Lesson 5.1 Light

8/3/2019 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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Ray 2


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


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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B u = f ( Object, O is at F )


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C f < u < 2f or f < u < r( Object O is between F and C.


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D u = 2f or u = r ( Object ,O is atC)


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E u > 2f or u > r ( Object, O isbeyond C )


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F u = ( Object ,O very far fromthe lens)

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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

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B f < u < 2f or f < u < r( Object O is between F and C)


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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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Lesson 5.1 Light