lesson 5.1 light
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LESSON 1 Understandingreflection oflight
Introduction :
Light is a form energy that enables usto see.Light is emitted or given by hotobjects, like a candle flame, or theglowing filament of light bulb ,or thesun.An object which produces its own lightis said to be luminous.Most of the objects we can see do notproduce their own light they are non-luminous. What we can see is the lightis reflected off them.Light seems to travel in straight lines.A picture of light as rays traveling instraight lines from a luminous sourceor bounching off objects, or beingabsorbed or transmitted by them, isvery useful one. We can think of lightray as a very thin beam of light. A realbeam contains many rays. But bydrawing just a few of them we can geta clear idea of what happening to thewhole beam.When rays of light strike any surfacethe rays are reflected , unless thesurface is black, when they areabsorbed.
The reflection depend on how smooththe surfaces are. Good mirrors reflectwell over 90% of the light that reachesthem, with only a small amount beingabsorbed.
The surface of most objects are veryirregular. Light hitting this sort ofsurface is scattered in all directions.
This is called diffuse reflection.On the other hand, the surface of apolished sheet of glass or metal canbe made extremely smooth. Thereflection from this is the regular.
Common terminology of reflectionof light on a plane mirror
Normal : A line at rightangles to the
mirrors surface.
Incident ray : A ray of light thatis directed
onto the mirrors
surface.
Reflected ray : A ray that isreflected by the
mirrors surface.
Angle of : The angle betweenthe incidentincidence ray and thenormal
Angle of : The angle betweenthe reflectedreflection ray and thenormal
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AO : Incident rayON : NormalOB : Reflected rayi : Angle of incidencer : Angle of reflection
Laws of reflection
The first law : The angle of incidence is equalto the angle of
reflection.
The second law: The incident ray,the normal and
the reflected ray all liein the
same plane.
Characteristics of an ImageFormed by a Plane Mirror bydrawing Ray Diagram
(1) Draw two normals at thesurface of the mirror.
(2) Draw two incident rays andeach of them
is directed to the normals.(3) Measure the angles of
incidencerespectively by using
protractor.(4) By using the Laws of reflection
(The angle of incidence isequal to the angle of reflectiondraw two reflected rays infront the mirror.
(5) Extrapolate the reflected raysbehind the with dotted line.
The point of intersection is thelocation of the image .
The characteristics of an image
formed by a plane mirror are :(1) The size of the image= the
size of the object(2) The distance of image = the
distance of object(3) Virtual image (appears to be
behind the mirror)(4) Laterally inverted (left and
right are interchanged)(5) Upright
Example 1
A boy is at a distance 5 m from aplane mirror. The mirror is movedaway from the boy by 2 m. What is thedistance between the boy and hisimage.
Solution
Example 2
The diagram shows two plane mirrorsset an angle 100o to each other. A raystrikes one of the mirrors as shown atangle of incidence of 45o . Redraw andcomplete the diagram showing thepath of the ray and calculate theangle of reflection at which it leavesthe second mirrors.
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Solution
Common terminology of reflectionof lighton a curved mirror
Concave mirror
Convex mirror
Pole of mirror , P : Thecentre point on
the curvedmirror
Centre of curvature , C : Thegeometric centre
of a hollowsphere ofwhich theconcave orconvex mirroris a part.
Principal axis ,PFC : A linewhich passes
through the
centre ofcurvature ,Cand the pole ofa curvedmirror, P
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Principal focus , F : A pointthrough
which all raystravelingparallel to theprincipal axisconverge to orappear todiverge fromafter reflectionby the mirror.
Radius of curvature, r :Distance between
the pole, P andthe
centre ofcurvature, C.
Focal length , f : Thedistance
between theprincipal
focus ,F andthe pole of thecurved mirror,P.
Relationship between f and r
Relationship curvature of mirrorand f
The more curved the mirror , theshorter its focal length, f.
Characteristics of an ImageFormed by a curved Mirror bydrawing Ray Diagram
We must draw any two of three rays todetermine the position of the image.
There are three
Ray 1
For concave mirror :
A ray parallel to the principalaxis is
reflected to pass through F.
For convex mirror :
A ray parallel to the principalaxis is
reflected as if it came from F. F
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r =
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Ray 2
For concave mirror :
A ray through F is reflectedparallel to
the principal axis.
For convex mirror :
A ray towards F is reflected
parallel to the principal axis.
Ray 3
For concave mirror :
A ray through C is reflectedback along its own path.
For convex mirror :
A ray towards C is reflectedback along its own path.
Image formed by a Concave Mirror
A u < f ( Object between F and P)
Characteristics of image :
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Application :
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B u = f ( Object, O is at F )
Characteristics of image :
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Application :
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C f < u < 2f or f < u < r( Object O is between F and C.
Characteristics of image :
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Application :
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D u = 2f or u = r ( Object ,O is atC)
Characteristics of image :
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Application :
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E u > 2f or u > r ( Object, O isbeyond C )
Characteristics of image :
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Application :
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F u = ( Object ,O very far fromthe lens)
OR
Characteristics of image :
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Application :
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The distanceobject, u
Characteristics ofimage
u < fu = f
f 2f
u =
Conclusion :
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Image formed by a Convex Mirror
A u < f ( Object between F and P
)
Characteristics of image :
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Application :
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B f < u < 2f or f < u < r( Object O is between F and C)
Characteristics of image :
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Application :
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The objectdistance, u
Characteristics ofimage
u < ff
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(1) Convex mirror are mounted ata sharp corner of a road so thatthe drivers are able to seeoncoming cars on the blind sideof the corner.
(2) Convex mirror hung on the
corner of ceilings insupermarkets serve to providea wide field of view activitieshappening in the shoppingarea.
TUTORIAL 1
1 Thediagram shows a single ray oflight being directed at a planemirror.
What are the angles of incidenceand reflection?
Angle of Angle of incidence reflection
A 40o 40o
B 40o 50oC 50o 40o
D 50o 50o
2 The diagram shows amirror istitled at an angle of 40o to thebench. A ray of light is directed so
that it hits the mirror at an angleof 10o to the surface of the mirror.
What is the angle of reflection?
A 10o B 20o
C 40o D 500
E 80o
3 The diagram shows an image isformed by a mirror.At which point the object is
placed?
4 The diagram shows a ray of lightfrom a small bulb strikes a planemirror.
Where is the image of the bulbformed and its characteristic?
A At P and virtualB At Q and realC At R and virtual
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5 The diagram showsa girl stands atthe side of a boy in front of amirror. They are both the samedistance from the mirror.Where does the girl see the boysimage?
6 The diagram shows four lamps infront of a plane mirror. The cardprevents the observer at M fromseeing the lamp directly.Which lamps image can be seen?
7 The diagram shows a wall clock .
What is the time shown by theimage of the wall clock in a planemirror as seen by a student.
A 01:40 B 04:50C 07:10 D 10:20
8 Diagram shows a student lookingat a plane mirror.
The image observed by thestudent is
9 Diagram shows an object O at adistance in front of a plane mirror
CD. The plane mirror is moved bya distance 4 cm at LM.
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What is the distance betweenobject and the
new image of the object?
A 12 cm B 16 cmC 24 cm D 32 cmE 64 cm
10 A boy stands in front of a planemirror a distance 5 m . When theboy moves toward the mirror by 2m , what is the distance betweenthe boy and his new image?
A 2m B 4mC 6m D 8 mE 10 m
11 The diagram shows a patientsitting in a testing room in front ofa chart.
What is the distance of image ofthe chart from the patient?
A 3 m B 4 mC 7 m D 10 mE 14 m
12 The diagram shows a half-metrerule has a hole at 15 cm markand is hung vertically. A planemirror is placed in front of theruler.
What is the region of the image ofthe ruler seeing by an observer ,
when the observer look throughthe hole?
A between 0 cm mark and 30cm mark
B between 0 cm mark and 45cm mark
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C between 5 cm mark and 35cm mark
D between 10 cm mark and 25cm mark
E between 0 cm mark and 50cm mark
13 The diagram showsa point object,O is positioned in front of a planemirror .
In order to view the image of O inthe mirror, you must be betweenthe points
A U and V B U and YC W and Y D Y and Z
14 The figure shows a light ray beingincident to the surface of planemirror MN. The mirror is thenrotated anti-clock wise through anangle 10o.
What is the new angle of reflectionof the light ray?
A 300 B 150
C 100 D 50
15 The diagram shows five light raysA, B,C ,D and E from an object, O andis reflected by a concave mirror toform an image I. Which of the light
ray is not correct?
16 The characteristics of an image ,formed by a convex mirror for allpositions of the object are
A diminished, real and invertedB magnified , real, and uprightC diminished ,virtual and uprightD magnified , virtual and
inverted
17 A concave mirror has a focallength 20 cm. What happen to thesize of image when an object isplaced at a distance of 40 cm infront of the mirror?
A diminished B magnifiedC same size of object
18 An object is placed at a distance18 cm from a concave mirror offocal length 10 cm. What is thecharacteristics of the imageformed?
A real , inverted and magnifiedB virtual , inverted and
magnifiedC real , upright and diminishedD virtual , upright and
diminished
19 A concave mirror has a focallength 15 cm. The image formedby the mirror is virtual, upright
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and magnified. What is the objectdistance?
A 8 cm B 15 cmC 20 cm D 30 cmE 40 cm
20 The figure shows a candle placedin front of aconcave mirror of focal length, f.
The image formed is
A real, upright and magnifiedB real, inverted and diminishedC virtual, inverted and magnifiedD virtual, upright and diminished
21 Which of the following states theright reason for replacing a planemirror are used as rear- viewmirrors in motor vehicles with aconvex mirror ?
A To shine the objectB To widen the field of viewC To produce a brighter image
D To produce a sharper image
22 (a) Figure shows the positions ofan object O
and the image I of the objectsis formed by a plane mirror M.
Figure(a)
Complete figure (a) showingthe path of the ray, includingthe in Incident ray, reflectedray and normal.
(b) Figure(b) shows two planemirrors JK andLM which have been placed ata right angles to each other.
Complete figure (b) to show theray diagram path until it emergesfrom the both the mirrors.
(c) Write down the word "REFLECTION " when the wordis seen by a person in a planemirror.
23 Figure (a) shows the positions ofthe eye of a car driver , the wingmirror of the car and the front of alorry.
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Figure (a)
(a) Draw rays to indicate the partof the front of the lorry whichcan be seen in the mirror bythe car driver.
(b) Figure (b) shows a planemirror and a curved mirror.
The mirrors have the samesize. The wing mirror of a caris often curved .
Draw rays from observer tothe plane mirror and thecurved mirror to explain howthe curved mirror give moreadvantage than the plane isused as a wing mirror of a car.
Figure (b)
24 An object is placed at a distance ,ufrom a concave mirror, M. Thetable shows the characteristics of
image , when the distance u isvaried.
Objectdistan
ce ,u /cm
ImageCharacteristics
10
20
30 Real, inverted, samesize as the object
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(a) Name the light phenomenonthat is involved to form theimage by the mirror..................................................
...................(b) Using the information in the
table, determine the focallength of the mirror M.
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(c) Give the reason for youranswer in (b).
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(d) Complete the table(e) An object is placed at a
distance 50 cm from theconcave mirror M.(i) Draw a ray diagram in the
space below to show theformation of the image.
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(ii) What is the characteristicsof the image?
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(iii) What is the characteristicsof the image when theconcave mirror M is
replaced by a convexmirror of focal length sameas the mirror M.
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25 Figures(a) and (b) show theparallel rays of light directed
toward the curved mirrors, J and K.CP is the radius of curvature, and Fis the focal point of the mirrors.
(a) What is meant by focal length.(b) Using Figure(a) and (b) ,compare the
curvature of the mirrors andtheir focal lengths.
(c) Figure(c) shows a crosssection of a parabolic reflectorused in a car headlamp.
The headlamp has two bulbs ,X and Y. Bulb X is located atthe focal point of the reflectorwhile bulb Y is above bulb X.
Figure(c)
Explain what happens to thelight rays from the bulb whenonly:(i) bulb X is switched on(ii) bulb Y is switched on
You may use diagrams toillustrate your answers.
(d) You are given two pieces ofplanemirror measuring 5 cm x 6 cmto make a periscope.(i) Using these mirrors and
other materials, describehow you would make theperiscope.
(ii) Explain why the image
observed through theperiscope using planemirrors are not very clear.
(iii) Suggest modification thatcan be made to theperiscope to produceclearer and bigger images.
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