Course: Combined Science + Separate Physics (black body radiation is S only) | Tier: Foundation + Higher | Linked equation: v = fλ
Notes
The electromagnetic spectrum
Electromagnetic (EM) waves are transverse, transfer energy from a source to an absorber, and all travel at the same speed through a vacuum or air: 3 × 108 m/s. They form a continuous spectrum. Our eyes only detect visible light.
| Wave (long λ → short λ) | Uses | Dangers |
|---|---|---|
| Radio waves | Television and radio | – |
| Microwaves | Satellite communications, cooking food | Can heat body tissue |
| Infrared | Electrical heaters, cooking food, infrared cameras | Burns |
| Visible light | Fibre optic communications | Very bright light can damage eyes |
| Ultraviolet | Energy-efficient lamps, sun tanning | Skin ages prematurely; increased risk of skin cancer |
| X-rays | Medical imaging and treatments | Ionising: can cause mutation of genes and cancer |
| Gamma rays | Medical imaging and treatments | Ionising: can cause mutation of genes and cancer |
Frequency increases (and wavelength decreases) from radio waves to gamma rays. The higher the frequency, the more energy the wave carries.
Properties
- Different substances may absorb, transmit, refract or reflect EM waves in ways that vary with wavelength.
- (HT) Refraction happens because the wave changes speed at a boundary: one side of the wavefront slows first, so the wave changes direction.
- (HT) Radio waves are produced by oscillations in electrical circuits. When absorbed, they can create an alternating current with the same frequency.
- Gamma rays come from changes in the nucleus of an atom.
Black body radiation (S only)
All bodies emit and absorb infrared radiation. The hotter the body, the more radiation it emits each second, and the shorter the wavelength of the peak emission. A perfect black body absorbs all the radiation that hits it – it’s also the best possible emitter. A body at constant temperature absorbs radiation at the same rate as it emits it. If it absorbs more than it emits, its temperature rises. The Earth’s temperature depends on the balance between the radiation it absorbs from the Sun and the radiation it emits into space.
Questions
Q1 (F) List the EM spectrum in order, starting with the longest wavelength.
Show answer
Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
Q2 (F) Give one use each of microwaves and infrared.
Show answer
Microwaves: satellite communications or cooking. Infrared: electrical heaters, cooking, or infrared cameras.
Q3 (F) Give two harmful effects of ultraviolet radiation.
Show answer
It makes the skin age prematurely and increases the risk of skin cancer.
Q4 (F) Which two EM waves are ionising, and why is this dangerous?
Show answer
X-rays and gamma rays. Ionising radiation can cause mutation of genes, which can lead to cancer.
Q5 (F/H) A radio wave has a wavelength of 1500 m. Calculate its frequency. (Speed of EM waves = 3.0 × 108 m/s)
Show answer
F: v = f λ
I: 3.0 × 108 = f × 1500
F: f = 3.0 × 108 ÷ 1500
A: 2.0 × 105 Hz
Q6 (F/H) Why are X-rays used for images of bones?
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X-rays pass through soft tissue but are absorbed by dense bone, so bones show up as shadows on the detector.
Q7 (H) Explain, using wavefronts, why light changes direction when it enters glass at an angle.
Show answer
Light slows down in glass. One side of each wavefront reaches the glass first and slows down while the other side is still moving faster in air. This makes the wavefront turn, so the wave changes direction – towards the normal.
Q8 (H) How are radio waves produced and detected?
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They are produced by oscillations (alternating current) in an electrical circuit. When absorbed by a receiving aerial, they create an alternating current with the same frequency as the radio wave.
Q9 (S only) What is a perfect black body?
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An object that absorbs all the radiation incident on it. It does not reflect or transmit any radiation, and it is the best possible emitter.
Q10 (S only) Explain why an object’s temperature stays constant even though it is emitting radiation.
Show answer
It is absorbing radiation at the same rate as it is emitting it, so there is no net change in its energy and its temperature stays the same.
