LENSES (chapter one) Flashcards
Describe the ray point of view
- Light travels out in straight lines from a small source
- Light in a parallel beam/from a very distant source has rays approximately parallel to each other
Describe the wave point of view
- Light spreads out in spherical waves from a small source(like ripples)
- Wave fronts from a parallel beam/distant source have zero curvature (do not say they are parallel)
What does a lens do in the ray point of view?
Bends the rays to bring them to a focus
What does a lens do in the wave point of view?
Adds curvature to the wave, centering it on the focus
How does a lens add curvature to a wave?
The middle is slowed down more than the edges as the middle of the lens is thicker than the edges. This ‘bends’ the wave.
What is the distance from lens to focus called?
focal length, f
Does a powerful lens have a small or large focal length?
small
What is the power of a lens?
How much curvature it has added
=1/focal length
What is the power of a lens measured in?
Dioptre
no s
What is v?
image distance
distance of image formed from lens
What is u?
Object distance
distance of original object from lens
What is the rule for curvature and lenses?
1/v=1/u+1/f
Curvature of wave after lens=curvature before lens+ curvature added by lens
What is the cartesian sign convention?
All distances to the left of the lens are negative (u) and all to the right are positive (v)
What does a negative magnification mean?
The image will be inverted
What does a magnification of less than one mean?
The image will become smaller
What is the formula for linear magnification?
image height/object height
m=v/u
What causes light to slow down when travelling through a lens?
Refraction
What is the formula for refractive index?
speed of light in free space/speed of light in that medium
What happens when an object is placed at the focal length?
No image will be formed
The rays radiate from the source with curvature, the lens will curve them more to parallel, meaning no focus point will ever be reached
When will an image be formed at the focal length?
When the source is distant
so that wavefronts are parallel
What happens when light is from a nearby source?
The wave still has some curvature when it goes through the lens, meaning the image will be formed FURTHER than the focal point
What happens when the source is closer than the focal length?
The image distance will be negative and so a virtual image will be formed on the left side of the lens
What is a virtual image?
An image which forms on the same side of the lens as the source (eg. in a magnifying glass)
When is f positive?
In a converging lens
