Virtual Focal Point

Concave Thin Lens

Diverging Lens

Concave Thin Lens

Virtual Image

o

i

o

id

d

h

h io d

1d1

f1

No matter where the object is: Image is always virtual, smaller and upright

FF

The Human Eye

Web Links: Eye lens,

Vision and Eyesight

Near Point – Closest distance the eye

can focus on (about 25 cm when we are

young)

Far Point – Farthest distance the eye can focus on (should be )

Someone who is Nearsighted cannot focus on far away objects. (Their far

point is not at infinity.)

Nearsightedness can be corrected with diverging lenses

Here’s how it works

Ex:

Without my contact lenses, I need to stand 35 cm or less from the TV in order to see it in focus. Find the focal length of the contact lenses that correct my

vision.

                              

Someone who is Farsighted cannot focus on objects too near.

Farsightedness can be corrected with converging lenses

Here’s how it works

The man has a near point of 48 cm. His reading glasses are 2.0 cm from his eyes, and with them on, he can read the newspaper as

close as 25 cm to his eyes. Find the focal length of his glasses

Ex:

Lens Aberrations

1) Spherical Aberration

Can you think of two ways that this problem could be eliminated?

2) Chromatic Aberration

How to correct this problem?

Compound (Achromatic)

Lens

Do mirrors ever experience Chromatic Aberration?A. Yes B. No

Two people who wear glasses are camping. One is nearsighted, and the other is farsighted.

Whose glasses would be useful in starting a fire by focusing the sun’s rays?

A. The nearsighted person

B. The farsighted person

C. Any piece of glass will do the trick

Try these problems A convex lens has a focal length of 5cm. An object is placed 7 cm from the lens. Determine the image distance and the image size. A convex lens has a focal length of 10cm. An object is placed 7 cm from the lens. Determine the image distance and the image size. A concave lens has a focal length of 8cm. An object is placed 12 cm from the lens. Determine the image distance and the image size. A concave lens has a focal length of 12cm. An object is placed 23 cm from the lens. Determine the image distance and the image size.

lens

book

13cm

If the image of the book is 5.0 cm below the lens, find the focal length of the lens.

Challenge problem…Hint di = -(f-5) cmSolve a quadratic to find f…

Sign conventions for Lenses

Focal length (f)

+ converging

- diverging

Object distance (do)

+ object on the left

Image distance (di)

+ image on the right (real)

- image on the left (virtual)

Magnification (m)

+ upright

- inverted