Chapter 1 - Lenses Flashcards
What is the shape of a converging lens?
Convex (thicker in the middle)
What is the shape of a diverging lens?
Concave (thinner in the middle)
What does a converging lens do?
Causes incoming parallel light rays to converge at a single point
What does a diverging lens do?
Causes incoming parallel light rays to diverge away from each other
Define “focal length” (f)
The distance between its optical centre and its principal focus
Define “object distance” (u)
The distance from the object to the optical centre of the lens
Define “image distance” (v)
The distance from the optical centre of the lens to the image
Define “optical centre”
A point in a lens where light rays go through without refraction
Define “principal axis”
A line that passes through the optical centre and is perpendicular to the lens
Define “principal focus” / “focal point”
A point where rays of light parallel to the principal axis meet after going through the lens
Define “focal plane”
A plane that passes through the principal focus and is perpendicular to the principal axis
(All parallel beams of light meet at a point on the focal plane after going through a lens)
What happens to light rays passing through the optical centre?
Rays are not deviated
What happens to light rays parallel to the principal axis?
Rays will be refracted by the lens to pass through the focal point on the other side of the lens
What happens to light rays passing through the focal point?
Rays will be refracted by the lens to emerge parallel to the principal axis on the other side of the lens
What happens to light rays parallel to each other?
Rays will converge to a point on the focal plane
What happens to all light rays coming from the same point on the object (real image)?
Rays will meet at the corresponding point on the real image after passing through the lens
What happens to all light rays coming from the same point on the object (virtual image)?
Rays will appear to originate from the corresponding point on the virtual image after passing through the lens
Formula to calculate linear magnification
m = hi / ho = v/u
[ linear magnification = image height / object height = image distance / object distance ]
u = ∞
Characteristics of image
Real, inverted, diminished
u = ∞
Location of image
v = f
u = ∞
Uses
Objective lens of telescope
2f < u < ∞
Characteristics of image
Real, inverted, diminished
2f < u < ∞
Location of image
f < v < 2f
2f < u < ∞
Uses
Camera
u = 2f
Characteristics of image
Real, inverted, same size of object
u = 2f
Location of image
v = 2f
u = 2f
Uses
Photocopier
f < u < 2f
Characteristics of image
Real, inverted, magnified
f < u < 2f
Location of image
2f < v < ∞
f < u < 2f
Uses
Projective lens of microscope
u = f
Characteristics of image
Depends on usage
u = f
Location of image
v = ∞
u = f
Uses
Spotlight
u < f
Characteristics of image
Virtual, upright, magnified
u < f
Location of image
Same side as object
u < f
Uses
Magnifying glass
Lens formula
1/f = 1/u + 1/v
Lens formula: sign convention
- Real image: u and v are positive
- Virtual image: u is positive and v is negative (image on same side of lens as object)
A person with perfect vision will have blurred vision under water: Explain why
The bending effect for light travelling from water into the eye is different from light travelling from air into the eye. The rays of light no longer meet on the retina of the eye.
A person with perfect vision will have blurred vision under water: How can the person overcome this?
Wear goggles so that the light is still entering the eye from air
A person with a particular degree of short-sightedness will have perfect vision under water: Explain why
People with short-sightedness have the light focused in front of the retina. The difference in refractive indices between water and the eye being less than that between air and the eye, will mean that the bending effect on the rays entering from water is weaker. So the rays will meet further away (i.e. on the retina) and the person sees a sharp image.
A converging lens with a focal length of 20.0cm is used to create an image of the sun on a paper screen.
How far from the lens must the paper be placed to produce a clear image? Why?
The lens must be at a distance of f (20.0cm).
Rays of distant objects (considered to be parallel) will converge on a point along the focal plane on the opposite side of the lens.
