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Grade 10-11 (age 15-17)

high school physics Light and Electromagnetic Waves

Light is an electromagnetic wave: electric and magnetic fields oscillating at right angles to each other as it spreads out. It needs no medium and travels through a vacuum at about 3 × 10⁸ m/s, and its name changes with wavelength, from gamma rays to radio waves. The visible light our eyes can see is only a very narrow part of the electromagnetic spectrum, and the shorter the wavelength, the larger the energy of a single photon, E = hf. Move the wavelength slider and the marker slides along the spectrum, changing the band's name, the frequency and the photon energy, while in the lower figure the wave's crowding and the energy bar change together.

Light Is an Electromagnetic Wave

💡 What light is
①Light is a wave of electric and magnetic fields oscillating at right angles to each other as it spreads out
②It needs no medium, so it crosses empty space, moving at c = 3 × 10⁸ m/s in a vacuum
③Its name changes with wavelength, from gamma rays to radio waves
④The visible light we see is only a very narrow part of the electromagnetic spectrum

The Electromagnetic Spectrum

550 nm
🌈 The range of visible light
①Visible light runs from 380 nm (violet) to 780 nm (red)
②Violet light has a short wavelength and high photon energy; red light has a long wavelength and low energy
③Ultraviolet carries more energy than visible light, which is why it burns skin
④Infrared is used in remote controls and thermal cameras
⑤The spectrum is drawn on a log scale, so equal lengths mean equal factors

Summary

EM Spectrum Characteristics
Gamma rays
Given off in nuclear reactions; used to sterilize medical equipment
λ < 0.01 nm
X-rays
Passes easily through matter; used for bone images
0.01 ~ 10 nm
Ultraviolet
Sterilization, sunburn, vitamin D production
10 ~ 380 nm
Visible Light
The only band the human eye can sense
380 ~ 780 nm
Infrared
Carries heat; used in remote controls and thermal cameras
780 nm ~ 0.1 mm
Microwave
Microwave ovens and communications
0.1 mm ~ 10 cm
Radio waves
TV and radio broadcasting, mobile phones
> 10 cm

Photon Energy E = hf

Photon energy
E = hf = hcλ
h = 6.63 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s
✨ The dual nature of light
①Light is a wave that shows interference and diffraction, and also a particle that causes the photoelectric effect
②In the photoelectric effect, light shining on a metal knocks electrons out
③Below a threshold frequency, no electrons come out however strong the light is
④This is because light acts as packets of energy hf, called photons
⑤Einstein received the 1921 Nobel Prize in Physics for explaining the photoelectric effect

Worked Examples

Example 1
In a vacuum, what is the wavelength of light with frequency 5 × 10¹⁴ Hz? Use c = 3 × 10⁸ m/s.
1
From c = fλ, λ = c/f.
λ = cf
2
Put in c = 3 × 10⁸ and f = 5 × 10¹⁴.
λ = 3×1085×1014 = 6×10-7 m
▸ 6×10⁻⁷ m (600 nm)
In a vacuum every light travels at the same speed c, so the wavelength is λ = c/f. 600 nm is orange light.
Example 2
Between light A (ultraviolet) and B (infrared), which has greater photon energy, and why?
1
Photon energy is E = hf, so the higher the frequency, the larger the energy.
E = hf
2
By c = fλ, ultraviolet, with its shorter wavelength, has a higher frequency than infrared.
λUV < λIR ⇒ fUV > fIR ⇒ EUV > EIR
▸ A (ultraviolet) has greater energy
The shorter the wavelength, the higher the frequency and the photon energy. That is why ultraviolet and X-rays are harmful to the body.

Summary

Speed
c = fλ
3 × 10⁸ m/s in a vacuum
Energy
E = hf
The energy of one photon
CSAT-style problem, adapted
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 the longest wavelength: radio > microwave > infrared > visible > ultraviolet > X-ray > gamma.
Radio (long λ) → gamma (short λ)
2
Putting the four given waves in this order gives ①.
Radio > Infrared > Visible > X-ray
🎯 Exam Points
①All electromagnetic waves travel at the same speed c = 3 × 10⁸ m/s in a vacuum
②The shorter the wavelength, the higher the frequency and photon energy; gamma rays are the highest
③Visible light runs from 380 nm (violet) to 780 nm (red)
④Interference and diffraction show wave behavior, and the photoelectric effect shows particle behavior
⑤The maximum kinetic energy of photoelectrons is Ek = hf − W, where W is the metal's work function
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Properties of Waves
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Total Internal Reflection

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