Thin Converging Lenses Notes

Coral Secondary School Secondary Two LSS Science (Physics) 2009 Notes Topic Light IV: Thin Converging Lenses Lenses •

A lens is a piece of clear plastic or glass with curved surfaces.

•

Lenses are widely used in many optical instruments such as spectacles, cameras and projectors.

•

When parallel light rays hit on different parts of a lens surface, the rays will have different incident angles (due to the curved surface). Each ray will thus refract by different amount. Light refracts the most at the outermost part of the lens while no refraction occurs in the middle. Depending on the curvature of the lens, light rays will either converge or diverge.

Converging lens

•

Diverging lens

Converging lenses brought parallel light rays to focus at a point (light rays converge). Diverging lenses spread out the parallel light rays (light rays diverge).

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Types of lenses:

double convex

pianoconvex

convex meniscus

2009, Ronald Tan, Coral Secondary School (Modified from Kuah H.M 2008)

double concave

pianoconcave

concave meniscus

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Converging lens

Diverging lens

Ray diagrams of thin converging lens

Definitions (1)

Optical centre C :

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the midway between the lens surfaces on its principle axis. Rays passing through optical centre are not refracted (2)

Principal axis: the horizontal line passing symmetrically through the optical centre of the lens

(3)

Focal point F (also known as the principle focus) the point at which all rays parallel to the principle axis converge to after refraction by the lens

(4) Focal length, f the distance between the optical centre C and the focal point F 2009, Ronald Tan, Coral Secondary School (Modified from Kuah H.M 2008)

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(5)

Focal plane the plane which passes through F and P (the focal plane is perpendicular to the principal of axis)

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For thin converging lens, 1.

Light rays passing through the optical centre are not refracted.

2.

light rays parallel to the principle axis will converge to pass through the focal point F.

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Constructing ray diagrams of thin converging lens (1) Draw the principle axis (a horizontal line) (2) Draw the lens (a line perpendicular to the horizontal line – with arrowheads) (3) Mark the optical centre C (4) Mark the focal point F on the axis (each lens will have 2 focal points; one focal point on each sides of the lens) (if the lens is symmetrical, the focal lengths will be the same)

(2) (1)

(4)

(3)

(4)

F

C

F f

(5)

Place the object on the left of the lens and mark the object as O (light rays are usually drawn as travelling from left to right)

2009, Ronald Tan, Coral Secondary School (Modified from Kuah H.M 2008)

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(2)

The distance between the object and the optical lens are usually marked as u.

(5) O C

F u (6) (7)

F f

Draw the light paths as shown (light rays passing through the optical centre are not refracted; light rays parallel to the principle axis will converge to pass through the focal point F)

(8)

The point where the two paths intercept on the right side of the lens is the position of the image. Draw the image (dotted) and mark the image’. The image distance, v, is measured from the lens to the image.

O

v

(7) (7)

F C

F

(8) image

u

f

2009, Ronald Tan, Coral Secondary School (Modified from Kuah H.M 2008)

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2009, Ronald Tan, Coral Secondary School (Modified from Kuah H.M 2008)

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