4/12: Applying the Lens/Mirror Formula Today we will review problems 9-12 on the Light III calculation WS and then you will prepare for tomorrow’s test.

4/12: Applying the Lens/Mirror Formula4/12: Applying the Lens/Mirror Formula Today we will review problems 9-12 on the Light III Today we will review problems 9-12 on the Light III

calculation WS and then you will prepare for tomorrow’s calculation WS and then you will prepare for tomorrow’s test by completing the review.test by completing the review.

Glasses: What type of lens?Glasses: What type of lens? Pick up review.Pick up review. Tomorrow’s test will be applied to the 6Tomorrow’s test will be applied to the 6 thth six weeks grading six weeks grading

period.period. Friday you will turn in all diagrams (mirrors and lenses) and Friday you will turn in all diagrams (mirrors and lenses) and

the completed review.the completed review. I will be available today after school and next Monday, I will be available today after school and next Monday,

Tuesday and Thursday after school.Tuesday and Thursday after school. Skip question 13Skip question 13 Add Question 24 and 25: What is hi and M if ho in #23 Add Question 24 and 25: What is hi and M if ho in #23

was 8 cmwas 8 cm

4/154/15 Last week you took the Light II Test and completed Last week you took the Light II Test and completed

the mirror diagrams. You also were introduced to the mirror diagrams. You also were introduced to Lenses.Lenses.

Today we will complete lens diagramsToday we will complete lens diagrams You will need the lens diagram sheet (you should You will need the lens diagram sheet (you should

already have this) 2 colored pencils, and a ruler.already have this) 2 colored pencils, and a ruler. I will be available Monday, Tuesday and Wednesday I will be available Monday, Tuesday and Wednesday

before and after school this week for test before and after school this week for test corrections, make ups, and retakes. You must sign corrections, make ups, and retakes. You must sign into spiral to indicate day and whether you are into spiral to indicate day and whether you are retaking or making up a test.retaking or making up a test.

Incomplete Boat Group? Meet in room 443 today at Incomplete Boat Group? Meet in room 443 today at 2:30.2:30.

4/18 “Quest” today4/18 “Quest” today You will need a scantron, calculator, and pencil.You will need a scantron, calculator, and pencil. Yesterday I collected the Lens and Mirror Yesterday I collected the Lens and Mirror

Diagrams. We went over HW 7,8,11 & 12. You Diagrams. We went over HW 7,8,11 & 12. You worked on the Light III review sheet in class.worked on the Light III review sheet in class.

Tomorrow is last day to turn in Boat SlipsTomorrow is last day to turn in Boat Slips

Refraction and Lenses

The most common application of refraction in science and technology is lenses.

The kind of lenses we typically think of are made of glass. The basic rules of refraction still apply but due to the curved surface of the lenses, they create images.

Types of LensesConvex lenses:

aka converging lenses

they bring light rays to a focus.

for farsightedness (hyperopia)

Concave lenses:

aka diverging lenses

they spread out light rays.

for nearsightedness (myopia)

Parts of a Lens

All lenses have a focal point (f). In a convex lens, parallel light rays all come together at a single point called the focal point. In a concave lens, parallel light rays are spread apart but if they are traced backwards, the refracted rays appear to have come from a single point called the focal point.

ffReal

Virtual

CONCAVE LENSES

Virtual images: form where light rays appear to have crossed.

In lenses: form on the same side of the lens as the object.

Virtual images: always upright., reduced

CONVEX LENSES

Real images: form where light rays actually cross.

In lenses: they form on the opposite side of the lens from the object since light can pass through a lens.

Real images: always inverted

Real images: can be projected.

Convex lenses can also form virtual images. These are enlarged.

Rules for Locating Refracted Images

1. Start at top of object. Light rays that travel through the center of the lens (where the principle axis intersects the midline) are not refracted and continues along the same path.

2. Start at top of object. Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal point (f).

On homework change question 3 to an On homework change question 3 to an object 4cm from lens with focal point of object 4cm from lens with focal point of 8cm.8cm.

Graph it now while I check homework.Graph it now while I check homework. What is the difficulty?What is the difficulty?

Images formed by Convex lenses

Locating images in convex lenses

Convex Lenses with the Object located beyond

2f

fC f

C

Light rays that travel through the center of the lens are not refracted and continue along

the same path.

Convex Lens

Object located beyond C

f2f f

2f

Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal

point (f).

Convex Lens

Object located beyond 2f

f2f f

2f

Image:

Real

Inverted

Smaller

Convex Lens

Object located beyond 2f

The image is located where the refracted light rays intersect

Convex Lenses with the Object located at 2f

f2f f

2f

Light rays that travel through the center of the lens are not refracted and continue along

the same path.

Convex Lens

Object located at 2f

f2f f

2f

Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal

point (f).

Convex Lens

Object located at 2f

f2f f

2f

Image:

Real

Inverted

Same Size

Convex Lens

Object located at 2f

The image is located where the refracted light rays intersect

Convex Lenses with the Object located between

f and 2f

f2f f

2f

Light rays that travel through the center of the lens are not refracted and continue along

the same path.

Convex Lens

Object located between f and 2f

f2f f

2f

Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal

point (f).

Convex Lens

Object located between f and 2f

f2f f

2f

Image:

Real

Inverted

Larger

Beyond 2f

Convex Lens

Object located between f and 2f

The image is located where the refracted light rays intersect

Convex Lenses with the Object located at f

f2f f

2f

Light rays that travel through the center of the lens are not refracted and continue along

the same path.

Convex Lens

Object located at f

f2f f

2f

Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal

point (f).

Convex Lens

Object located at f

f2f f

2f

No image is formed.

All refracted light rays are parallel and do not cross

Convex Lens

Object located at f

Convex Lenses with the Object located between

f and the lens

f2f f

2f

Light rays that travel through the center of the lens are not refracted and continue along

the same path.

Convex Lens

Object located between f and the lens

f2f f

2f

Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal

point (f).

Convex Lens

Object located between f and the lens

f2f f

2f

Convex Lens

Object located between f and the lens

These to refracted rays do not cross to the right of the lens so we have to project them back behind the lens.

f2f f

2f

Image:

Virtual

Upright

Larger

Further away

Convex Lens

Object located between f and the lens

The image is located at the point which the refracted rays APPEAR to have crossed behind the lens

Images formed by concave lenses

Locating images in concave lenses

Concave Lenses with the Object located

anywhere

f2f f 2f

Light rays that travel through the center of the lens are not refracted and continue along

the same path.

Concave Lens

Object located anywhere

f2f f 2f

Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal

point (f).

Concave Lens

Object located anywhere

f2f f 2fImage:

Virtual

Upright

Smaller

Between f and the lens

Concave Lens

Object located anywhere

The image is located where the refracted light rays appear to have intersected

The eye contains a convex lens. This lens focuses images on the back wall of the eye known as the retina.

The distance from the lens to the retina is fixed by the size of the eyeball. For an object at a given distance from the eye, the image is in focus on the retina. Although the image on the retina is inverted, the brain interprets the impulses to give an erect mental image. If the object moved closer to the eye and nothing else changed the image would move behind the retina the image would therefore appear blurred. Similarly if the object moved away from the eye the image would move in front of the retina again appearing blurred. To keep an object in focus on the retina the eye lens can be made to change thickness. This is done by contracting or extending the eye muscles. We make our lenses thicker to focus on near objects and thinner to focus on far objects.

Someone who is nearsighted can see near objects more clearly than far objects. The retina is too far from the lens and the eye muscles are unable to make the lens thin enough to compensate for this. Diverging glass lenses are used to extend the effective focal length of the eye lens.

Someone who is farsighted can see far objects more clearly than near objects. The retina is now too close to the lens. The lens would have to be considerable thickened to make up for this. A converging glass lens is used to shorten the effective focal length of the eye lens. Today’s corrective lenses are carefully ground to help the individual eye but cruder lenses for many purposes were made for 300 years before the refractive behavior of light was fully understood.

Lens Equation

(1/f) = (1/do) + (1/di)

f = focal length

do = object distance

di = image distance

Lens Magnification Equation

M = -(di / do) = (hi / ho)

M = magnification

di = image distance

do = object distance

hi = image height

ho = object height

Lens Sign Conventions

f + for Convex lenses- for Concave Lenses

di + for images on the opposite side of the lens (real)- for images on the same side (virtual)

do + alwayshi + if upright image

- if inverted imageho + alwaysM + if virtual

- if real imageMagnitude of magnification

<1 if smaller=1 if same size>1 if larger