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AQA GCSE Specification
OPTICS13.4 What do mirrors and lenses do to light?
Using skills, knowledge and understanding of how science works:• to construct ray diagrams to show the formation of images by plane, convex and concave mirrors• to construct ray diagrams to show the formation of images by diverging lenses and converging lenses• to explain the use of a converging lens as a magnifying glass and in a camera• to calculate the magnification produced by a lens or mirror using the formula:magnification = image height / object height
Skills, knowledge and understanding of how science works set in the context of:
• The normal is a construction-line perpendicular to the reflecting/refracting surface at the point of incidence.
• The angle of incidence is equal to the angle of reflection.
• The nature of an image is defined by its size relative to the object, whether it is upright or inverted relative to the object and whether it is real or virtual.
• The nature of the image produced by a plane mirror.• The nature of the image produced by a convex mirror.• The nature of the image produced by a concave mirror
for an object placed at different distances from the lens.
• Refraction at an interface.• Refraction by a prism.• The nature of the image produced by a diverging lens.• The nature of the image produced by a converging
lens for an object placed at different distances from the lens.
• The use of a converging lens in a camera to produce an image of an object on a detecting device (eg film).
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
ReflectionNotes questions from pages 234 & 235
1. Explain how convex and concave mirrors differ from plane mirrors.
2. Copy Figure 2 on page 235 and state what is meant by (a) the law of reflection and (b) the ‘normal’ line.
3. Copy Figure 1(b) on page 234 and use it to explain what is meant by a ‘virtual’ image.
4. What is a real image? Give two examples.5. Copy and answer questions (a), (b) and (c) on pages 234
and 235. 6. Copy the ‘Key points’ table on page 235.7. Answer the summary questions on page 235.
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
convex mirror
Convex mirrorA convex mirror is like the outside of a spoon.
focal length, f
principal axis O
F
principal focus
centre of curvature
C
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
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.
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 heightQuestion:
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
SimulationsLens / 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
Curved mirrors Notes questions from pages 236 & 237
1. By referring to a copy of Figure 2 on page 236 define what is meant by (a) the principle focus and (b) the focal length of a concave mirror.
2. Under headings ‘Object beyond principle focus’ and ‘Object between focal point and the mirror’ copy Figures 3 and 4 on page 237 and describe the type of image formed in each case.
3. Define what is meant by magnification.4. Copy Figure 5a on page 237 and describe the type of image
formed.5. Copy the Key Points on page 237.6. Copy and answer questions (b) and (c) on pages 236 and 237.7. Answer the summary questions on page 237.
Curved mirrors ANSWERS
In text questions:(b) 2.5 (c) Upright, magnified, virtual.
Summary questions:1. (a) Real, inverted, in front. (b) Virtual, upright, behind.
(b) The image is virtual, upright and twice as large as the object.
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.
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
Refraction Notes questions from pages 238 & 239
1. Copy Figure 1 on page 238 and use it to explain what is meant by refraction. Also copy the two bullet points to the right of Figure 1.
2. Copy Figure 2 on page 238 and use it to explain the cause of refraction.3. Explain how the speed of light is related to the refraction of light.4. Draw a diagram and use it to explain why a swimming pool appears to be
shallower than it actually is.5. Explain, with the aid of a diagram, how a prism can be used to split white
light into its component colours. How does this experiment indicate that red light travels faster than blue light through glass?
6. Copy and answer questions (a), (b), (c) and (d) on pages 238 and 239.7. Copy the ‘Key points’ table on page 239.8. Answer the summary questions on page 239.
Refraction ANSWERS
In text questions:(a) Yes (b) The top part moves faster than the lower part of the wave so the wave topples
over at the top.(c) The same.(d) (i) Slower (ii) Faster
Summary questions:1. (a) Decreases, towards. (b) Increases, away from. (c) Decreases
(b) White light consists of all the colours of the spectrum. The beam is split into these colours because each colour is refracted slightly differently, owing to their different speeds in glass.
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.
(c) Rays passing through the principal focus before refraction are refracted parallel to the principal axis.
Fprincipal axis F
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
2. Object between F and 2F
object
imageF2F
F 2F
Use: ProjectorThe image formed is:Larger than the object (magnified)Beyond 2FInvertedReal
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.
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
Lenses Notes questions from pages 240 & 241
also uses pages 252 and 2531. Copy the diagram for Exam Style Question 2(a) on page 252 and
use it to explain what is meant by the principal focus is of a converging lens.
2. Copy the diagram for Exam Style Question 2(b) on page 253 and use it to explain what is meant by the principal focus is of a diverging lens.
3. For both types of lens define what is meant by focal length.4. Describe, with the aid of diagrams, how the image formed by a
converging lens changes as the distance to the object decreases.5. Copy and answer questions (a), (b) and (c) on pages 240 and
241.6. Copy the ‘Key points’ table on page 241.7. Answer the summary questions on page 241.
Lenses ANSWERS
In text questions:(a) A 5 cm focal length lens (b) Inverted(c) To make it appear
much larger so any flaws can be seen.
Summary questions:1. (a) Converging, real.
(b) Diverging, virtual. 2. (a) Upright, enlarged and
virtual. (b) (i) Inverted, enlarged
and virtual. (ii) The slide must be
moved towards the screen.
(c) The image real, inverted and enlarged. Magnification = 3.
Using lenses Notes questions from pages 242 & 243
1. Copy Figure 1 on page 242 with all of the labels and explain the rules for drawing the three rays shown.
2. Draw Figure 2 on page 242 and explain how the lens of a camera has to be adjusted to cope with a range of object distances.
3. Explain with the aid of Figure 3 on page 243 how a magnifying glass produces an image.
4. Explain with the aid of Figure 4 on page 243 how a concave lens produces an image.
5. Copy and answer questions (a), (b), (c) and (d) on pages 242 and 243.
6. Copy the ‘Key points’ table on page 243.7. Answer the summary questions on page 243.
Using lenses ANSWERS
In text questions: Summary questions:
(b) Towards the object.
(c) To inspect objects in detail.
(d) The image is always smaller than the object.
Light and sound issues Notes questions from pages 250 & 251
1. Answer questions 3(a) and 3(b) and on page 251.
Light and sound issues ANSWERS
3. (a) About 166 hours.
(b) The data is permanently stored on the disc.