Transcript Concave Thin Lens - King`s Senior Science

Concave Thin Lens
Diverging Lens
Virtual Focal Point
Concave Thin Lens
Virtual
Image
hi di

ho do
1
1
1


 f do  d i 
No matter where the object is:
Image is always virtual, smaller and upright
F
F
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
Ex:
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
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.
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) cm
Solve a quadratic to find f…
lens
13cm
book
Sign conventions for Lenses
Focal length (f)
+ converging
Object distance (do)
+ object on the left
- diverging
Image distance (di)
Magnification (m)
+ image on the right (real)
+ upright
- image on the left (virtual)
- inverted