• Quick retrieval from earlier learning (Wave speed = frequency × wavelength; Colour, prisms and refraction). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. A wave has a frequency of 4 Hz and a wavelength of 0.5 m. What is its speed, in m/s? → 2 m/sPrompt: v = f × λ
2. Sound travels at 340 m/s in air. What is the wavelength of a 680 Hz sound, in metres? → 0.5 mPrompt: Rearrange v = f λ to find λ.
3. A wave has a frequency of 5 Hz and a wavelength of 4 m. What is its speed, in m/s? → 20 m/sPrompt: v = f × λ

• Ask the class to make a wave with a rope or slinky and describe what moves and what doesn't.
• Share the objective: The electromagnetic spectrum. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Which type of electromagnetic wave is used for airport baggage scanners?
• Step 1: X-rays pass through soft tissue but not bone.
• Step 2: So the answer is: X-rays
• Answer: X-rays
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Which type of electromagnetic wave is used for TV remote controls? (infrared / microwaves / radio waves / X-rays) → infraredPrompt: Think about which waves can pass through the body, carry signals or heat things.
2. The electromagnetic spectrum: which statement is NOT true? (Electromagnetic waves are transverse. / All electromagnetic waves travel at the same speed in a vacuum. / Electromagnetic waves need a material to travel through. / Visible light is only a small part of the electromagnetic spectrum.) → Electromagnetic waves need a material to travel through.Prompt: Is a radio wave a sound wave?

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. The electromagnetic spectrum: which statement is NOT true? (Visible light is only a small part of the electromagnetic spectrum. / All electromagnetic waves travel at the same speed in a vacuum. / Ultraviolet can damage skin cells and cause sunburn. / Radio waves are a type of sound.) → Radio waves are a type of sound.Prompt: Is a radio wave a sound wave?
2. Which type of electromagnetic wave is used for treating some cancers? (infrared / gamma rays / ultraviolet / X-rays) → gamma raysPrompt: Think about which waves can pass through the body, carry signals or heat things.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.