Thin LensChapter 15.2

Bending of Light• Any transparent object that is curved with affect

the path of light rays.• Ex:

o Glass bottle full of water will affect the image of an object.o Your eye’s muscles expand and contract causing the lens in your eye to

change shape and therefore allow people to focus on different objects.

Lens• Lens – A transparent object that refracts light

rays, causing them to converge or diverge to create an image.

Converging Lens• Converging Lens – A lens where the middle part

of the lens is thicker than the rim.• Also known as a convex lens.

Diverging Lens• Diverging Lens – A lens where the middle part of

the lens is thinner in the middle and thicker around the rim.

• Also known as a concave lens.

Focal Length• Each lens has two focal points (F), therefore there

are two focal length (f).• A focal length is measured from the focal point to

the center of the lens (normal).• The focal points lay on the principal axis.

Converging & Diverging Focal Points• For converging lens, the focal point is located at

the point where the rays of light meet at a single point.

• For diverging lens, the focal point is located at a point from which the diverged rays appear to originate.

Rules For Drawing Ray Diagrams of Converging

LensRay From object to lens From lens to imageParallel Ray Parallel to Principal

AxisPasses through focal Point

Central Ray To the center of lens (Where normal meets principal axis)

Straight through from the center of the lens

Focal Ray Passes through front focal point

Parallel to principal axis

Ray Diagrams for Converging Lens

Ray Diagrams for Converging Lens

Ray Diagrams for Converging Lens

Rules For Drawing Ray Diagrams of Diverging LensRay From object to lens From lens to imageParallel Ray Parallel to Principal

AxisDirected away from front focal point

Central Ray To the center of lens (Where normal meets principal axis)

Straight through from the center of the lens

Focal Ray Proceeding towards back focal point

Parallel to principal axis

Ray Diagrams for Diverging Lens

Characteristics of Converging Lens

• Image is always real, unless object is in front of the focal point.

• Image is always on the opposite side the lens, unless the object is in front of the focal point.

• Image is always inverted, unless object is in front of the focal point.

Characteristics of Diverging Lens

• Image is always virtual (Imaginary).

• Magnification is less than 1.

• Image is always upright.

• Image is formed on the same side as the object.

Thin-Lens Equation

p = Object Distance- Always Positive

q = Image Distance- Negative ( Object side of lens)- Positive (Opposite side of Lens)

f = Focal Length- Positive (Converging Lens)- Negative (Diverging Lens)

Magnification Equation

M = Magnification- Positive (Upright Image)- Negative (Inverted Image)- M>1 (larger Image) M<1 (Smaller

Image)h = Object Heighth‘ = Image Heightp = Object Distanceq = Image Distance

Example Problem• An object is placed 30.0cm in front of a

converging lens that has a focal length of 10.0cm. Find the image distance and magnification.

Example Problem Answer