Waves - Lenses Flashcards
what is a convex lens
a lens which bulges outwards. causes rays of light that are parallel to converge at the principal focus
what is the principle focus of a convex lens
where the rays hitting the lens parallel to the axis all meet
what is the focal length
distance from the centre of the lens to the principal focus
there are principal focuses on each side of a convex lens. which focus will rays focus on
the one to the far side of the lens in relation to where the rays are coming from
an incident ray that passes through the principal focus before meeting the lens will refract how
refracts through the lens are travels parallel to the axis
what is a concave lens
causes parallel rays of light to diverge
what is the principal focus point of a concave lens
the point where rays hitting the lens parallel to the axis appear to all come from. So if you trace back the refracted rays its the point where they all meet
what happens if an incident ray passes through the centre of a concave lens
carriers on in same direction
what happens if an incident ray is traveling towards the far side principal focus point of a concave lens
refract to travel in parallel to the axis
how is a real image formed
light rays from a point on an object come together to form an image. The light rays pass through the same point. real images can be captured on a scree.
how are virtual images formed
light rays from a point on an object are diverging after they have left the lens. so light from the point on the object appears to be coming from a different place. Rays don’t pass through that point, they just appear to.
what 3 things do you have to say when you are describing an image
how big it is compared to the object
whether its upright or inverted
whether its real or inverted
what is a ray diagram
a diagram that shows the path taken by light rays through a lens
how are convex and concave lens shown on a ray diagram
convex is a double sided arrow
concave is a double sided arrow with the arrow head going the other way
what is the first thing you need to do when drawing a ray diagram for convex
draw a line from the top of the object that passes through the middle of the lens without changing direction
after you have drawn a line from the top of the object going through the middle of the lens what do you need to do in your ray diagram for convex
draw a second ray from the top of the object that travels parallel to the axis. The ray should refract at the lens so it passes through the principle focus point.
If the 2 rays don’t meet you need to extend the refracted rays back on the near side of the lens as virtual rays ( dotted lines)
what are the 3ed and 4th rays you draw in a ray diagram for convex
you do the same as the first 2 rays but from the bottom of the object
where do you draw your image on a ray diagram for convex
between the 2 meeting points of the four rays
if the bottom of your object is on the axis then the bottom of your image will be too
what does the type of image produced by a convex lens depend on
where the object is placed in relation to the priciple focus point
what is beyond 2F
an object beyond 2F will produce a real, inverted image that is is smaller than the object. The image will sit between f and 2f on the far side of the lens
the point 2F is the length of 2 principle focus points behind the lens
what is at 2F
an object that is placed at 2F will produce a real inverted image that is the same size as the object. The image will sit at 2F on the far side to the lens
Between F and 2F
an object placed between F and 2F will produce a real inverted image thats bigger than the object. It will sit beyond 2F on the far side of the lens
between f and the lens
virtual image that is the right way up but bigger than the object. It will be on the same side of the lens
how do you draw a ray diagram for a concave lens
draw ray from the top of the object that passes straight through centre of the lens. Then draw a ray from top of object parallel to axis. This ray should refract from the lens as if it has come straight from the principle focus.
draw a dotted ( virtual ray) from the focus to this point
repeat the process at the bottom of the object.
draw the image. The top is where the ray that passes through the middle of the lens meets the virtual ray going to the top. The same principle applies to the bottom of the object
