Light

Complete Physics pages for IGCSE 139 to 168

IGCSE SYLLABUSOPTICSLens3.2 (c) Thin converging lensCore• Describe the action of a thin converging lens on abeam of light• Use the term principal focus and focal length• Draw ray diagrams to illustrate the formation of areal image by a single lens3.2 (d) Dispersion of lightCore• Give a qualitative account of the dispersion of lightas shown by the action on light of a glass prism

3.2 (e) Electromagnetic spectrumCore• Describe the main features of theelectromagnetic spectrum and state that

all e.m. waves travel with the same high speed in vacuum

• Describe the role of electromagnetic waves in:

– radio and television communications (radio waves)

– satellite television and telephones(microwaves)– electrical appliances, remote

controllers for televisions and intruder alarms (infrared)

– medicine and security (X-rays)• Demonstrate an awareness of safety

issues regarding the use of microwaves and X-rays

Reflection

Law of Reflection

normal

angle of incidence, i

incident ray

reflected ray

angle of reflection, r mirror

The angle of incidence (i) is equal to the angle of reflection (r)

Note: Both angles are measured with respect to the ‘normal’. This is a construction line that is perpendicular to the reflecting surface at the point of incidence.

The image formed by a plane mirror

plane mirror

imageobject

normalsconstruction lines (virtual light rays)

The image produced by the plane mirror is:1. The same size as the object2. The same distance behind the mirror as the object is

in front3. Upright (the same way up as the object)4. Back-to-front compared with the object (lateral

inversion)5. Virtual

Real and virtual images

REAL images are formed where light rays cross after reflection by a mirror or refraction by a lens.

Real images can be cast onto a screen. Example: A projector image

VIRTUAL images are formed where light rays only appear to come from.

A virtual image cannot be cast onto a screen.Example: The image formed by a plane mirror

Choose appropriate words to fill in the gaps below:

The law of reflection states that the angle of _________ is always _______ to the angle of incidence.

Both angles are measured relative to the _________, a line that is at _______ degrees to the reflecting surface at the point of reflection.

A plane mirror forms a _______ image which is unlike a _____ image in that it cannot be cast onto a screen. The image in a plane mirror is also the same ______ and the same way up as the object.

realvirtual reflectionsizenormal equal ninety

WORD SELECTION:

real

virtual

reflection

size

normal

equal

ninety

SimulationsOptical illusions - by eChalk BBC KS3 Bitesize Revision: KS3 Light Contents Page What is light? Seeing Test bite on KS3 Light Law of Reflection - NTNU - features a movable plane mirror Reflection in a plane mirror - eChalk Balloon blasting game - eChalk Height of mirror and image seen - NTNU Virtual image formation - eChalk Image formed by a plane mirror - NTNU Reflection and images from two mirrors at 90 degrees to each other - NTNU Law of Reflection - Crocodile Clip Presentation BBC KS3 Bitesize Revision: Reflection & Scattering

Concave mirrorA concave mirror is like the inside of a spoon.

centreof the mirror

concave mirror

principal axis O

focal length, f

F

principal focus

C

centre of curvature

radius of curvature, r

Mirror definitionsThe principal axis is a construction line that is perpendicular to and passes through the centre of the mirror, O.

The principal focus, F is the point through which all rays travelling parallel to the principal axis before reflection pass through or appear to come from after reflection.

The focal length, f is the distance from the centre of the mirror, O to the principal focus, F.

The centre of curvature, C is the centre of the circle of which the surface of the mirror is a part.

The radius of curvature, r is the distance along the principal axis between the centre of the mirror, O and the centre of curvature and equal to TWICE the focal length, f of the mirror.

Standard rays – concave mirror

F

concave mirror

principal axis

(a) Rays incident parallel to the principal axis pass through the principal focus after reflection.

(b) Rays passing through the centre of curvature before reflection are reflected back along their initial path.

Fprincipal axis

C

centre of curvature

(c) Rays striking the centre of the mirror are reflected as if the mirror was flat.

O

Concave mirror images1. Object more than the radius of curvature away from the mirror.

object

image FC O

Use: Satellite receiver (with microwaves)The image formed is:

Smaller than the object (diminished)Between F and CInverted (upside down)Real

2. Object between F and C

FC O

image

object

Use: Satellite transmitter (with microwaves)The image formed is:

Larger than the object (magnified)Beyond CInvertedReal

F

C

3. Object nearer than the principal focus

Use: Makeup and shaving mirrorsThe image formed is:

Larger than the objectOn the other side of the mirror from the objectUprightVirtual

image

object

observer

Standard rays – convex mirror

F

convex mirror

principal axis

(a) Rays incident parallel to the principal axis appear to come from the principal focus after reflection.

C

convex mirror

principal axis F

(b) Rays heading for the centre of curvature before reflection are reflected back along their initial paths.

(c) Rays striking the centre of the mirror are reflected as if the mirror was flat.

O

CF

Convex mirror imagesObjects at all distances from a convex mirror

object

observer

image

Use: Security mirrorsThe image formed is:

Smaller than the objectOn the other side of the mirror from the objectUprightVirtual

Convex mirrors give a wide field of view

Magnification

magnification = image height

object height

Question:

Calculate the magnification if a mirror produces an image of 40cm from an 8cm sized object.

magnification = image height / object height

= 40cm / 8cm

magnification = 5 x

Complete:

object height image height magnification

3 cm 24 cm

45 cm 3 x

20 cm 0.25 x

300 mm 0.10 m

Answers

3 x

5 cm

15 cm

8 x

Choose appropriate words to fill in the gaps below:

There are two types of curved mirror, __________ and convex. Concave mirrors look like the _____ of a spoon.

Concave mirrors __________ light parallel to the principal axis so that all the rays pass through the __________ focus. Makeup mirrors can be concave in order to provide a __________ view of the face.

Light is diverged by _________ mirrors. These always produce _________ images and are used to provide a wide field of view which is especially useful for ____ rear view mirrors.

convexprincipalconcavemagnified inside virtualconverge

WORD SELECTION:

car

convex

principal

concave

magnified

inside

virtual

converge

car

Simulations

Lens / mirror effect on a beam of light - NTNU

Tiger image formation by a plane or curved mirror - NTNU

Mirage of pig formed by a concave mirror - includes UTube clip - NTNU

Curved mirror images / ray diagrams - NTNU

Refraction

Refraction occurs when a wave changes speed as it passes from one region to another.

This speed change usually causes the wave to change direction.

Water waves slow down as they pass over from a deeper to a shallower region.

Light slows down as it passes from air into glass, perspex or water.

Refraction of light at a plane surface(a) Less to more optical dense transition (e.g. air to glass)

angle of incidence

normal

AIR GLASS

angle of refraction

Light bends TOWARDS the normal.

The angle of refraction is LESS than the angle of incidence.

(b) More to less optical dense transition (e.g. water to air)

angle of refraction

angle of incidence

normal

WATER AIR

Light bends AWAY FROM the normal.

The angle of refraction is GREATER than the angle of incidence.

Refraction experimentTypical results:

angle of incidence / °

angle of refraction / °

deviation / °

0 0 0

15 10 5

30 19 11

45 28 17

60 35 25

75 40 35

No deviation occurs when the angle of incidence is zero.

Increasing the angle of incidence increases the deviation.

object at the bottom of a pool

AIR

WATER

Why a pool appears shallow

image

normals

observer

Complete the paths of the RED light rays:

A

B

C

D

E

F

prism

Dispersion A prism splits the colours of white light into the spectrum.

This is called dispersion.

Violet light deviates the most,

red the least.

spectrum

whitelight

Choose appropriate words to fill in the gaps below:

Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes.

Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________.

Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________.

most speeddirectiondeviation coloursperspextowards

WORD SELECTION:

dispersion

most

speed

direction

deviation

colours

perspex

towards

dispersion

SimulationsRefraction - Powerpoint presentation by KT Light Refraction - Fendt Reflection & Refraction at a boundary - NTNU Refraction animation - NTNU - Does not show TIR effect Law of Refraction - Crocodile Clip Presentation Prism - non dispersive reflections and refractions - NTNU Prism/Lens - non dispersive refraction and reflections - NTNU BBC KS3 Bitesize Revision: Refraction Light moving from water to air - NTNU Where is the fish? - refraction by water - NTNU The appearance of an object under water / ray diagram - NTNU How a fish sees the world - NTNU Fibre optic reflection - NTNU BBC Bitesize Revision: Optical fibres Dispersion - Powerpoint presentation by KT Dispersion of light using a prism - NTNU - prism apex angle can be changed Prism showing light dispersion for different colours - Explore Science Dispersion - Crocodile Clip Presentation Sequential Puzzle on Colour Spectrum order- by KT  - Microsoft WORD Prism - multishape prism and single light ray - no extra reflections - netfirms BBC KS3 Bitesize Revision: Dispersion

Lenses

Converging lens

Converging lens with a parallel beam of light

focal length, f

centreof the lens

converging lens

principal axis O F

principal focus

With glass and plastic lenses a converging lens has a convex shape.

O

diverging lens

principal axis

Diverging lens

Diverging lens with a parallel beam of light

With glass and plastic lenses a diverging lens has a concave shape.

focal length, f

Fprincipal focus

Lens definitionsThe principal axis is a construction line that is perpendicular to and passes through the centre of the lens.

The principal focus, F is the point through which all rays travelling parallel to the principal axis before refraction pass through or appear to come from after refraction.

The focal length, f is the distance from the centre of the lens, O to the principal focus, F.

Standard rays – converging lens

F

principal focus

principal axis

(a) Rays incident parallel to the principal axis pass through the principal focus after refraction.

(b) Rays passing through the centre of the lens are not deviated.

centreof the lens

O

(c) Rays passing through the principal focus before refraction are refracted parallel to the principal axis.

Fprincipal axis F

The image is real (the light rays really go there).The image is inverted (it is upside down).The image is smaller than the object. (Diminished)

object

imageF

2F O F 2F

Converging lens images1. Object more than twice the focal length distant from a converging lens

Converging lens images1. Object more than twice the focal length distant from a converging lens

object

imageF

2F O F 2F

Uses: Camera and EyeThe image formed is:Smaller than the object (diminished)Between the F and 2FInverted (upside down)Real

• As above, you can see that the image is not the same as the object.The image is still real and inverted but it is now bigger than the object.

object

imageF2F

F 2F

2. Object between F and 2F

2. Object between F and 2F

object

imageF2F

F 2F

Use: ProjectorThe image formed is:Larger than the object (magnified)Beyond 2FInvertedReal

• The image is called virtual because the light rays never really go there(compare this with a real image).The image is called upright because it is the right way up

observer

objectimage

F F

3. Object nearer than the principal focus

observer

objectimage

F F

3. Object nearer than the principal focus

Uses: Magnifying glassThe image formed is:Larger than the objectOn the same side of the lens as the objectUprightVirtual

diverging lens

principal axisF

Standard rays – diverging lens

(a) Rays incident parallel to the principal axis appear to come from the principal focus after refraction.

(b) Rays passing through the centre of the lens are not deviated.

O

O F principal axis

(c) Rays heading for the principal focus before refraction are deviated parallel to the principal axis.

F

Diverging lens imagesObjects at all distances from a diverging lens

object

imageobserver

Use: Correction of short sightThe image formed is:

Smaller than the objectOn the same side of the lens as the objectUprightVirtual

The Camera

The camera uses a converging lens to produce an image on a light detecting surface such as a CCD (Charge Coupled Device) chip or photographic film.

In most cases the subject (object) is well more than twice the focal length of the lens away from the camera.

The image produced is therefore:

diminished, inverted and real.

F

2F

subject (object)

image produced on CCD or film

F

Structure of a simple camera

Focussing is achieved by moving the lens away from or towards the light detector. The further the subject is away from the camera the closer the lens is moved towards the CCD / film.

The amount of light reaching the light detector is controlled by adjusting the size of the ‘stop’ aperture and the length of time that the shutter opens.

lensaperture

shutter CCD or film

Choose appropriate words to fill in the gaps below:

Lenses work by the process of _________. A converging lens is made by using a _______ shaped piece of glass or perspex.

A magnifying glass requires the object to be placed _______ than its focal length. The image formed is _______.

A projection lens produces a __________ and inverted image whereas a camera lens normally produces a _________ image. In both cases the image is ______.

Diverging lenses are used to correct ______sight.

realshort refraction diminishednearerconvex virtual

WORD SELECTION:

magnified

real

short

refraction

diminished

nearer

convex

virtual

magnified

SimulationsGeometric Optics with Lenses - PhET - How does a lens form an image? See how light rays are refracted by a lens. Watch how the image changes when you adjust the focal length of the lens, move the object, move the lens, or move the screen. Prism/Lens - no dispersive refraction and reflections - NTNU Lens images / ray diagrams - NTNU How an image is formed by a convex lens / effect of stopping down lens - NTNU Lens / mirror effect on a beam of light - NTNU