5.1 reflection of light

8/12/2019 5.1 Reflection of Light

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5.1 Reflection of Light

Reflection of Light on a plane mirror

The phenomenon of reflection of light obeys the laws of

reflection:

i) the incident ray, the reflected ray, and the normal all lieon the same plane

ii) the angle of incidence is equal to the angle of reflection,

i.e. i= r


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Image form on plane mirror, convex mirror and concave

mirror is due to the reflection of light from the surface of

mirror to our eyes

The characteristics of the image formed by aplane mirror:Virtual, Upright, Laterally inverted, same size as the object

Distance of the image from the mirror is the sameas the

distanceof the object from the mirror


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What is real and virtual image?

An image which can be formed on a screen is a real

image, whereas an image which cannot be form on to a

screen is called virtual image

A real image is formed where rays actually meet

Whereas a virtual image is formed by the apparent

intersection of rays


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Ray Diagrams for Plane Mirrors

1. The reflected rays

obeys the laws ofreflection

2. Extend the reflected

rays backwards to meet

at I. This is the position

of the image

3. The rays behind the mirror do not exit. They are

virtual rays and are represented by dotted lines


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Three steps to draw the position of the image

i). Locate the position of the image

ii). Locate the reflected rays

iii). Locate the incident rays


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Example:

Figure below shows an ambulance car. Why the word

ambulance is purposely inverted laterally on an

ambulance car ?

Answer:

.

Images seen through the real mirror of a car is laterally

inverted


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The diagram below shows student looking into a plane

mirror

The image as observed by the student is

Answers:-

C(The image formed

is laterally inverted)


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Experiment to verify angle of incidence is the same as

the angle of reflection


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Experiment to determine the position of an image in a plane

mirror and to show that the image distance is equal to the

object distance

Ray box is position at a distance do. The image of the bulb

in the ray box is view from position P3. Two pins, P1and

P2are erected in front of the mirror such that they are in

line with the image of the object (state of non- parallax).

Likewise for Q3.P1P2and Q1Q2respectively are extended


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Example:

A candle is placed at a distance x in front of a plane

mirror. What is the distance moved by the image if

a). the candle moves toward the mirror by 1 meterb). the plane mirror is moved towards the candle by 1

meter


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Answer:

Distance moved by the image of candle, d = 1 m

Distance moved byimage, d

= 2x-2(x-1)

= 2m

[When a mirror is moved towards

the viewer at a velocity of v, the

image will move with a velocityof 2v towards the viewer]


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The figure (a) shows a light ray reflected by a plane

mirror. What is the angle of deviation of the reflected ray

in figure (b), if the mirror is rotated 10clockwise about

point O ?

Answer:

Before rotation:Angle of incidence = Angle of reflection

=30

angle POQ = 60


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After rotation of 10:

Angle of incidence =40

angle reflection =40

Angle POQ=80

angle of deviation (QOQ) = angle POQ- angle POQ

= 80-60

= 20[ 2 x angle of rotation of the mirror]

[plane mirror and normal line will rotate but the incidence

angle is fixed]


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Jennifer is standing at a distance of 10 m from a plane

mirror. If Jennifer and the mirror move to the left by 1 m

and 3 m respectively, what is the reduction in the

distance between Jennifer and her image?

Initially, the distance between Jennifer and her image

=10 m + 10 m = 20 m

After they moved, the distance = 10+1-3 = 8 m

Distance between she and the new image position

= 2 x 8 = 16 m. Reduction = 20-16 = 4 m


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Draw and show the formation of the image I

Answer:


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Common Terminology of curved mirrors

1. Centre of curvature, C: the centre of the sphere that

forms the curved mirror

2. Pole of mirror, P : The centre point of the curved mirror


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3. Principal axis: A line which passes through the centre of

curvature, Cand the pole of a curved mirror, P.

4. Radius of curvature, r(=CP): Distance between the pole, P

and the centre and the centre of curvature, C.5. Principal focus, F: A point through which all rays

travelling parallel to the principal axis converge to or appear

to diverge from after reflection by the mirror.

6. Focal length, f : The distance between the principal focus,

F and the pole of the curved mirror, P.

7. Aperture of mirror: The portion of the surface of the mirror

that reflects light.8. Object distance, u : Distance of object from the pole of the

mirror, P

9. Image distance, v : Distance of image from the pole of the

mirror, P.


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Concave mirror

Rays travelling parallel to the principal axis will converge

to a point, called the real principal focus, F,ontheprincipal axis

Distance FP = Focal length, f


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Convex Mirror

Rays travelling parallel to the principal axis appear todiverge from a point behind the mirror.

This point is called the virtual principal focus

l i hi b h di f C d h


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Relationship between the Radius of Curvature, r and the

Focal Length, f

AM, parallel to and near principal axis, AM is reflected at

M, where CM is the normal.

According to the laws of reflection: Angle i= Angle r.

AMC =

MCF =

, FCM is isosceles triangle andMF = CF. Ray is parallel and M is very near to P

hence by close approximation FM FP CF= FP

Thus for concave mirror CF=FP = 1/2 PC

That is :- Focal length, f = 1/2 Radius of curvature, r


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Determine the Position and Characteristics of an Image

with a Ray Diagram

Concave Mirror

1. A ray parallel to the

principal axis is reflected

to pass through F


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2. A ray through F is

reflected parallel to the

principal axis

3. A ray through C is

reflected back along its

own path.


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Convex Mirror

1. A ray parallel to the

principal axis is reflectedas if it came from F

2. A ray towards F is

reflected parallel to theprincipal axis


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St F D i R Di


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Steps For Drawing Ray Diagram

1. Draw a horizontal line to represent the principal axis

and a curve line to represent a curve mirror. A dotted

vertical line is drawn at the pole of the curve mirror torepresent the construction line

2. Mark the position of F and C on the principal axis.Make sure CF=FP

3. Draw an arrow as object standing upright on the

principal axis at a given distance from the mirror. This is

the object distance, u


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4. Draw rays coming from the top of the object using

any 2 of the 3 construction rules {a. Ray that is parallel

to the axis, b. Ray passes through the focus, F, c. Ray

passes through the centre of curvature, C}

5. The top of the images is

where the two reflected rays

meet again after reflection. Use

arrows to indicate the direction

of the rays

6. Draw the image as a vertical arrow with its tail on the

principal axis and the head at the point where the two

reflected rays meet. The distance between the image and

the mirror is the image distance, v.


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Characteristic of the image formed:

a). real or virtual

b). inverted or uprightc). magnified, diminished or of the same size.

Dotted lines are used for extending rays in backwards

behind the mirror to locate the virtual image.

A virtual image is also drawn using dotted line with an

arrow


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Images Formed by a Concave Mirror

a). Object is between F and Pvirtual

uprightmagnified

behind the mirror

Application: Make-up

mirror, shaving mirror

b). Object is at F virtual

upright

magnifiedat infinity

Application: Reflectorto

produce aparallel beam

of light rays (spot light)


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c). Object between F and C

real

inverted

magnified

Beyond point C

d). Object at CReal

inverted

same size as object

At C

Application:- Reflector in

a projector


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e). Object is beyond C

f). Object at infinity

real

inverted

diminished in sizebetween C and F

real

inverted

diminished in size

at FApplication:-Reflecting

telescope(To form the first

image of a distant object)

Image Formed by a Convex Mirror


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Image Formed by a Convex Mirror

a). Object at a point beyond F

b). Object at a point before F

c) Object at infinity


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c). Object at infinity

The characteristic of an image, formed by a convex

mirror are:

a). virtualb). upright

c). diminished in size

d). the image is seen at a distance v f behind the

mirror for all positions of the object. (v is negative)


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3. Rear View Mirrors of Cars

Reflection of light on Curved MirrorsConcave mirror with long focal

lengths can be used as shaving mirrors

or make-up mirrors and as they

produced magnified and upright

images.

Concave mirror also used by dentist


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Convex mirror are used as rear-view mirrors in

motor vehicle.

A convex mirror has a wider view than a plane mirror. Itis also used as shop security mirrors.


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Example

You are given 3 mirrors- a plane mirror, a concave

mirror, and a convex mirror. Which mirror can be used

to ignite a match head? Explain your choice

Answer:

Sunlight will be convergedat the focus point, when the

match head is placed at the

focus of the concave mirror

it will get heated up and

ignited.


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Explain the difference between the images formed

a). Holding a shiny spoon with its back facing your eyes at

a distance of approximately half a metre from your eyesb)holding the other side of the spoon facing you

Answer:

For the back of the spoon image is upright for the otherside the image is inverted

(Back of the spoon is similar to a convex mirror)


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The radius of curvature of a concave mirror is 20 cm. At

what distance must an object be placed to produce

a). an image of the same size?

b). an image at infinity?

Answer:

a). v = 20 cm

b). v = 10 cm


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Based on the diagram below:

a). Draw the reflected rays of P, Q, and

b). State the nature of the image formed

c). What will happen to the image if the object is movedaway from the mirror?

Answer:

b). The image is real, inverted and magnified.

c). The image moves towards mirror and become smaller

and smaller

a)

B d th di b l


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Based on the diagram below:

a). draw the reflected rays

b). Where is the image located?

c). What is the characteristic of the image?

Answer:

b). Image is located at F

c). Image is virtual, upright and diminished

a)

5.1 reflection of light

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