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Physics 1230: Light and Color

Chuck Rogers, Charles.Rogers@colorado.eduRyan Henley, Valyria McFarland, Peter Siegfried

physicscourses.colorado.edu/phys1230

Lecture 17:

Lenses and ray tracing

EXAM 2 is next week Thursday in-class

Online and Written_HW9 due TONIGHT

Last Time: Refraction all the way through block

What was happening in Activity 8?

U2L05 3

Convex and concave lenses

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• Each of the two surfaces has a spherical shape.

• Light can penetrate through the lenses and bend at the air-lens interface.

Last Time: Concave and convex lenses

Glass

We build lenses out of glass with non-parallel sides

Which ray of light will have changed direction the most upon exiting the glass?

Glass

A

C

B

If slabs aren’t parallel - lens

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We build lenses out of glass with non-parallel sides

Put film, Retina here!

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• Light rays bent towards each other… CONVERGING LENS. • The less parallel the two sides, the more the light ray changes

direction.• Rays from a single point, converge to a single point on the other

side of the lens (and then start diverging again).

We build lenses out of glass with non-parallel sides

Put film, Retina here!

Focus

f

optical axis

Light rays coming in parallel focus to a point, called the focal point

Converging (convex) lens

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a good light collector or solar oven; can also fry ants with sunlight (but

please don’t do that)

Light focusing properties of converging lens

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The “backwards” light collector:create a collimated light beam

Light focusing properties of converging lens

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n>1

Rays entering

“slower” material bend toward

normal

Rays entering

“faster” material bend away from

normal

n=1

Ray tracing:

1. As long as ray stays in

same medium, it goes

straight.

2. At each interface to a

different medium,

calculate how it will

bend. Go back to 1.

Ray tracing to understand lenses and images

This gets tedious!

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FF

foci}focal lengthIf the glass surface is nearly a section of a

sphere, it will FOCUS

parallel rays.

A THIN LENS is very thin

compared to the focal

length. Then we can

simplify the treatment with

THREE SIMPLE RULES.

Thin convex (converging) lens

Group Activity!

• Come on down and get a handout.

• Your group should have a small focusing lens

• You should have a ruler or come get one.

• IT IS AN EXCELLENT IDEA TO HAVE A RULER FOR THE COMING EXAM 2.

Please get started on Part 1.

A Question:

You hold up a focusing lens and look through

it at an image of a distant object that is far on

the other side of the lens.

You find that the image is located at:

A)The object side of the lens

B)On your side of the lens

C)Exactly inside the lens

The image is:

A) Virtual B) Real

Group Activity!

Please get continue on Part II.

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1) A ray parallel to the axis

is deflected through the

focus on the other side

2) A ray through the center

of the lens continues

undeviated

3) A ray coming from the

focus on one side goes

out parallel to the axis

on the other

F’F

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2

3 3

foci

}focal lengthThin convex (converging) lens: Ray tracing rules

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foci (focuses?)

Where will this ray go?

Ray tracing

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Ray Tracing

Where will this ray go?

Suppose it’s emitted from this object

foci (focuses?)

Ray tracing

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Ray Tracing

foci (focuses?)

We know where these 3 rays

go, using the simple ray rules

Ray tracing

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Ray Tracing

foci (focuses?)

We know where these 3 rays

go, using the simple ray rules

Amazing property of

this lens: all rays from

the tip of the arrow will

converge to the same

point

Ray tracing

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Amazing property of

this lens: all rays from

the tip of the arrow will

converge to the same

point (the image)

See how the rays emerge

from this point (the image)?

Ray tracing: Thin lens, object outside focus

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Amazing property of

this lens: all rays from

the tip of the arrow will

converge to the same

point (the image)

Eye sees an image

here.The Lens acts as our

“Magic Ray Machine”, creating the rays to

produce an image.

Ray tracing: Thin lens, object outside focus

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In this case, the image is:

Eye sees an image

here.

A)Virtual

B)Real

Clicker question

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In this case, the image is:

Eye sees an image

here.

A)Virtual

B)Real

Real because the light rays really

go through the image. You can put

a screen there to see it.

Clicker question

Remember the Pinhole?

Pinhole = selecting one set of rays. Dim but always in focus!

Many pinholes = many images

Lens gathers and bends light from multiple rays to focus to one image

Demo and PHET Geometric-optics

http://phet.colorado.edu/en/simulation/geometric-optics

Idea of magnification and focused image

Group Activity!

Please get continue on Part II and

start to get to G.

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Ray tracing a convex lens: object inside focus

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Ray tracing a convex lens: object inside focus

The image appears larger (and farther away) than the object.

This is a magnifying glass.

(Remember: a magnifying glass is a convex lens.)

Aside: near-sighted people need concave/diverging lenses; can a marooned

myopic start a fire with his eye-glasses?

Converging/convex lens:Near objects are magnified and upright.Far objects are upside down and smaller

Near object

Far object

Magnifying glass

Group Activity!

Please get continue on Part 3.

Good place for a break!

Enjoy your day.

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Ray tracing exercise

Ray tracing exercise

Come get some handouts!