1. Complete Physics pages for IGCSE 139 to 168
Light
Complete Physics pages for IGCSE 139 to 168

2. IGCSE SYLLABUS OPTICS 3.2 (e) Electromagnetic spectrum Lens Core
3.2 (c) Thin converging lens
Core
• Describe the action of a thin converging lens on a
beam of light
• Use the term principal focus and focal length
• Draw ray diagrams to illustrate the formation of a
real image by a single lens
3.2 (d) Dispersion of light
• Give a qualitative account of the dispersion of light
as shown by the action on light of a glass prism
3.2 (e) Electromagnetic spectrum
Core
• Describe the main features of the
electromagnetic 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

3. Reflection

4. Law of Reflection
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.
mirror
angle of
incidence, i
incident ray
normal
reflected ray
angle of
reflection, r

5. The image formed by a plane mirror
object
image
plane mirror
construction lines (virtual light rays)
normals
The image produced by the plane mirror is:
The same size as the object
The same distance behind the mirror as the object is in front
Upright (the same way up as the object)
Back-to-front compared with the object (lateral inversion)
Virtual

6. 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

7. 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.
reflection
equal
normal
ninety
virtual
real
size
WORD SELECTION:
normal
virtual
size
reflection
real
equal
ninety

8. Simulations Optical 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
Reflection & Scattering

9. A concave mirror is like the inside of a spoon.
principal focus
principal axis O C
centre of curvature
radius of curvature, r
focal length, f
centre
of the mirror

10. Mirror definitions
The 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.

11. Standard rays – concave mirror
(a) Rays incident parallel to the principal axis pass through the principal focus after reflection.
concave mirror F
principal axis

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

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

14. Concave mirror images Use: Satellite receiver (with microwaves)
1. Object more than the radius of curvature away from the mirror. F C O
object
image
Use: Satellite receiver (with microwaves)
The image formed is:
Smaller than the object (diminished)
Between F and C
Inverted (upside down)
Real

15. Use: Satellite transmitter (with microwaves) The image formed is:
2. Object between F and C F C O
object
image
Use: Satellite transmitter (with microwaves)
The image formed is:
Larger than the object (magnified)
Beyond C
Inverted
Real

16. Use: Makeup and shaving mirrors The image formed is:
3. Object nearer than the principal focus F C
object
image
observer
Use: Makeup and shaving mirrors
The image formed is:
Larger than the object
On the other side of the mirror from the object
Upright
Virtual

17. Standard rays – convex mirror
(a) Rays incident parallel to the principal axis appear to come from the principal focus after reflection.
convex mirror
principal axis F

18. (b) Rays heading for the centre of curvature before reflection are reflected back along their initial paths. C
convex mirror
principal axis F

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

20. Convex mirror images Use: Security mirrors The image formed is:
Objects at all distances from a convex mirror C F
object
image
observer
Use: Security mirrors
The image formed is:
Smaller than the object
On the other side of the mirror from the object
Upright
Virtual
Convex mirrors give a wide field of view

21. 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

22. Complete: Answers 8 x 15 cm 5 cm 3 x object height image height
magnification
3 cm
24 cm
45 cm
3 x
20 cm
0.25 x
300 mm
0.10 m
8 x
15 cm
5 cm
3 x

23. 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.
concave
inside
converge
principal
virtual
convex
magnified
car
WORD SELECTION:
converge
magnified
concave
principal
car
convex
inside
virtual

24. 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

25. Refraction

26. 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.

27. 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.

28. Light bends AWAY FROM the normal.
(b) More to less optical dense transition (e.g. water to air)
angle of incidence
normal
WATER AIR
angle of refraction
Light bends AWAY FROM the normal.
The angle of refraction is GREATER than the angle of incidence.

29. Refraction experiment
Typical results:
angle of incidence / °
angle of refraction / °
deviation / ° 15 10 5 30 19 11 45 28 17 60 35 25 75 40
No deviation occurs when the angle of incidence is zero.
Increasing the angle of incidence increases the deviation.

30. Why a pool appears shallow
observer
normals
object at the bottom of a pool
AIR
WATER
image

31. Complete the paths of the RED light rays:B C D E F

32. Dispersion
A prism splits the colours of white light into the spectrum.
This is called dispersion.
Violet light deviates the most,
red the least.
prism
white
light
spectrum

33. 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 _________.
speed
direction
towards
perspex
deviation
colours
most
dispersion
WORD SELECTION:
towards
deviation
most
dispersion
direction
perspex
speed
colours

34. Simulations Refraction - 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
Dispersion

35. Lenses

36. Converging lens
With glass and plastic lenses a converging lens has a convex shape.
Converging lens with a parallel beam of light F
principal focus
centre
of the lens
converging lens
principal axis O
focal length, f

37. Diverging lens
With glass and plastic lenses a diverging lens has a concave shape.
Diverging lens with a parallel beam of light O
diverging lens
principal axis F
principal focus
focal length, f

38. Lens definitions
The 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.

39. Standard rays – converging lens
(a) Rays incident parallel to the principal axis pass through the principal focus after refraction. F
principal focus
principal axis

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

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

42. Converging lens images
1. Object more than twice the focal length distant from a converging lens F 2F O
object
image
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)

43. Converging lens images
1. Object more than twice the focal length distant from a converging lens F 2F O
object
image
Uses: Camera and Eye
The image formed is:
Smaller than the object
(diminished)
Between the F and 2F
Inverted (upside down)
Real

44. 2. Object between F and 2F F 2F
object
image
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.

45. 2. Object between F and 2F Use: Projector The image formed is:
Larger than the object (magnified)
Beyond 2F
Inverted
Real

46. 3. Object nearer than the principal focus
image
object
observer
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

47. 3. Object nearer than the principal focus
image
object
observer
Uses:
Magnifying glass
The image formed is:
Larger than the object
On the same side of the lens as the object
Upright
Virtual

48. Standard rays – diverging lens
(a) Rays incident parallel to the principal axis appear to come from the principal focus after refraction.
diverging lens
principal axis F

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

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

51. Diverging lens images Use: Correction of short sight
Objects at all distances from a diverging lens F
observer
object
image
Use: Correction of short sight
The image formed is:
Smaller than the object
On the same side of the lens as the object
Upright
Virtual

52. 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.

53. The image produced is therefore: diminished, inverted and real.
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

54. Structure of a simple camera
CCD or film
shutter
lens
aperture
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.

55. 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.
refraction
convex
nearer
virtual
magnified
diminished
real
short
WORD SELECTION:
convex
magnified
short
refraction
nearer
real
virtual
diminished

56. Simulations
Geometric 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