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High School. Optics. PHYSICS. InClass. by SSL Technologies. with S. Lancione. Exercise-52. Lenses. Part 1 /2. Concave. Convex. Lenses. PART-1 /2. LENSES Whereas mirrors produce images by reflection, lenses produce

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High School

Optics

PHYSICS

InClass

by SSL Technologies

with S. Lancione

Exercise-52

Lenses

Part 1 /2

Concave

Convex

Lenses

PART-1 /2

LENSES

Whereas mirrors produce images by reflection, lenses produce

images by refraction. Depending upon their “thickness”, lensesare classified as “thick” or “thin”. In this course, we will onlyconsider “thin” lenses. In effect, therefore, we disregard thethickness of the lens assuming it to be a line.

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Lenses

PART-1

There are two main types of lenses called convex (also knownas converging) and concave(also known as diverging).

Convex lenses are thicker in the middle than at the edges while

concave lenses are thicker at the edges than in the middle.

As illustrated below, both convex and concave lenses each have

three variations.

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Convex Lens

Concave Lens

Lenses

PART-1

When representing “thin” lenses in a diagram, it is sometimes more

convenient to simply draw an arrow rather than the actual lens.

Using this method, an arrow is used to represent the lens as

illustrated below.

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REMEMBER

Convex lenses are thicker in the middle and thus converge light rays.

Concave lenses are thinner in the middle and thus diverge light rays.

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Lenses

PART-1

Just as with concave mirrors, the characteristics of the image

formed by a converging lens depend upon the location of the object.

There are six "strategic" locations where an object may be placed.

For each location, the image will be formed at a different place and

with different characteristics. We will illustrate the six different

locations and label them as CASE-1 to CASE-6.

Case-1: Object at infinity

Case-2: Object just beyond 2 F’

Case-3: Object at 2F’

Case-4: Object between 2F’ and F’

Case-5: Object at F’

Case-6: Object within focal length (f)

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Convex (converging)

Lenses

Infinity simply means

“far away”.

CASE-1: Object at “infinity”

No image

Object

NOTE

Since the object is at “infinity”,

all rays arrive parallel.

No image formed(All rays pass through F)

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Note-1

A ray that comes parallel

is refracted through F.

Note-2

A ray that goes through the

vertex goes right through.

Note-3

A ray that goes through F’

is refracted parallel.

This ray is extrain locating theimage.

Convex (converging)

Lenses

NOTE

In order to establishan image point, allwe need are twointersecting rays.

CASE-2: Object just beyond 2F’

Object

Image

Image is real (formed by refracted rays)

Inverted (upside down)

Reduced (smaller than object)

Located between F and 2F

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This ray is extra.

Convex (converging)

Lenses

Again:

In order to establishan image point, allwe need are twointersecting rays.

CASE-3: Object at 2F’

Image

Object

Image is real (formed by refracted rays)

Inverted (upside down)

Same size as object

Located at 2F

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Convex (converging)

Lenses

CASE-4: Object between 2F’ and F’

Object

Image

Image is real (formed by refracted rays)

Inverted (upside down)

Magnified (larger than object)

Located beyond 2F

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Convex (converging)

Lenses

CASE-5: Object at F’

No image

Object

No image is formed

(rays refract parallel)

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Convex (converging)

Lenses

CASE-6: Object is within focal length

Image

Object

Image is virtual

(formed by extended rays)

Upright

Magnified

Located on same side as object

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Now let's consider

Concave lenses

delay

Concave (diverging)

Lenses

One case only!

Object

Image

Image is always virtual

(formed by a direct ray and an extended ray)

Always upright

Always reduced

Always located on same side as object

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Lenses

PART-1

Be sure to follow the following sign convention whensolving problems concerning lenses.

Sign convention:

1- Object distance is always positive.

2- Image distance is positive if the image is on the side of the

lens where light emerges (from lens).

3- Image distance is negative if the image is on the side of the

lens where the light enters (the lens).

4-The focal length of a convex (converging) lens is positive.

5- The focal length of a concave (diverging) lens is negative.

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Question-1

A lens or a lens.

A lens that is thickerin the middle than at the ends is known as:

Convex

Converging

Click

Click

Question-2

A lens or a lens.

A lens that is thickerat the ends than in the middle is known as:

Concave

Diverging

Click

Click

Question-3

Trace the rays that emerge from the following glass mediums:

a)

b)

c)

d)

Click

Click

Question-4

For each case below, draw the appropriate lens that will produce

the indicated rays.

a)

Concave (diverging)

b)

Convex (converging)

Click

Click

Question-5

How is the image formed by a mirror different from the image

formed by a lens?

A mirror forms an image by reflection whereas

a lens forms a an image by refraction.

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Click

Question-6

For each convexlens illustrated below, draw the image.

a)

Image

Image is real, inverted, reducedand located between F and 2F.

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Click

Question-6

For each convexlens illustrated below, draw the image.

b)

Image

Image is real, inverted, samesize object and located at 2F.

Click

Click

Question-6

For each convexlens illustrated below, draw the image.

c)

Image

Image is real, inverted, magnifiedand located beyond 2F.

Click

Click

Question-6

For each convexlens illustrated below, draw the image.

d)

Parallel

All rays refract parallel,

no image is formed.

Click

Click

Question-6

For each convexlens illustrated below, draw the image.

e)

Image

Image is virtual, upright, magnifiedand located on same side as object.

Click

Click

Question-7

For each concavelens illustrated below, draw the image.

REMEMBER

Concave lenses always produce imagesthat are virtual, upright and reduced.

Image

Image is virtual, upright, reducedand located on same side as object.

Click

Click

Question-7

For each concavelens illustrated below, draw the image.

Image

Image is virtual, upright, reducedand located on same side as object.

Click

Click

Question-8

An object that is 7 cm high is placed 20 cm in front of a convex(converging) lens whose focal length is 15 cm. Determine the

characteristics of the image:

Real

Type (real or virtual): _______________

Location: _______________

Magnification: _______________

Height: _______________

60 cm

-3 (inverted and 3x larger than object)

21 cm

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Click

Question-9

Note that diverging lenseshave a negative focal length.

An object whose height is 4 cm is placed 50 cm from a concave

(diverging) lens. If the focal length of the lens is 30 cm, determine

the characteristics of the image:

Virtual

Type (real or virtual): _______________

Location: _______________

Magnification: _______________

Height: _______________

18.75 cm

0.375 (upright and smaller than object)

1.5 cm

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Click

Question-10

An object that is 5 cm high is placed 70 cm in front of a convex

(converging) lens whose focal length is 20 cm. Determine

the characteristics of the image:

Real

Type (real or virtual): _______________

Location: _______________

Magnification: _______________

Height: _______________

28 cm

-0.04 (inverted and smaller than object)

2 cm

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Click

Question-11

A lens produces the following optical effect.

Which group of lens produces the above effect?

A)

Double concave

B)

Plano concave

C)

Double convex

D)

Plano convex

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Click

Question-12

Beams of light rays are traveling through air parallel to theprincipal axis of four different lenses. The light rays enterthe lenses and are refracted.

In which diagram are the light rays correctly illustrated?

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