Thin Converging Lenses

L E N S

LENS

Learning Objectives • Describe the action of a thin converging lens on a beam of light. • Define the term focal length. • Draw ray diagrams to illustrate the formation of real and virtual images of an object.

What is a Lens? • Is any piece of transparent glass or plastic • Is specially moulded such that it either converges light rays passing through it to a point or makes them more divergent • Via this process forms images.

Converging VS Diverging Lens Converging Lens •Thicker in the middle than at the ends Eg:

Diverging lens •Thinner in the middle than at the ends Eg:

Converging VS Diverging Lens Converging Lens •Brings light rays travelling through it to one point. i.e. converges the light rays passing through it. Diverging lens •Spreads out the light rays travelling through it. i.e. diverges the light rays passing through it.

Main Features of a Thin Converging Lens •Optical centre For a symmetrical lens, this is the point midway between the lens surfaces on its principal axis. Optical Centre

Main Features of a Thin Converging Lens •Principal axis This is the imaginary line passing symmetrically through the optical centre of the lens.

Principal axis

c

Main Features of a Thin Converging Lens •Principal focus (F) All rays travelling close to and parallel to the principal axis will converge to a point on the principal axis.This point is known as the principal focus or focal point.

F

Main Features of a Thin Converging Lens •Focal length (f)This is the distance between the optical centre C and the principal focus F Optical centre

F Focal length (f)

Refraction due to Thin Converging lens

Refraction Rules for a Converging Lens RULE 1 A light ray travelling parallel to the principal axis refracts through the lens and travels through the focal point on the opposite side of the lens.

Refraction Rules for a Converging Lens RULE 2 A light ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

Refraction Rules for a Converging Lens RULE 3

Any ray that is either non-parallel to the principal axis or that which does not pass through the focal point will converge at the same point as the rays mentioned in rules 1 and 2 provided they all emanate from the same point.

Object and Image Distance

Object Distance

Image Distance

Exercise • Click on the link below and follow the instructions on the next slide.

LENS

Instructions for using Lens Applet Firstly, • click and change the light rays to “Principal rays” on the tab. • Click and drag the Object to any position • Observe how the image changes as your objects is moved: 5. Towards the lens; 6. Away from the lens.

Instructions for using Lens Applet Secondly, • click and change the refractive index of the lens by dragging on the tab. • Observe how the image changes as your refractive index of the lens 4. Is increased; 2. Is decreased.

Instructions for using Lens Applet Finally, • click and change the curvature radius of the lens by dragging on the tab. • Observe how the image changes as your radius of the lens 4. Is increased; 5. Is decreased. You may play with it and use what you have learnt to answer the questions in Worksheet Set A.

To Conclude • Read the “Thin Converging Lenses Notes” • Attempt Worksheet Set A , print it out and submit on next lesson.