image formations in lenses and thin lens equations

8/7/2019 IMAGE FORMATIONS IN LENSES AND THIN LENS EQUATIONS

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IMAGE FORMATIONS IN LENSESAND THIN LENS EQUATIONS


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The image formed by a converging

lens can be made using only threeprincipal rays.

Ray 1 is the ray which travels parallel to the

axis and after going through the lens it passesthrough the focal point.

Ray 2 passes through the center of the lens.

Ray 3 goes through the focal point and thentravels parallel to the axis after passingthrough the lens.


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The image formed by a diverging

lens can be made using only threeprincipal rays:

Ray 1 is the ray which travels parallel to the

axis and after going through the lens it passesthrough the focal point.

Ray 2 passes through the center of the lens.

Ray 3 goes through the focal point and thentravels parallel to the axis after passingthrough the lens.


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Thin LENS Equation

You can find the distance of the image from the

lens by the following equation:

1/q + 1/p = 1/f,

where q is the distance from the lens to theimage, p is the distance from the object to

the lens, and fis the focal distance of the lens


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Thin LENS Equation


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Thin LENS Equation

Example:

A converging lens of focal length 10.0 cm

forms images of objects placed 30.0 cm.Construct a ray diagram, find the imagedistance and describe the image.


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Thin LENS Equation

Solution:

Construct a ray diagram as shown below.


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Thin LENS Equation

Solution:

Use the thin lens equation to find the image

distance.


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Thin LENS Equation

Solution:

Substituting values,


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Thin LENS Equation

Solution:

Solving for the magnification

The image is reduced by one half and the negative

sign for M tells us that the image is inverted.


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Thin LENS Equation

Example:

A diverging lens of focal length 10.0 cm forms

images of objects placed 30.0 cm. Constructa ray diagram, find the image distance anddescribe the image.


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Thin LENS Equation

Solution:

Construct a ray diagram as shown below.


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Thin LENS Equation

Solution:

Use the thin lens equation to find the image

distance.


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Thin LENS Equation

Solution:

Substituting values,


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Thin LENS Equation

Solution:

Solving for the magnification

The image is reduced by one fourth and the positive

sign for M tells us that the image is upright.


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EXERCISE (1 whole by pair)

1. Find graphically the image location for an objectat each of the following distances from aconverging lens that has a focal length of 20cm.

Check your results by calculating the imageposition and the lateral magnification,respectively.

A. do = 50 cm

B. do = 20 cm C. do = 15 cm D. do = - 40 cm


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EXERCISE (1 whole by pair)

2.)You are given a thin diverging lens.You findthat a beam of parallel rays spreads out afterpassing through the lens, as though all therays came from a point 20.0 cm from thecenter of the lens.You want to use this lens toform an erect virtual image that is 1/3 theheight of the object. (a) Where should the

object be placed? (b) Draw the principal raydiagram.

image formations in lenses and thin lens equations

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