE SHAPR OF THIS PENCIL

    • There are no particles in space, what happens when light strikes particles?  It slows down : Bends towards the normal.
    • As a result… The denser the new medium = the more the light slows down and refracts.
    • May occur due to the number of particles in the material

    • Light goes from air to water (or glass) =  bends towards normal
    • Light goes from water (or glass) to air = bends away from normal

    Lenses Refract and Focus Light

    Lens:

    Curved piece of glass or other transparent material that refracts light in a predictable way, useful because the light rays that refract can form images.

    •  Lens ability depends on:
    1. Shape of lens
    2. Material of the lens

    Types of Lenses

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    Convex Lens:

    Piece of transparent material that is thicker in the middle than at the edges; light rays passing through it converge or come together. 

    • Technical name for a lens that curves out on both side = double convex lens.
    • Recall: Light rays refracted through a lens meets at the focal point.
      • Useful for two reasons:
    1. Good light collector  (ex. Refracting telescope)
    2. Forms a Real Image
    • Produces a 'real image' - an image that looks like a real object.
    • Size of an image depends on the distance of the object from the lens.
    • Main Drawback is that the image produced is upside-down.

    Concave Lens: 

    Piece of transparent material that is thinner in the middle than at the edges; light rays passing through it diverge or spread out.

    • Much like a concave mirror had 4 possibilities, so does a convex lens
    • Image formed depends on how far the object is from the lens

                Position of Object                                                       Image

    (I)  Object far from the focal point                            Upside-down image

    Screen Shot 2018-01-22 at 11.25.36 AM.png

                                                                                        Smaller Size

                                                                                        Real Image

    (II)  Object closer to the focal point                          Upside-down image

                                                                                        Larger Size

                                                                                        Real Image

    (III)  Object on the focal point                                   No Image

    (IV)  Object between the focal point & lens             Upright image

                                                                                        Enlarged

                                                                                        Same side as object


    Eye structure

    A brief introduction

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    How do glasses work

    Nearsightedness:

    Can see near but not far because focal point is in front of retina

    Farsightedness:

    Can see far but not near because focal point is behind retina