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Unit 3 Optics. What is Light?. Electromagnetic Spectrum. is energy that moves in the form of waves electromagnetic spectrum ranges from short (x-ray and gamma) wavelengths to long (radio) wavelengths

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Unit 3 Optics

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Unit 3 optics

Unit 3 Optics

What is light

What is Light?

Electromagnetic spectrum

Electromagnetic Spectrum

  • is energy that moves in the form of waves

  • electromagnetic spectrum ranges from short (x-ray and gamma) wavelengths to long (radio) wavelengths

  • The majority of the electromagnetic spectrum (other than radio and visible light) is absorbed and deflected by Earth’s atmosphere

Colours of visible light

Colours of Visible Light

  • Light is made up of particles (Photons)

  • eyes evolved to detect only 1 part of spectrum

  • Light travels in a straight line in same medium

  • Travels at same speed in a vacuum

  • Slows in a medium...

  • Red slows the least and violet the most

  • Light separates in a prism (ROYGBIV)

Proof of light

Proof of Light

  • Proof of light as a particle

    • Light is pulled by gravity…bends around stars

    • Light can move and cut mass. Cutting lasers and lasers

  • Proof of light as a wave

    • Light reflects the same way that waves do

    • Light refracts according to the same rules as any other type of wave

    • Light will slow down in a dense medium then speed up again as it leaves that medium…not possible if mass

    • Light has a frequency and wavelength as do waves.

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Light production

Light Production?

Unit 3 optics

  • Luminescence: produce and emit light

  • Non-Luminescence: reflect light

Unit 3 optics

  • Incandescence: is the emission of light from a hot body due to its temperature.

  • Chemiluminescence: emission of light and heat as the result of a chemical reaction.

  • Bioluminescence: chemiluminescence that occurs in animals

Unit 3 optics

  • Fluoresence: emission of light by a substance that has absorbed other electromagnetic radiation...

  • emitted light has a longer wavelength, and therefore lower energy

  • Phosphoresecence: energy absorbed by a substance is released relatively slowly in the form of light.

Unit 3 optics

  • Electroluminescence: a material emits light in response to the passage of an electric current.

  • Triboluminescence: light generated when material pulled apart, scratched or crushed...through the breaking of chemical bonds



  • LED: light emitting diodes: ..no filament and light is produced as electricity moves through a semiconductor

  • CFL compact fluorescent lights:

  • Inside the  glass tube is a partial vacuum and a small amount of Hg. An electric discharge causes Hg atoms to emit UV light

  • tube is lined with a coating of a fluorescent material which absorbs the ultraviolet and re-emits visible light.

Laser application

Laser Application

  • light amplification by stimulated emission of radiation

  • one specific wavelength of light (one specific color).

  • light released is “organized” -- each photon moves in step with the others

  • The light is very directional.

  • A very tight concentrated beam

  • A flashlight, on the other hand, releases light in many directions, and the light is weak--diffuse.

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Reflection ray model

Reflection &Ray model

Plane mirrors

Plane Mirrors

  • Angle of incidence equals the angle of reflection.

  • Measured from incident ray to normal and reflected ray to normal

Images in plane mirrors

Images in Plane Mirrors

  • Locate by perpendicular lines:

  • Distance of perpendicular to the reflective surface = the distance to image behind the mirror

Describing images salt

Describing Images – SALT

  • S: SizeSmaller, larger, same size

  • A: AttitudeUpright or inverted

  • L: LocationImage is closer, farther or equal to reflecting surface as the object.

  • T: TypeReal or virtual

The characteristics of an image formed by all plane mirrors

The characteristics of an image formed by all plane mirrors:

  • The image is the same size as the object.

  • The image is the same distance behind the mirror that the object is in front of the mirror.

  • The image is always virtual.

  • The image is always upright vertically.

  • The image is always inverted horizontally; if you lift your right hand your reflection lifts its left.

Images using rays

Images using Rays

Curved mirrors

Curved Mirrors



Concave rules

Concave Rules

  • Draw a principal axis…and trace mirrored surface

  • 5 rays at concave mirror (with middle ray directed along the principal axis

  • Reflect back and meet at the focus point

  • Measure 2x distance F to mirror = center of curvature


    1. Single ray parallel to principal axis. What happens?

    2. Single ray through the focus point. What happens?

    3. Single ray through the center of curvature. What happens?

Rules for determining images in concave mirrors

Rules for Determining Images in Concave Mirrors

  • Any incoming incident ray that is parallel to the principal axis will be reflected through the focus point.

  • Any incident ray that goes through the centre of curvature will strike the mirror at a 90° angle and will reflect back onto itself.

  • An incident ray that goes through the focus point and then strikes the mirror will reflect parallel to the principal axis.

  • An incident ray that strikes the vertex will follow the law of reflection, since the vertex is perpendicular to the surface of the mirror.



  • Center of the circle for a curved mirror is called the curvature

  • Half the distance between the centre of curvature and mirror is called the focus or focus point.

  • The focus is given the symbol of F and the centre of curvature is located at 2F.

Convex mirror

Convex Mirror

Convex mirror rules

Convex Mirror Rules

  • An incoming incident ray that is parallel to the principal axis strikes the mirror and reflects as if it had come from the focus point on the other side of the mirror.

  • An incoming incident ray that is aimed at the centre of curvature will strike the mirror and reflect back upon itself.

  • An incoming incident ray that is aimed at the focus point will strike the mirror and reflect parallel to the principal axis



Unit 3 optics

  • Mini Activities:

  • Penny in crucible...angle so you can’t see...add water

  • Penny under the beaker and observe it from an angle down through the side of the beaker...add water

Unit 3 optics

  • Refraction is the change in direction of light as it passes at an angle from one medium into another...e.g.air into water

Unit 3 optics

  • As light (or any wave) changes speed, it changes direction…unless it hits the new medium straight on at a perfect angle of 90.

Unit 3 optics

  • When a ray of light passes into a slower, more optically dense medium, the ray of light bends towards the normal. (example air  glass)

  • When a ray of light passes into a faster, less optically dense medium, the ray of light bends away from the normal. (example glass  water)

  • When the angle of incidence is zero, there is no refraction. (No change in direction, yes a change in speed)

Index of refraction

Index of Refraction

Index of refraction1

Index of Refraction

  • The ratio of the speed of light in a vacuum (c = 3.00 x 108) to the speed of light in a given material (v).

  • Question:

  • The speed of light in a liquid is 2.25 x 108 m/s. What is the liquid?

  • Calculate the speed of light in diamond with an index of refraction of 2.417

Snell s law of refraction

Snell’s Law of Refraction

Internal reflection

Internal Reflection

Total internal reflection

Total Internal Reflection

  • Critical Angle

Unit 3 optics

  • Diamonds have a very high index of refraction that makes total internal reflection easier. But the cut of the diamond is also important. 45 angle!







  • There are two types of lenses, converging (convex) and diverging (concave).

  • Due to refraction, light rays bend as they pass into and out of the lens.

Unit 3 optics

  • ray that is parallel to the principal axis is refracted through the principal focus (F – opposite side of lens from the object)

  • A ray that passes through the secondary principal focus (F’ – same side of lens as object) is refracted parallel to the principal axis.

  • A ray that passes through the optical centre goes straight through without bending.

Object beyond 2f





Object beyond 2F’

Object beyond 2f1

Object beyond 2F’

Object on 2f





Object on 2F’

Object between 2f and f





Object between 2F’ and F’

Object on f





Object on F’

Between f and lens





Between F’ and lens

Lens equations

Lens Equations

Unit 3 optics

  • Distances of virtual images are negative, real are positive.

  • Heights are positive if upright and negative if inverted.

  • Converging lens has a real principal focus and a positive focal length.

  • Diverging lens has a virtual principal focus and negative focal length.



An upright image has a positive magnification



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