Physics: waves and electromagnetic radiation (grade 11) · lesson

Speed, frequency, and wavelength

💡 Wave speed = frequency × wavelength (v = fλ).

Frequency is how many waves pass a point each second, in hertz (Hz). Wavelength is the distance from one crest to the next. Amplitude is the height of a wave above its rest position. Period is the time for one wave: period = 1 ÷ frequency.

When a wave enters a new material, its frequency stays the same but its speed changes. Since v = fλ, the wavelength changes in the same direction as the speed.

🧠 How to think about it

Sound at 340 m/s and 170 Hz: λ = v ÷ f = 340 ÷ 170 = 2 m. If sound moves into water (1,500 m/s) with the same frequency, the faster speed means a longer wavelength.

frequency: waves per second, in hertz
wavelength: distance from crest to crest
period: time for one full wave

The electromagnetic spectrum

💡 Electromagnetic waves all travel at the same speed in a vacuum, so higher frequency means shorter wavelength.

From longest to shortest wavelength: radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma ray.

Electromagnetic waves can cross empty space. Sound and ocean waves are mechanical waves: they need matter to travel through.

In a vacuum, every EM wave moves at the same speed. So if frequency doubles, wavelength must halve.

🧠 How to think about it

With v fixed, f and λ move in opposite directions. A radio wave at 1.0 × 10⁸ Hz: λ = (3.0 × 10⁸) ÷ (1.0 × 10⁸) = 3 m.

electromagnetic wave: a wave of electric and magnetic fields that can cross empty space

🔗 Khan Academy: Electromagnetic radiation · Khan Academy (free) ↗

Energy and matter

💡 Higher-frequency light carries more energy per photon, and absorbed waves can heat matter.

Light comes in packets called photons. A photon's energy depends on its frequency. Ultraviolet photons carry enough energy to break bonds in DNA. Radio photons do not.

When food absorbs microwaves, its water molecules move faster. Faster particles mean a higher temperature. Microwaves can't make food radioactive or split atoms.

🧠 How to think about it

To compare the danger of two kinds of radiation, compare frequency, not brightness or speed. Higher frequency means more energy in each photon.

photon: one packet of light energy

Wave or particle?

💡 Light acts as a wave in some experiments and as a particle in others, so scientists use both models.

Light makes interference patterns, just like water waves. That is wave behavior.

Light also knocks electrons off metals in single packets of energy, called the photoelectric effect. That is particle behavior.

🧠 Let's work it out

When two good experiments support different models, don't throw one away.
Ask which model explains which result.
For light, both are needed.

What do you think comes next?