Reflection: light bounces
When light hits a surface and bounces back, that's reflection — and it obeys one tidy rule: the angle of incidence equals the angle of reflection, both measured from the normal (an imaginary line perpendicular to the surface). Smooth surfaces like mirrors reflect in an organized way and form images; rough surfaces like paper scatter light in all directions, which is why you can read a page from any angle.
Plane (flat) mirrors produce images that look real enough but are actually virtual: they appear to be behind the mirror, upright, the same size, and flipped left-right. That flip is why text on your shirt reads backward in the mirror.
- Angle of incidence = angle of reflection (measured from the normal)
- Normal = perpendicular line to the surface at the point of impact
- Smooth = organized reflection (images); rough = scattered reflection
- Plane mirror image: virtual, upright, same size, left-right flipped
A laser hits a flat mirror at 35° from the normal. By the law of reflection it bounces off at 35° from the normal, on the other side of the normal. Total turn between incoming and outgoing beams: 180 − 35 − 35 = 110°.
Refraction: light bends
When light crosses from one medium to another — air to water, air to glass — its speed changes and its path bends. That's refraction. The rule of thumb: entering a slower medium (like water), light bends toward the normal; exiting into a faster one, it bends away. This bend is why a straw in a glass looks snapped at the waterline and why pools look shallower than they are.
Refraction is also why prisms and raindrops split white light into a rainbow. Different colors travel at slightly different speeds in glass, so each bends by a slightly different amount — violet most, red least — fanning the beam into the spectrum: red, orange, yellow, green, blue, indigo, violet.
- Refraction = bending when light changes speed between media
- Into a slower medium → bends toward the normal
- Into a faster medium → bends away from the normal
- Dispersion: each color bends differently → rainbows (ROYGBIV)
- Lenses are refraction on purpose — curved glass bending rays to a plan
Lenses and color
A lens uses refraction to focus or spread light. Convex lenses (thicker in the middle) converge rays to a focal point — magnifying glasses, camera lenses and your eye's lens all work this way. Concave lenses (thinner in the middle) spread rays apart and correct nearsightedness in glasses.
Color: white light is all colors mixed. An object's color is the light it reflects — a red apple absorbs most colors and bounces red back to your eye. The sky is blue because air molecules scatter shorter blue wavelengths across the sky; at sunset, light travels a longer path, the blue scatters away entirely, and reds and oranges are what's left.
- Convex lens: converges light — magnifying glass, eye, camera
- Concave lens: diverges light — nearsighted glasses
- Focal point: where parallel rays cross after a convex lens
- Object color = the light it reflects; the rest is absorbed
- Blue sky = scattered blue light; red sunset = long path, blue filtered out
A magnifying glass forms an image three times the size of a 5 mm ant. Magnification = image size ÷ object size = 15 ÷ 5 = 3×. If the same ant's image measured 20 mm, magnification would be 4× — same object, stronger effective lens.
Key concepts to memorize
🎯 Study tips for this topic
- Always draw the normal before any angle — most errors come from measuring from the surface instead.
- Remember 'slow toward, fast away': entering a slower medium bends light toward the normal.
- Connect lenses to your own eyes — glasses are just corrective refraction you wear.
- Quiz yourself with household items: spoon reflections, a straw in water, a garden hose mist rainbow.
- Drill the ROYGBIV order and which color bends most (violet) and least (red).
