physics 2225 optics 2: spherical lenses and optical instruments purpose of this minilab use lens...

Physics 2225 Optics 2: Spherical Lenses and Optical Instruments

Purpose of this Minilab

• Use lens formula to determine focal length of a lens.

• Learn about image magnification in magnifying glasses, microscopes, and telescopes.


Activity 1: Focal Length of a Lens

Flashlight or table lamp at the end of classroom (long distancecompared to focal length).

Light rays enter lens approximately parallel.

Screen or sheet ofpaper to see image.

Lensf

Method 1:

Move sheet until image is in focus.Then measure f.


Move sheet until image is in focus.Then measure so and si and calculate f with:

Activity 1: Focal Length of a Lens

Object (illuminated crosson the light source)

Screen or sheet ofpaper to see image.

si

Method 2:

s

o

io ssf

111


The Imaging Equation for Lenses

fss io

111

so: object distancesi: image distancef : focal length


Sign Rules For Lenses

Convex lenses: f is positiveConcave lenses: f is negative

Real objects: so is positiveVirtual objects: so is negative

Real images: si is positiveVirtual images: si is negative

Most objects are real.

Virtual images cannot be picked up with a screen.


Virtual or Real Image?

fsifimagevirtuals

fsifimagereals

sfs

oi

oi

oi

)(0

)(0

:lens)(convex f positivefor

111

In Activity 1.2 (using a converging lens) place the object at a distance larger than f away from the lens to get a real image.

Hint: To answer Q1, do a similar analysis for the concave lens (f < 0).


Activity 2: Magnification

sizeobject

size image :ionMagnificat

'

h

hM

sosi

h’

h a

a

io s

h

s

h'

tan o

i

s

s

h

h

'

o

i

s

s

h

hM

'

An inverted imagemeans that h’and h haveopposite sign. M < 0


2.2 Virtual image magnification (magnifying glass)

Without the magnifying glass:

eye

25 cm (typical nearest distance a human can focus on)

With the magnifying glass:

eye

125

f

cmM

f

virtual image


Verifying this magnification of a magnifying glass

1) Tape linear graph paper on viewing screen.2) Place lens about 25cm away from screen.3) Hold a second piece of graph paper close to lens.4) Move your eye close to the lens.5) Move the second piece of graph paper so it is in focus.6) Compare the size of graph paper seen through the lens

with the size of the graph paper on the screen (seen not through the lens). See next page for illustration.

your eye (closeto lens)

25 cm

Optical Bench

viewing screen

linear graphpaper

lensf = +100mm

hand held linear graphpaper (close to lens)


What you should see ….

lens

viewing screen

graph paper onviewing screen

hand heldgraph paper

hand held graph paperseen through lens

Compare:3.5 divisionson the graph papertaped to the screen= 1 division on thehand held graphpaper seenthrough the lens.

M=3.5 (in this example)

…then check whether this agrees with 125

f

cmM


Remarks to formula for magnifying glass….

The actual magnification depends on exactly where the object is placed:

If the object you magnify is placed exactly at the focal point of themagnifying glass, then

If you move the object even closer to the lens, the magnification can getas high as

You could get a theoretical value anywhere between those two magnifications, depending on where exactly you hold the paper.

f

cmM

25

125

f

cmM


Activity 3: Microscope

eye

siso

objectivefo=200mm

eyepiecefe=100mm

125

f

cmM e

o

io s

sM

1

25

f

cm

s

sMMM

o

ieo

need so > fo

real imagebetween lenses

virtual image


Microscope: Building Instructions

Step 1: Install light source and objective lens.

optical bench

light source

illuminated arrowon this side

200mm lens(objective)

so 30cm measure si

handheld piece of paper:move so that the image of the arrow is in focus.

record so and si



Step 2: Install eyepiece lens.

optical bench


so 30cm Si (as previously determined)

100mm lens(eyepiece)

some smalldistance further



Step 3: Replace light source with white viewing screen

optical bench


so 30cm Si


whiteviewingscreen

Viewing screen must be placed where the arrow used to be.

Cover the viewing screen with linear graph paper.

linear graphpaper



Step 3: Look through eyepiece and adjust it’s position.

optical bench


so 30cm Si


whiteviewingscreen

linear graphpaper

eye

Adjust eyepiece positionso that the image of thegraph paper is in focus.


Microscope: Measuring the magnification

optical bench



linear graphpaper

eye

Step 1: Hold a second piece of graph paper approximately 25cm from your eye.That extra graph paper should be a bit to the side so you can still see the image of the graph paper that is on the viewing screen.

25cm

hand held linear graphpaper


Step 2: What you should see ….and measure

viewing screen

graph paper onviewing screen

hand heldgraph paper(25cm fromeye)

image of graph paperon viewing screen

Compare:2.8 divisionson the hand held graph paper= 1 division on the image ofGraph paper taped to the screen.

M=2.8 (in this example) …then check whether this agrees with

1

25

f

cm

s

sM

o

i


Activity 3: Telescope

eye

si fo

so (looking at far away objects)

objectivefo=350mm

eyepiecefe=100mm

e

o

f

fM

virtual image

fe

fo + fe


Telescope: Building Instructions

Install objective and eyepiece

optical bench



Separate objective and eyepieceby fo+fe (=450mm)


Telescope: Measuring the Magnification



optical bench

View white board throughtelescope from the back ofthe room.

White board in the front of the room.Draw a thick scale on the white board.Illuminate the scale with a lamp.

lamp

eye


What you should see …

white board

Compare scale seen throughtelescope with scale seendirectly to determine M.

Here: Magnification looks like M - 2.3 (negative because inverted)

telescope eyepiece


Using the Desk Lamp

Dimmer

Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.

On/Offswitchof lamp

physics 2225 optics 2: spherical lenses and optical instruments purpose of this minilab use lens...

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