LENSES

BELLWORK

Get out notes and practice from yesterday

Pick up ruler and finish practice from yesterday

PRACTICE 1

A 1.5 cm object is placed 6.0 cm from a double convex lens, which has a focal length of 1.5 cm.

PRACTICE 2

A 1.5 cm object is placed 3.0 cm from a convex lens, which has a focal length of 1.5 cm.

LENSES

A lens is a piece of transparent material, such as glass or plastic, that is used to bend light and produce an image.

When light passes through a lens, refraction occurs.

TYPES OF LENSES

A convex lens is thicker at the center than at the edges.

A convex lens is a converging lens because it refracts parallel light rays of light so that they meet at a focal point.

A concave lens is a thinner in the center than at the edges.

A concave lens is a diverging lens because rays passing through the lens spread out.

Lenses

Converging Lens

Diverging Lens

F F

f f

LENS CONSTRUCTION

Lenses consist of two faces Each face of the lens can be either

curved or flat. A lens consisting of two curved

surfaces will have two focal points.

ANATOMY OF A LENS

Double Convex Lens

IMAGES FORMED BY LENSES

Virtual images formed by lenses are on the same side of the lens as the object.

Virtual images are always upright. Real images formed by lenses are on

the opposite side of the lens from the object.

Real images are always inverted.

IMAGES BY CONCAVE LENSES

Concave lenses produce virtual, upright, reduced images.

IMAGES OF CONVEX LENSES

Convex lenses produce either real or virtual images.

If the image is virtual, it will also be upright and enlarged.

If the image is real, it can be enlarged, reduced, or the same size as the object.

LIGHT BEHAVIOR

The lens is more optically dense, so light slows down.

When light passes from air through a lens, it bends toward the normal.

IMAGE CONSTRUCTION FOR CONVEX LENS

1. Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.

2. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

THIRD RULE FOR IMAGE CONSTRUCTION An incident ray that passes through the

center of the lens will in affect continue in the same direction that it had when it entered the lens.

IMAGE CONSTRUCTION FOR CONCAVE LENSES

1. The ray that travels parallel to the principal axis on the way to the lens will refract and travel in a direction such that its extension passes through the focal point on the object's side of the lens.

2. Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

THIRD RULE FOR IMAGE CONSTRUCTION An incident ray that passes through the

center of the lens will in affect continue in the same direction that it had when it entered the lens.

This is true for both concave and convex lenses.

Ray Tracing for Lenses

Light passes through a lens

There is a focal point on both sides of a lens

Converging Lens:

Ray #1: Parallel to the axis Refracts through F

Ray #2: Through F

Refracts parallel to axis

Ray #3: Through Center of lens undeflected

For a lens, a real image is on the opposite side as the object

1d

1d

1fo i

The Thin Lens Equation

mdd

i

o

These equations also work on lenses:

The Magnification Equation

But the variables are defined slightly differently now because……….

For a mirror, a real image was on the same side as the object

THE HUMAN EYE

The eye act as a lens. The cornea is the transparent covering

that protects the eye. The cornea is also the part of the eye

that refracts light. The iris is the colored part of the eye

surrounding the pupil and regulates the amount of light that enters the eye.

The pupil is the opening through which light enters.

The pupil appears black because light is entering but not leaving. All light entering the eye is absorbed.

The eye lens is a crystalline lens. Muscles change the shape of the lens which changes its focal length.

The retina is the inner surface of the eye.

The retina contains rods and cones that detect the intensity and frequency of incoming light.

The rods and cones send nerve impulses to the brain along the optic nerve.

VISION

The eye can change the focal length through a process called accommodation.

This process occurs automatically and allows us to see objects nearby and far away.

Based on the focal length of our eye (approximately 1.8 cm) images formed on our retina are reduced, inverted, and real.

An image is produced by the cornea lens system on the retina.

The brain interprets the signal from the retina and we see an object as right side up.

http://www.bing.com/videos/search?q=how+the+eye+works&view=detail&mid=1D185809FDBF88EEF4941D185809FDBF88EEF494&first=0&FORM=NVPFVR

IMPERFECT VISION

http://www.wolframalpha.com/widgets/gallery/view.jsp?id=65dc55ab724bbb85a9ebeb5c53943bb7

FAR-SIGHTEDNESS

Far-sightedness (hyperopia)- allows objects far away to be seen clearly, but objects close up cannot be focused clearly.

This occurs because the focal length is longer than the distance from the lens to the retina.

The image forms behind the retina.

A convex lens is used to correct the problem because it is a converging lens.

The corrective lens causes light to refract before it reaches the eye, decreasing the image distance, and focusing the image on the retina.

This condition normally occurs as you age because the muscles in your eye become rigid.

NEAR-SIGHTEDNESS

Near-sightedness (myopia) occurs when nearby objects can be seen clearly but far away objects are not focused clearly.

Occurs because the focal length is shorter than the distance from the lens to the retina.

Images form in front of the retina. http://vnatsci.ltu.edu/s_schneider/physl

ets/main/nearsighted.shtml

A concave lens is used to correct the problem because it is a diverging lens.

The corrective lens causes light to diverge before it hits the eye lens. (The light is then converged by the cornea and eye lens)

More common in young people due to a bulging cornea.

http://vnatsci.ltu.edu/s_schneider/physlets/main/nearsighted.shtml

ASTIGMATISM

Astigmatism occurs when the eyeball is not spherical.

The result is that vertical lines of images can focus while horizontal lines are not.

Eyeglasses having a non-spherical shape correct the problem.

OPTICAL DEVICES

Microscopes allow the eye to see extremely small objects

Most microscopes use two convex lenses an objective lens with a short focal length an eyepiece or ocular lens (magnifies the image)

Telescopes allow the eye to see very distant objects Telescopes use two convex lenses

Objective lens has a long focal length The eyepiece lens has a short focal length Allows more light to enter the eye

http://www.youtube.com/watch?v=tpKWHSsBpnE

LENSES REVIEW

Lenses have 2 focal points Convex lenses form the same type of

images as concave mirrors Convex lenses are converging Concave lenses form the same type of

images as convex mirrors Concave lenses are diverging

VIRTUAL IMAGES

Form on the same side of the lens as the object are virtual

Have a negative di Have a positive hi (upright images)

REAL IMAGES

Images that form on the opposite side of the lens as the object are real

Have positive di Have a negative hi (inverted

images)

For each problem below, draw the ray diagram for the lens.

Draw each diagram to scale and include the principal axis, lens, f, and 2f.

Measure the location of the image and the height of the image.

Calculate these values and compare with the measured values.

1. A 2.0 cm object is placed 10.0 cm from a double convex lens, which has a focal length of 3.0 cm.  

2. A 2.0 cm object is placed 6.0 cm from a double convex lens, which has a focal length of 3.0 cm.