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Cheat Sheet: Mirrors And Lenses

1. Spherical Mirrors

1.1 Definition & Types

TermDefinition
Spherical mirrorMirror whose reflecting surface is part of a hollow glass sphere; surface may curve inwards or outwards
Concave mirrorReflecting surface curved inwards
Convex mirrorReflecting surface curved outwards
Plane mirrorFlat reflecting surface

1.2 Manufacture & Coating

FeatureDetail
ShapePart of an imaginary hollow sphere
ManufactureMade by grinding and polishing a flat piece of glass into a curved surface
Coating on outer curved surfaceProduces a concave mirror
Coating on inner curved surfaceProduces a convex mirror

1.3 Image Characteristics & Comparison

MirrorImage characteristics
Concave (object close)Image erect and enlarged
Concave (object far)Image inverted; size decreases as distance increases
ConvexImage always erect and diminished; size decreases as distance increases
PlaneImage always erect and same size as object
Common featureLateral inversion (left-right reversal)

1.4 Uses

Mirror typeApplication
Concave mirrorReflectors in torches
Concave mirrorHeadlights and scooter reflectors
Concave mirrorDental mirrors provide enlarged view when held close
Concave mirrorMain mirror in reflecting telescopes
Convex mirrorSide-view mirrors on vehicles
Convex mirrorMirrors at road intersections and sharp bends for visibility
Convex mirrorSecurity mirrors in big stores for monitoring

2. Laws of Reflection

2.1 Terms and Meanings

TermMeaning
Incident rayIncoming light ray that strikes the mirror
Reflected rayOutgoing light ray that bounces off the mirror
NormalImaginary line perpendicular to the mirror at the point of incidence
Angle of incidence (i)Angle between incident ray and the normal
Angle of reflection (r)Angle between reflected ray and the normal

2.2 The Two Laws

2.2.1 First Law of Reflection

LawStatement
First law of reflectionAngle of incidence equals angle of reflection; i = r
Special caseIf incident ray is along the normal then i = r = 0

2.2.2 Second Law of Reflection

LawStatement
Second law of reflectionIncident ray, normal, and reflected ray lie in the same plane

2.3 Application to Mirrors (Parallel Beams)

MirrorEffect on parallel beams
Plane mirrorReflected beams remain parallel
Concave mirrorReflected beams converge
Convex mirrorReflected beams diverge

3. Lenses

3.1 Definition & Types

TermDefinition
LensPiece of transparent material with curved surfaces
Convex lensThicker at the middle than at the edges
Concave lensThicker at the edges than at the middle
Magnifying glassLens that enlarges small print
Light interactionLenses transmit light; objects are seen through a lens rather than by reflection

3.2 Image Characteristics

LensImage characteristics
Convex lens (small object distance)Image erect and enlarged
Convex lens (increasing object distance)Image becomes inverted and size decreases as distance increases
Concave lens (any distance)Image always erect and diminished; size decreases as distance increases

3.3 Convergence and Divergence

ElementEffect on parallel beams
Thin glass plateParallel beams pass through unchanged
Convex lensConverges parallel beams (converging lens)
Concave lensDiverges parallel beams (diverging lens)

3.4 Burning by Concentrated Light

ElementObservation
Concave mirrorConverges sunlight to a small point; can burn paper
Convex lensConverges sunlight to a small point; can burn paper

3.5 Uses of Lenses

UseApplication
EyeglassesHelp people see clearly
Cameras, telescopes, microscopesUse lenses to capture or magnify images
Human eyeContains a convex lens that changes shape to focus on near or far objects
The document Cheat Sheet: Mirrors And Lenses is a part of the Class 8 Course Science Curiosity Class 8 - New NCERT.
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