Lenses Flashcards
focal length
distance from the lens to the principal focus
Concave lens used to treat..
Short Sight (seeing distant objects)
Convex lens used to treat..
Long sight (seeing near objects)
Principal focus
the point where light’s parallel rays meet or appear to meet (drawn on both sides of the lens)
Shorter the focal length…
the more powerful the lens (as the light rays have been refracted more!)
Convex lens
Makes parallel rays CONVERGE (focus) to a point
- rays focused AFTER lens
- lens looks like an oval
Concave lens
makes parallel rays DIVERGE (spread out)
- rays focused BEFORE lens VIRTUALLY
- inverted shape
Real image
- Image formed where light rays meet!!!
- object OUTSIDE the focal length of a converging lens
Virtual image
- Image formed where light rays APPEAR to come from
- inside the focal lens of a converging lens (magnified), formed by a diverging lens
When do images appear magnified?
When an object is placed inside the focal of a converging lens
magnification < 1
image diminished
mag > 1
image magnified
concave lens image
Virtual (before lens) Upright Diminished - Ray 1 - parallel to lens and then upwards in line w focal point before lens (draw dotted line before lens) - Ray 2 diagonal through centre of lens where dotted + ray 2 join is image
Convex AFTER focal point image
Real
Inverted
Diminished
- Ray 1 parallel and down to focal point after lens
- Ray 2 through the centre
- Ray 3 through focal point before lens and then parallel
= where all these meet up = image
Convex INSIDE focal point
Virtual
Upright
Magnified
Ray 1 - parallel then to focal point after lens
Ray 2 - through the centre
W dotted line join up where they meet BEFORE the lens = image formed there
