Lenses Flashcards
what is a lens?
a shaped piece of transparent glass or plastic that refracts light
describe the structure of a concave lens:
thicker in the middle than it is at the edges
describe the behaviour of light in convex lenses: (2)
-Parallel light rays that enter the lens converge.
-They come together at a point called the principal focus.
what is the principal focus?
focal point: the focus of a lens where light rays appear to converge or to diverge from.
how is a convex lens drawn in a ray diagram?
a convex lens is drawn as a vertical line with outward facing arrows to indicate the shape of the lens.
what is the focal length?
The distance from the lens to the principal focus
define ‘converge’
Move towards each other.
define ‘diverge’
When a light ray splits up or spreads out.
describe the structure of a concave lens:
-how does this affect the rays? (2)
thinner in the middle than it is at the edges
-this causes parallel rays to diverge.
-They separate, but appear to come from a principle focus on the other side of the lens.
how is a concave lens drawn in a ray diagram?
-a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.
what can the images formed by a lens be? (3)
-upright or inverted (upside down compared to the object)
-magnified or diminished (smaller than the object)
-real or virtual
what is the difference between a real and virtual image? (2)
-A real image is an image that can be projected onto a screen.
-A virtual image appears to come from behind the lens.
how could you draw a ray diagram? (2)
-Draw a ray from the object to the lens that is parallel to the principal axis. Once through the lens, the ray should pass through the principal focus.
-Draw a ray which passes from the object through the centre of the lens.
what does an image formed by a convex lens depend on? (2)
-the lens used
-the distance from the object to the lens.
what do cameras and human eyes contain?
convex lenses
For a distant object that is placed more than twice the focal length from the lens, the image is: (3)
inverted
diminished
real
what di projectors contain?
convex lenses
For an object placed between one and two focal lengths from the lens, the image is:
inverted
magnified
real
what must a film that is being loaded into a projector be in order for the projected image to be the right way up?
film must be loaded into the projector upside down
describe the behaviour of light rays for an object that is viewed through a concave lens:
-light rays from the top of the object will be refracted and will diverge on the other side of the lens.
light rays from the top of the object will appear to come from a point: (2)
-the same side of the principal axis, meaning the image will be upright
-closer to the principal axis, so the image will be smaller than the object
what is the correlation between powerfulness and focal lens?
If a lens has a shorter focal length it is said to be more powerful.
equation for power of a lens:
power of lens (D) = 1/ focal length (m)
what is the power of a lens measured in?
dioptres
