2.3 Lenses Flashcards
Define focal length
the distance from the centre if a lens to its principle focus
When the object is between F and the Lens, describe the image
- virtual
- upright
- magnified (use magnifying lens)
- formed further than lens than object on same side as object
When the object us at F, describe the image
- no image formed
- at infinity
- real
- enlarged
- inverted
When the object is between F and 2F, describe the image
- real
- enlarged
- inverted
- I formed beyond 2F
When the object is at 2F, describe the image
- I formed at 2F
- real
- same size as object
- inverted
When the object is beyond 2F, describe the image
- I formed between F and 2F
- real
- diminished
- inverted
When the object is at infinity, describe the image
- at F
- real
- diminished
- inverted
Describe the image formed by a diverging lens
- virtual
- upright
- diminished
Define a real image
one that rays of light actually pass throigh
Define a virtual image
one that rays of light only appear to pass through
Define linear magnification
the ratio of the image height to object height
Define accommodation
the ability of the eye to clearly see objects at different distances
Where does most of the refraction of light take place in the eye? Why?
the boundary between the air and the cornea because this is where the largest change of refractive index occurs
What is the near point of a normal eye
25cm
Define myopia
a person suffering from myopia (short-sightedness) is unable to see distant objects clearly
How does myopia work
the sufferer cannot make lens thin enough to view distant objects and so lit from distance objects converges at a point in front of the retina, making them see a blurry image
What kind of lens is used to correct myopia
concave (diverging) lens
What focal length does the concave lens to fix someones myopia have?
the distance to the person’s actual far point
Define hypermetropia
a long-sighted person sees distant objects clearly but does not see near objects clearlt
How does hypermetropia work?
The ciliary muscles are too weak to make the lens thick and so have a shorter focal length
The rays from the object are not bent sufficiently to form an image on the retina
What kind of lens are used to correct hypermetropia?
Convex (converging) lens
