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# Speed of Wave

### Speed of Wave

The speed of a wave is given by wavelength x frequency.

Waves are the **transfer of energy**. This transfer happens over a distance at a certain **speed** (v).

How do you calculate **speed** (v)?

The speed of a wave can be calculated with its **frequency** (f) and **wavelength** (λ). What is the frequency of a wave?

What is the SI unit for measuring the **wavelength** $(\lambda)$ of a wave?

What is the unit for **speed** (v)?

Now let's have a go at deriving the equation for the speed of a wave. We will do this, using **wavelength** and **frequency**.

We derived the SI unit for frequency from its equation. What is this? Remember Hertz is not the SI unit.

What is the SI unit for wavelength?

So, we know that the unit for speed is $^m/_s$ . This can also be written as $m\times \frac{1}{s}$ . Notice these are the two units for wavelength and frequency. So which one of these options if the correct formula for speed of a wave?

**A:** $speed = \frac{wavelength}{frequency}$

**B:** $speed=wavelength \times frequency$

Answer **A** or **B**.

What is the **equation for speed**? Given that wavelength (λ) is measured in metres $(m)$ , frequency (f) is measured in Hertz or $\frac{1}{s}$ , and speed (v) is measured in $^m/_s$ .

So the speed of a wave is given by $v=f\times\lambda$ where **v** is speed, **f** is frequency, and **λ** is wavelength.

What is the speed of a wave that has a frequency of 20 Hz and a wavelength of 250m?

The speed of sound in air is $343m/_s$ What is the wavelength of a sound wave with a frequency of $49Hz$ ?

What is the frequency of red light, that has a wavelength of $700 nm$ ? The speed of light is always $300,000,000m/_s$ or $3\times10^8m/_s$ .

The speed of a wave depends on the **medium** the wave is travelling through. We will look at this more closely in later lessons.