By Mark Jordan © The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan.

By Mark Jordan

©

The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan

OUTLINE OF THE DAY

To Be Completed by Class Teacher

The Professional Development Service for Teachers is funded by the Department of Education and Science under the National

Development Plan

• Light is stated to travel at a of 299 792 458 m / s. so how long does it take light to come from the Sun to Earth?

• How often could light travel around the earth in one second?

• With the advent of new innovative technology is it likely that light will be made travel faster in the future?

• Dave Grennan, Irish astronomer, recently discovered a supernova that exploded nearly 300,000 years ago yet the light from that explosion is now only reaching Earth. How is this possible?

(see notes for more information)

What do you think?

Longest Shortest

Electromagnetic waves (including light) travel at a speed of 3 x 108 ms-1

Light is part of Electromagnetic Spectrum

– the part we can see, i.e. the visible spectrum

(see notes for more information)

The visible spectrum is made up of seven colours. • Can you explain why we can see these different colours.• Is black a colour?

http://www.teachersdomain.org/asset/lsps07_vid_lightreflect/

Light bounces of surfaces. Click the link below (must have Quicktime installed) to find more about bouncing light and ……. photons.

A ray of light is an extremely narrow beam of light.

All visible objects emit or reflect light rays in all directions.

Our eyes detect light rays.

We see images when

light rays converge in our eyes.

Light can be reflected. Reflection is the bouncing of light of a solid object

object

image

It is possible to see images in mirrors.

Mirrors are good at reflecting light rays.

Plane Mirrors

image

Light reflected off the mirror converges to form an image in the eye.

How do we see images in mirrors?

The eye perceives light rays as if they came from the mirror.

The image is virtual since it is formed by the apparent intersection of light rays.

(apparent rays are indicated on the diagram as broken lines and actually don’t exist)

Laws of Reflection Exp.- Follow steps in animation

The normal is a line right angles to the mirror where the ray of light hits it. (A ray of light striking the mirror at 900 is reflected back along the same path).

Mirror

normal

incident ray

reflected ray

θi

Angle of incidence

θr

Angle of reflection

Law 1

When light is reflected off a mirror, it hits the mirror

at the same angle (the incidence angle, θi) as it

reflects off the mirror (the reflection angle, θr).

Law 2

The incident ray, the reflected ray and the normal are all lie on the

same plane.

• A driver in a parked car has 2 views of the car parked behind him – ‘rear view mirror’ (right) & in the ‘side mirror ‘(left). o How is it that each mirror gives a different view? o Which view represents the true distance the parked car is from the drivers car?

(see notes for more information)

Points to ponder

Concave Mirror- Part of a sphere reflective surface on inside

C: the center point of the sphere

r: radius of curvature (just the radius of the sphere)

F: the focal point of the mirror (halfway between C and the mirror)

f: the focal distance, f = r/2

r

f

•C

•F

optical axis

Concave Mirrors(caved in)

•F

•Light rays that come in parallel to the optical axis reflect through the focal point•Light rays that come in along the optical axis strike the mirror at 90 so reflect back along optical axis through the focal point.

Principal axis

Concave Mirror

•F

•c

Image formed in a concave mirror object placed outside centre of curvature

FocusCentre of Curvature

Object

Image:- Real, Inverted & diminished

f

u

v

Principal axis

Concave Mirror

•F

•c

Image formed in a concave mirror when object placed at centre of curvature

FocusCentre of Curvature

Object

Image:- Real, Inverted & diminished

v

f

u

Principal axis

Concave Mirror

•F

•c

Image formed in a concave mirror when object placed between centre of

curvature & focus

FocusCentre of Curvature

Object

Image:- Real, Inverted & Enlarged

v

f

u

Principal axis

Concave Mirror

•F

•c

Image formed in a concave mirror when object placed at focus

FocusCentre of Curvature

Object

Image:- At Infinity

f

u

Principal axis

Concave Mirror

•F

•c

Image formed in a concave mirror when object placed inside focus

Focus

Centre of Curvature

Object

Image:- Virtual, Erect & Enlarged

vf

u

Equation

ƒ = focal lengthu = object distancev = image distance

vuf

111

if distance is negative the image is behind the mirror

Magnification Equation

v

vm

m = magnificationv = image heightu = object height

if the magnification is negative the image is inverted (upside down)

Sign Convention for Mirrors

Quantity Positive (+) Negative (--)

Object location (u) Object is in front of the mirror

Object is behind the mirror

Image location (v) Image is front mirror

Image is behind of mirror

Focal length (f) Mirror is concave Mirror is convex

Magnification (M) Image is upright Image is inverted

Concave mirror

Crosswire

Lamp-box

Screen

u

v

TO FIND THE FOCAL LENGTH OF A CONCAVEMIRROR

Procedure• Get the approx. focal length of mirror by focusing distant object on screen – why?• Place the lamp-box well outside the approximate focal length – why?• Move the screen until a clear inverted image of the crosswire is obtained.• Measure the distance u from the crosswire to the mirror, using the metre stick. • Measure the distance v from the screen to the mirror. • Calculate the focal length of the mirror using - - - - - -

• Repeat this procedure for different values of u. • Calculate f each time and then find an average value.

vuf

111

Convex Mirrors

Light rays that come in parallel to the optical axis reflect from the focal point.

optical axis

•F

The focal point is considered virtual since sight lines, not light rays, go through it.

principal axis

•C

•F

Convex Mirrors

Image:- Virtual, Erect & Diminished

Focus

Centre of Curvature

Object

v

fu