# Light and waves

> Wavelength and frequency, ready for the electromagnetic spectrum


## Objectives

- State that light travels as waves that carry energy.
- Define wavelength and frequency.
- Explain that, for light, a shorter wavelength means a higher frequency.

## 1. Light and waves: from zero




## 2. Energy from far away

What carries the Sun's energy to you?

- Sound waves from the Sun
- Hot air flowing from the Sun
- Light, travelling as waves ✓

Light travels as waves that carry energy, even through empty space. The space between the Sun and Earth has no air, so neither hot air nor sound can carry the energy across it.

[ar-EG] الضوء بيتنقل على شكل waves شايلة energy، حتى في الفضا الفاضي. والمسافة بين الشمس والأرض مفيهاش هوا، فلا الهوا السخن ولا الصوت يقدروا يوصّلوا الـ energy.
## 3. Wavelength

Light travels as a wave. A wave has high points, called **crests**. One light wave. The arrow marks one wavelength. Wavelength The distance between two neighbouring crests of a wave. For light, wavelengths are tiny. We measure them in nanometres (nm): 1 nm = $10^{-9}$ m.


## 4. Frequency

Frequency The number of complete waves that pass a point in one second. Its unit is the hertz (Hz). In empty space, all light travels at the same speed: $3 \times 10^8$ m/s. Slide to change the wavelength. Watch the frequency. Wavelength Frequency speed of light = frequency × wavelength


## 5. Your turn

Red light has a wavelength of about 700 nm. Blue light has about 450 nm. Which has the higher frequency?

- Blue light ✓
- Both the same, because all light has the same speed
- Red light, because its waves are longer

All light has the same speed in empty space, so a shorter wavelength means more waves pass each second. Blue light (about 450 nm) has the shorter wavelength, so it has the higher frequency.

[ar-EG] كل الضوء ليه نفس السرعة في الفضا الفاضي، فالـ wavelength الأقصر معناه waves أكتر بتعدّي كل ثانية. والضوء الأزرق (حوالي 450 nm) الـ wavelength بتاعه أقصر، فالـ frequency بتاعته أعلى.
## 6. Summary

Light travels as waves that carry energy, even through empty space. **Wavelength**: the distance between two neighbouring crests. **Frequency**: the number of waves that pass a point in one second (Hz). For light, a shorter wavelength means a higher frequency. Same speed in empty space: shorter wavelength, higher frequency (values from the slider above). Key words: **waves**, **energy**, **wavelength**, **frequency**, **shorter wavelength → higher frequency**. Next: light is only one member of a bigger family of waves. Can you explain why blue light has a higher frequency than red light?



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-phys.waves-light