# Lenses.

## Presentation on theme: "Lenses."— Presentation transcript:

Lenses

Lenses

Focal Point Focal Length

Animation of Light in a Lens

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

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

4. An image is formed where the rays cross.
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
A parallel beam of light is sent through an aquarium. If a convex glass lens is held in the water, it focuses the beam… closer to the lens than… at the same position as… farther from the lens than… …before.

A parallel beam of light is sent through an aquarium
A parallel beam of light is sent through an aquarium. If a convex glass lens is held in the water, it focuses the beam… 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. closer to the lens than… at the same position as… farther from the lens than… …before.

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

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:
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.

Diverging (not doing diverging lenses in this class)
Sign Conventions Quantity symbol Front Back Object Location p + - Image Location q Focal Length f Diverging (not doing diverging lenses in this class) Converging

Real Image Front of lens Back of lens
Quantity symbol Front Back Object Location p + - Image Location q Focal Length f Image is behind the lens so q = + Object is in front of the lens so p = + Real Image Front of lens Back of lens

Front of lens Back of lens Virtual Image
Quantity symbol Front Back Object Location p + - Image Location q Focal Length f Front of lens Virtual Image Back of lens Image is on front of the lens so q = - Object is in front of the lens so p = +

Lens Equations: Magnification
Note the negative sign

Signs for magnification
Quantity symbol Upright Image Inverted Image Image Height h’ + Magnification M If M is < the image is smaller than the object If M is > 1 the image is larger than the object

Image is inverted (upside down) therefore h’ = -
Quantity symbol Upright Image Inverted Image Image Height h’ + Magnification M Image is inverted (upside down) therefore h’ = - Real Image

The lens projects an image of the candle on a wall
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