Light

What you'll learn

Laws of reflection; images in plane mirrors.
Concave and convex mirrors — uses, image formation.
Refraction; convex and concave lenses — uses, ray diagrams.
Structure of the human eye; defects of vision and corrections.

Key concepts

Nature of light

Light is a form of energy that travels in straight lines (rectilinear propagation).
Speed of light in vacuum: 3 × 10⁸ m/s.
Light is a transverse electromagnetic wave — does not need a medium.

Reflection of light

Reflection: bouncing back of light from a surface.

Laws of reflection:

The angle of incidence = angle of reflection (∠i = ∠r).
The incident ray, reflected ray, and normal at the point of incidence all lie in the same plane.

Types of reflection:

Regular (specular) reflection: smooth surface → parallel rays reflect parallel → clear image (mirrors).
Diffuse (irregular) reflection: rough surface → parallel rays scatter → no clear image (paper, walls). This is why we can read a book without glare.

Plane mirror — image properties

Property	Value
Size	Same as object
Distance	Same as object (behind mirror)
Nature	Virtual (cannot be projected on screen)
Orientation	Laterally inverted (left↔right flipped)
Upright/inverted	Erect (same way up)

Curved mirrors

Feature	Concave mirror	Convex mirror
Shape	Curves inward (cave)	Curves outward
Focus	Real focus (in front of mirror)	Virtual focus (behind mirror)
Image (distant object)	Real, inverted, diminished	Virtual, erect, diminished
Image (nearby object)	Can be virtual, erect, magnified	Virtual, erect, diminished (always)
Uses	Shaving/makeup mirror, dentist mirror, torch/headlights, solar cooker, telescope	Rear-view mirror, side mirror, security/surveillance mirrors, street lights
Key advantage	Magnifies; concentrates light	Wider field of view

Terms:

Principal axis: line through centre of curvature and pole.
Focus (F): point where parallel rays converge (concave) or appear to diverge from (convex).
Centre of curvature (C): centre of the sphere; C = 2F from pole.
Focal length (f): distance from pole to focus; f = R/2 (R = radius of curvature).

Refraction of light

Refraction: bending of light as it passes from one medium to another (due to change in speed).

Light bends towards the normal when going from less dense to more dense medium (e.g., air → glass).
Light bends away from the normal when going from dense to less dense medium (e.g., glass → air).

Everyday examples of refraction:

A pencil in water appears bent/broken.
A swimming pool appears shallower than it is.
Stars twinkle (light refracts through different air density layers).
Mirages (hot desert air refracts light from sky).
Rainbows (water droplets refract and disperse sunlight into spectrum).

Refractive index (n): ratio of speed of light in vacuum to speed in medium.

n = c / v; for glass n ≈ 1.5; for water n ≈ 1.33.
Higher refractive index → light slows more → bends more.

Lenses

Feature	Convex (converging) lens	Concave (diverging) lens
Shape	Thicker at centre	Thinner at centre
Effect on rays	Converges parallel rays	Diverges parallel rays
Focus	Real focus (on other side)	Virtual focus (same side as object)
Image (distant object)	Real, inverted, diminished	Virtual, erect, diminished
Image (nearby — inside F)	Virtual, erect, magnified	Virtual, erect, diminished
Uses	Camera, microscope, telescope, magnifying glass, projector, correcting hypermetropia (long sight)	Correcting myopia (short sight), peephole in doors

Lens formula: 1/f = 1/v − 1/u (not required at class 8, but useful) Power of lens (P) = 1/f (in metres); unit: Dioptre (D). Convex = +ve; Concave = −ve.

The human eye

Part	Function
Cornea	Transparent front surface; does most of the focusing (~70% of total)
Pupil	Opening in iris; controls amount of light entering
Iris	Coloured part; muscles control pupil size
Lens	Fine-focuses image on retina; can change shape (accommodation)
Retina	Light-sensitive layer; rods (dim light/B&W) and cones (colour/bright)
Optic nerve	Carries signals to brain
Blind spot	No photoreceptors where optic nerve exits; no vision here
Vitreous humour	Jelly-like substance filling eyeball

Persistence of vision: image stays on retina for ~1/16 s after light is removed → basis of cinema (24 fps).

Defects of vision

Defect	Problem	Cause	Correction
Myopia (short sight / near-sightedness)	Can see near, not far	Eyeball too long OR lens too curved → image forms in front of retina	Concave lens (diverges rays before entering eye)
Hypermetropia (long sight / far-sightedness)	Can see far, not near	Eyeball too short OR lens too flat → image forms behind retina	Convex lens (converges rays before entering eye)
Presbyopia	Can't focus close up (old age)	Lens loses elasticity	Convex lens (reading glasses) or bifocals
Astigmatism	Blurred at all distances	Irregular shape of cornea/lens	Cylindrical lenses
Cataract	Cloudy vision	Lens becomes opaque	Surgery (lens replacement)
Colour blindness	Can't distinguish red/green	Absent cone type	No cure; managed with special lenses

Quick check

State the two laws of reflection.
What type of image is formed by a convex mirror? Why is it used as a rear-view mirror?
What is refraction? Why does a pencil look bent in water?
Compare convex and concave lenses — shape, effect, and one use each.
A person can see objects clearly only up to 1 m. What defect does this suggest? Which lens corrects it?

Open the Practice tab for graded questions on Light.