Waves, sound & light · Topic 12 of 17

Light & the Electromagnetic Spectrum

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Physics · light

EM waves, light included, travel at 3.00 × 10⁸ m/s in a vacuum; light reflects at equal angles and bends when its speed changes.

01 · Speed of lightc ≈ 3.00 × 10⁸ m/s

02 · EM spectrumRadio→micro→IR→visible→UV→X-ray→gamma

03 · ReflectionAngle in = angle out (from the normal)

Refraction = light bending as it enters a medium of different speed must know

All EM waves: transverse, no medium needed, speed c in a vacuum

Light travels in straight lines (rays) until it reflects or refracts

Light outruns sound — you see lightning before thunder

Speed of light in a vacuum c ≈ 3.00 × 10⁸ m/s must know

Index of refraction n = c ÷ v (always ≥ 1) must know

Visible ≈ 400–750 nm: red = longest λ, violet = shortest λ

n: air ≈ 1.00, water 1.33, glass ≈ 1.5, diamond 2.42

Snell's law: n₁ sin θ₁ = n₂ sin θ₂ (angles from the normal)

EM spectrum (long λ → short λ): radio, micro, IR, visible, UV, X-ray, gamma must know

Reflection: angle of incidence = angle of reflection (from the normal) must know

Higher frequency = shorter λ = more energy and penetration

Slows down → bends toward the normal; speeds up → bends away

n = 3.00 × 10⁸ ÷ 2.25 × 10⁸ ≈ 1.33 (water)

v = c ÷ n = 3.00 × 10⁸ ÷ 2.4 = 1.25 × 10⁸ m/s

25° from the surface = 65° from the normal → reflects at 65°

Commonly confused

	Reflection	Refraction
What happens	Light bounces off a surface	Light bends entering a new medium
The rule	Angle of incidence = angle of reflection	Snell's law: n₁ sin θ₁ = n₂ sin θ₂
Speed of light	Unchanged	Changes (v = c ÷ n)
Angles measured from	The normal	The normal

Measure angles from the normal, not the surface (90° − surface angle)

Show 4 more facts

Dispersion: white light spreads into colors (prism, rainbow)

Smooth surface: mirror reflection; rough surface: diffuse (scattered)

UV, X-rays and gamma rays can damage cells (cancer risk)

Uses: radio/MRI, microwave ovens/radar, IR thermal imaging, X-ray diagnosis

Pearl

Slow down, bend toward the normal; speed up, bend away.

Practice this topic, then test the whole section.

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