Lenses

Lenses

Focal Point

Focal Length

Animation of Light in a Lens

Ray Diagrams for Converging Lenses1. Parallel incoming light is refracted through the focal point

2. Light coming in through the focal point is refracted out parallel

3. Rays passing through the center are refracted out in the same direction.

4. An image is formed where the rays cross.

A parallel beam of light is sent through an aquarium. If a convex glass lens is held in the water, it focuses the beam…

1. closer to the lens than…

2. at the same position as…

3. farther from the lens than…

…before.

A parallel beam of light is sent through an aquarium. If a convex glass lens is held in the water, it focuses the beam…

1. closer to the lens than…

2. at the same position as…

3. farther from the lens than…

…before.

The index of refraction (n) between water and glass is less than it is between air and glass. Therefore the light in the water bends less and if focused farther away.

A real image is one where the light rays actually come from the image location.

Real Image

Virtual Image

A virtual image is one where the light rays do not actually come from the image location, but rather only seem to.

Lens Equations: Distances & Focal Points:

1p 1

q1

f

•Focal Length for a converging lens is positive (+).

•There is something called a diverging lens which has a negative focal length, but we have not (& will not) talk about that type of lens.

Sign Conventions

Quantity symbol

Front Back

Object Location

p + -

Image Location

q - +

Focal Length

f + -

Converging Diverging (not doing diverging lenses in

this class)

Quantity symbol Front Back

Object Location

p + -

Image Location

q - +

Focal Length f + -

Real Image

Front of lens

Back of lens

Object is in front of the lens so p = +

Image is behind the lens so q = +

Front of lens

Back of lens

Object is in front of the lens so p = +

Image is on front of the lens so

q = -

Virtual Image

Quantity symbol Front Back

Object Location

p + -

Image Location

q - +

Focal Length f + -

Lens Equations: Magnification

M h h

qp

Note the negative sign

Quantity symbol Upright Image

Inverted Image

Image Height

h’ +

Magnification

M +

If M is < 1 the image is smaller than the objectIf M is > 1 the image is larger than the object

Signs for magnification

Real Image

Quantity symbol Upright Image

Inverted Image

Image Height

h’ +

Magnification

M +Image is inverted

(upside down) therefore h’ = -

The lens projects an image of the candle on a wall. How will the image differ if the top half of the lens is covered with a red filter and the bottom half with a green filter?

Lens