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Grade 10-11 (age 15-17)

Light and Electromagnetic Waves

Light and Electromagnetic Waves

Light is an electromagnetic wave: electric and magnetic fields oscillate perpendicular to each other as they travel. It needs no medium and moves at about 3×10⁸ m/s in vacuum, with names from gamma rays to radio waves by wavelength. Visible light is only a tiny band of that spectrum. Slide the wavelength here to see which name each region of the spectrum gets.

Light is an EM Wave!
💡 Nature of Light
①Light is a wave: electric and magnetic fields oscillate perpendicular to each other and propagate
②Propagates without medium — even through space (vacuum c = 3×10⁸ m/s)
③By wavelength, named from gamma to radio
④Visible light is just a tiny part of the EM spectrum!
Electromagnetic Spectrum
550 nm
🌈 Range of Visible Light
①Visible: 380 nm (violet) ~ 780 nm (red)
②Violet (short λ) → high energy / Red (long λ) → low energy
③UV burns skin because its energy is higher than visible light
④Infrared used in remote controls and thermal cameras

Summary

ListEM Spectrum Characteristics
Gamma rays
Emitted by nuclear reactions; medical sterilization
λ < 0.01 nm
X-rays
Bone imaging; strong material penetration
0.01 ~ 10 nm
Ultraviolet
Sterilization, sunburn, vitamin D synthesis
10 ~ 380 nm
Visible Light
The only band detectable by human eyes
380 ~ 780 nm
Infrared
Heat energy, remote controls, thermal imaging
780 nm ~ 0.1 mm
Microwave
Microwave ovens, communications
0.1 mm ~ 10 cm
Radio waves
TV, radio, mobile communications
> 10 cm
Photon Energy E = hf
Photon Energy
E = hf = hcλ
h = 6.63×10⁻³⁴ J·s, c = 3×10⁸ m/s
✨ Wave-Particle Duality
①Light is both a wave (interference, diffraction) and a particle (photoelectric effect)
②Photoelectric effect: light hits metal, electrons emit
③Below threshold frequency, no emission no matter how intense
④This is because light acts as energy quanta hf (photons)
⑤Einstein won the 1921 Nobel for explaining the photoelectric effect
Worked Examples
Example 1
What is the wavelength of light with frequency 5×10¹⁴ Hz in vacuum? (c = 3×10⁸ m/s)
1
From c = fλ, λ = c/f.
λ = cf
2
Substitute c = 3×10⁸, f = 5×10¹⁴.
λ = 3×1085×1014 = 6×10-7 m
▸ 6×10⁻⁷ m (600 nm)
In vacuum every light wave travels at c. Use λ = c/f. 600 nm is in the orange range.
Example 2
Between light A (ultraviolet) and B (infrared), which has greater photon energy, and why?
1
Photon energy E = hf, so higher frequency means greater energy.
E = hf
2
Ultraviolet has a shorter wavelength than infrared, hence higher frequency (c = fλ).
λUV < λIR ⇒ fUV > fIR ⇒ EUV > EIR
▸ A (ultraviolet) has greater energy
Shorter wavelength means higher frequency and photon energy — why UV and X-rays are hazardous.
Summary
Speed
c = fλ
3×10⁸ m/s
Energy
E = hf
Photon energy
CSAT-style
Which lists these electromagnetic waves correctly from longest to shortest wavelength?
①Radio > Infrared > Visible > X-ray
②X-ray > Visible > Infrared > Radio
③Visible > Infrared > Radio > X-ray
④Infrared > Radio > X-ray > Visible
⑤Radio > Visible > Infrared > X-ray
▸ ① Radio > Infrared > Visible > X-ray
1
From longest wavelength: radio > microwave > infrared > visible > ultraviolet > X-ray > gamma.
Radio (long λ) → gamma (short λ)
2
Arranging the four given waves in this order gives option ①.
Radio > Infrared > Visible > X-ray
🎯 Exam Points
①All EM waves travel at c = 3×10⁸ m/s in vacuum
②Shorter λ → higher f, higher E (gamma rays most dangerous)
③Visible: 380 nm (violet) ~ 780 nm (red)
④Duality: interference/diffraction → wave; photoelectric → particle
⑤Photoelectric: E = hf − W (work function) — no emission below threshold
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Properties of Waves
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Total Internal Reflection
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