Topic 5.4-5.6 — Lenses Flashcards
(14 cards)
What is focal length?
The distance between the lens and the focal point
What is the focal point?
The point where all horizontal rays meet after passing through the lens
How do you work out the power of a lens?
Power of the lens is the inverse of the focal length.
e.g. Thicker lens means shorter focal length, so greater power
Describe concave lenses
A concave lens is thinner in the middle than it is at the edges. This causes parallel rays to diverge and spread light outwards. Light appears to have come from the focal point.
In a ray diagram, a concave lens is drawn as a vertical line with inward pointing arrows either end to indicate the shape of the lens.
Describe how concave lenses can correct eyesight
It is used to spread light out further. Concave lenses correct short-sightedness (myopia) as light is focused in front of the retina, so needs to be spread out slightly to be able to be focused onto the retina.
How do you draw the image of an object through a concave lens?
-Draw a horizontal ray from the top of the object to the lens. This will be refracted:
* Draw a line from F (on the side of the object) through the top of the lens
-Draw a line from the top of the object through the centre of the lens
-The image is where the lines intersect
Describe convex lenses
They are fatter at the centre and focus light inwards. Horizontal rays focus onto the focal point.
In a ray diagram, a convex lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.
Describe how convex lenses can be used
They are used for magnifying glasses, binoculars and to correct long-sightedness as it focuses the rays closer.
How do you draw the image of an object through a convex lens?
-Draw a horizontal ray from the top of the object to the lens. This will be refracted:
* Draw a line from the top of the lens through F (on the opposite side of the lens to the object)
-Draw a line from the top of the object through the middle of the lens.
-The image is where the lines intersect
-The ray exits the lens along a line from the top of the image to the top of the lens
Define each descriptor for an image
-An image may be upright or inverted (upside down compared to the object)
-An image may be magnified or diminished (smaller than the object)
-An image may be real (can be projected onto a screen) or virtual (appears to come from behind the lens)
What images do diverging (concave) lenses produce?
Virtual, upright and diminished images
What images do converging (convex) lenses produce when the object is further from the lens than 2 focal lengths?
An inverted, diminished, real image.
This is used for a camera or human eye.
What images do converging (convex) lenses produce when the object is between 1 and 2 focal lengths from the lens?
AN inverted, magnified, real image.
This is used for projectors.
What images do converging (convex) lenses produce when the object is placed less than 1 focal lengths from the lens?
An upright, magnified, virtual image.
This is used for magnifying glasses.
