🧰 You need: pen, pencil, ruler, calculator, coloured pencils if you have them. Turn your page sideways (landscape). Time: 45 minutes. Course: Combined and Separate.
Information – read this first
- Electromagnetic (EM) waves are transverse. They transfer energy from a source to an absorber.
- All EM waves travel at the same speed in a vacuum: 300 000 000 m/s (3 × 10⁸ m/s).
- In order: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Memory aid: Rich Men In Vegas Use eXpensive Gadgets.
- Going from radio to gamma, the wavelength decreases and the frequency increases.
- Visible light is the only part we can see. Its colours are red, orange, yellow, green, blue, indigo, violet. Red has the longest wavelength.
- wave speed = frequency × wavelength (v = fλ)
| Wave | Typical wavelength | Use | Danger |
|---|---|---|---|
| Radio waves | 1 m to over 1 km | Radio and TV broadcasts | Low risk |
| Microwaves | about 1 cm | Satellite communications, cooking food | High intensity can heat body tissue |
| Infrared | about 0.01 mm | Electric heaters, cooking, infrared cameras | Can cause burns |
| Visible light | about 0.0005 mm (400–700 nm) | Fibre optic communications, seeing | Very bright light can damage eyes |
| Ultraviolet | about 0.00001 mm | Energy-efficient lamps, sun tanning | Skin ageing, increased risk of skin cancer |
| X-rays | about 0.0000001 mm | Medical imaging (seeing bones) | Ionising: can cause gene mutation and cancer |
| Gamma rays | smaller than 0.000000001 mm | Medical imaging and treatment, sterilising equipment | Ionising: can cause gene mutation and cancer |
What to do
- (2 min) Write the title The Electromagnetic Spectrum and today’s date. Underline both with a ruler.
- (5 min) Near the top of the page, rule a long rectangle about 21 cm long and 3 cm tall. Divide it into 7 boxes, each 3 cm wide.
- (3 min) Write the seven waves in the boxes in the correct order, from radio waves on the left to gamma rays on the right.
- (3 min) In the visible light box, draw thin stripes for red, orange, yellow, green, blue, indigo, violet (red nearest the infrared box). Colour them if you can.
- (4 min) Above the rectangle, draw a wavy line from left to right. Make the waves long and stretched on the left and short and squashed on the right.
- (3 min) Below the rectangle, rule two long arrows across the page:
• one pointing left, labelled “wavelength increases”
• one pointing right, labelled “frequency increases” and “energy increases”. - (3 min) Under each box, write its typical wavelength from the table.
- (8 min) Under the wavelengths, write one use and one danger for each wave. Draw a warning triangle ⚠️ next to the three ionising waves: ultraviolet, X-rays and gamma rays.
- (2 min) Copy the first two bullet points from the information box under your diagram.
- (10 min) Write the heading Calculations. Copy the worked example, then do Q1–Q5.
- (2 min) Check your answers and correct in a different colour.
Calculations
For all EM waves, v = 300 000 000 m/s (3 × 10⁸ m/s).
Worked example: A radio station broadcasts at 100 000 000 Hz (100 MHz). Find the wavelength.
λ = v ÷ f = 300 000 000 ÷ 100 000 000 = 3 m
Q1 A radio wave has a frequency of 200 000 Hz (200 kHz). Calculate its wavelength.
Show answer
λ = 300 000 000 ÷ 200 000 = 1500 m
Q2 A microwave has a wavelength of 0.03 m. Calculate its frequency.
Show answer
f = v ÷ λ = 300 000 000 ÷ 0.03 = 10 000 000 000 Hz (1 × 10¹⁰ Hz)
Q3 Green light has a frequency of 6 × 10¹⁴ Hz. Calculate its wavelength.
Show answer
λ = 3 × 10⁸ ÷ 6 × 10¹⁴ = 5 × 10⁻⁷ m (0.0005 mm, or 500 nm)
Q4 Which has the higher frequency: infrared or ultraviolet? Use your diagram to explain.
Show answer
Ultraviolet. It is further to the right of the spectrum (towards gamma), where frequency increases.
Q5 A satellite is 36 000 000 m above the Earth. How long does a microwave signal take to reach it? (time = distance ÷ speed)
Show answer
t = 36 000 000 ÷ 300 000 000 = 0.12 s
⭐ Challenge
Explain how an X-ray image of a broken arm is made. Why do radiographers stand behind a screen when taking X-rays?
Show answer
X-rays pass through soft tissue but are absorbed by bone. The X-rays that get through reach a detector behind the arm, so the bones show up as white shapes (where X-rays were absorbed) and a break shows as a gap. X-rays are ionising and can cause gene mutation and cancer. Radiographers take many X-rays every day, so they stand behind a screen (or leave the room) to reduce their dose.
