Lenses

📖 In-Depth Theory

Types of Lenses

A LENS is a shaped piece of transparent material that refracts light to form images.

CONVERGING (CONVEX) LENS:

Thicker in the middle, thinner at the edges.

Bends parallel rays INWARD to meet at the FOCAL POINT.

Focal length (f): distance from lens to focal point.

Can form REAL images (on the other side of the lens) — can be projected on a screen.

Can also form VIRTUAL images (if object inside focal length) — appear on same side as object.

DIVERGING (CONCAVE) LENS:

Thinner in the middle, thicker at the edges.

Bends parallel rays OUTWARD — they appear to diverge from a VIRTUAL focal point.

Focal length: the virtual focal point is on the SAME side as the incoming light.

Always forms VIRTUAL, upright, diminished images — cannot be projected.

Ray Diagrams for Converging Lenses

THREE STANDARD RAYS for constructing images:

1. RAY PARALLEL to the principal axis → refracts THROUGH the focal point on the other side.

2. RAY THROUGH the CENTRE of the lens → passes straight through UNDEVIATED.

3. RAY THROUGH the FOCAL POINT on the near side → refracts PARALLEL to the principal axis.

Where two (or more) rays meet → location of the IMAGE.

IMAGES FORMED BY CONVERGING LENS:

Object beyond 2f: real, inverted, diminished image between f and 2f.

Object at 2f: real, inverted, same size as object, image at 2f on other side.

Object between f and 2f: real, inverted, magnified image beyond 2f.

Object at f: no image — rays parallel, never meet.

Object inside f: virtual, upright, magnified image on same side as object (magnifying glass).

MAGNIFICATION:

Magnification = image height ÷ object height = image distance ÷ object distance

Applications of Lenses

MAGNIFYING GLASS:

Converging lens used with object inside focal length.

Virtual, upright, magnified image.

Used by jewellers, stamp collectors, to read small print.

CAMERA:

Converging lens focuses real, inverted image onto sensor/film.

Object far from lens → image close to lens (just beyond f).

Zoom lens: variable focal length.

EYE:

Cornea and eye lens (both converging) focus light onto retina.

CILIARY MUSCLES change lens shape → adjust focal length (accommodation).

NORMAL VISION: image forms on retina.

CORRECTING VISION DEFECTS:

MYOPIA (short-sightedness): eye lens too powerful → image forms in FRONT of retina.

Corrected with DIVERGING lens — spreads light before it reaches the eye.

HYPEROPIA (long-sightedness): eye lens too weak → image would form BEHIND retina.

Corrected with CONVERGING lens — helps converge light before it reaches the eye.

TELESCOPES: two converging lenses — objective (large, long focal length) + eyepiece (small, short focal length).

MICROSCOPES: two converging lenses in series — objective very close to specimen.

⚠️ Common Mistake

A converging lens does NOT always form a real image. If the object is inside the focal length, it forms a VIRTUAL image (like a magnifying glass). Diverging lenses ALWAYS form virtual images. Real images can be projected; virtual images cannot.

📐 Variables

fFocal length (f) is measured in metres (m)

MMagnification (M) is measured in ()

📐 Key Equations

Magnification = image height ÷ object height = image distance ÷ object distance

📌 Key Note

Converging (convex) lens: focuses parallel rays at focal point. Forms real images (object beyond f) or virtual (object inside f). Three rays: parallel→focal point, through centre undeviated, through focal point→parallel. Magnification = image/object height. Myopia: diverging lens. Hyperopia: converging lens.

🎯 Matching Activity — Lenses and Images

Match each lens/object position to the type of image formed. — drag the symbols on the right to match the component names on the left.

Converging, object beyond 2f

Drop here

Converging, object inside f

Drop here

Diverging lens

Drop here

Myopia correction

Drop here

Virtual, upright, diminished image — always, regardless of object position

Virtual, upright, magnified image — same side as object (magnifying glass)

Real, inverted, diminished image — between f and 2f on other side

Diverging lens — diverges light before it reaches the eye

⭐ Higher Tier Only

Draw accurate ray diagrams for converging and diverging lenses for objects at various positions. Calculate magnification from image and object distances. Explain short-sightedness and long-sightedness and how each is corrected. Describe the eye as an optical instrument.

🔬 Triple Science Only

Lenses (physics only) — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. A converging lens has focal length 10 cm. Where should an object be placed to use it as a magnifying glass?

2. A person is short-sighted (myopic). What type of lens corrects this and why?

⭐ How Well Do You Understand This Topic?

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Types of Lenses

Ray Diagrams for Converging Lenses

Applications of Lenses

Mr. Badmus AI