lenses preap physics. critical angle at a certain angle where no ray will emerge into the less dense...

Lenses

PreAP Physics

Critical Angle

At a certain angle where no ray will emerge into the less dense medium.– For water it is 48 which does not allow the

ray to enter the air

Light ray is only reflected inside the more dense medium.– Other Examples

Glass (43)Mirrors (90-95)Diamonds (24.6)

Total internal reflectionPhenomenon that involves the reflection of all the incident light off the boundary. Only takes place when both of the following two conditions are met:– Light is in the more dense medium and approaching the

less dense medium. – Angle of incidence is greater than the so-called critical

angle.

Critical Angle  n1 sin q1 =  n2 sin q2

              =  n2 sin 90

    sin q1  =  n2 / n1

Fiber Optics

A.K.A. Optical fibers or light pipesThin glass fibers – Use Total Internal Reflection

Developed for communication and data transmission– Replacing electric circuits and microwave

links

Thousands of miles of it in our phone lines

Fiber Optics

Other examples of Total Internal Reflection

THIN LENSES Lenses are an essential part of telescopes, eyeglasses, cameras, microscopes and other optical instruments. A lens is usually made of glass, or transparent plastic.

A converging (convex) lens is thick in the center and thin at the edges.

A diverging (concave) lens is thin in the center and thick at the edges.

NearsightednessYour eye focuses images in front of the retina– Objects far away appear fuzzy, but objects up

close can be seen

To correct Place a concave lens to refocus the light so it focuses on the retina

FarsightednessEye focuses the light behind the retina.– Objects close up are fuzzy and objects far away

are clearer.

To correct – Place a convex lens to refocus the light so it

focuses on the retina

IMAGE FORMATION BY LENSES There are three principal rays to locate an image.

Principal Rays

Review Ray

Diagram for both types of lenses.

2 Types of Images for Lenses

A real image is always formed on the side of the lens opposite to the object.

A virtual image will appear to be on the same side of the lens as the object.

26.3 a. Find the images formed by the following lenses

using the Ray Tracing method.

b. Write the characteristics of each image: -real or virtual, -larger, smaller or same size as object and-upright or erect.

Convex Lens with Object Beyond 2F

Convex Lens with Object at 2F

Convex Lens with Object Inside Focal Point

Convex Lens with Object Between 2F and F

No image is formed.

Convex Lens with Object at Focal Point

Concave Lens with Object Beyond 2F

Concave Lens with Object Inside Focal Point

THE LENS EQUATION The lens equation can be used to locate the image:

1 1 1

d d fo i

Mh

h

d

di

o

i

o

The ratio M is called the magnification, ho is the object’s size and hi is the image size.

Where do is the object’s distance, di is the image distance and f is the focal length.

R radius of curvature

+ converging

- diverging 

f focal length

+ converging

- diverging

doobject distance

+ real object

+ real object

diimage distance

+ real images

- virtual images

hoobject size

+ if upright

- if inverted

hiimage size

+ if upright

- if inverted

26.4 A 5 cm tall object is located 30 cm from a convex lens of 10 cm focal length. a. Find the location and nature of the image.

do = 30 cm

f = 10 cm

dd f

d fio

o

30 10

30 10

( )= 15 cm, real

b. What is the height of the image?

ho = 5 cm h

h

d

di

o

i

o

hd h

dii o

o

15 5

30

( )= - 2.5 cm, inverted