Electromagnetic waves transfer energy
Definition
Transverse wave
A wave in which the oscillations are perpendicular to the direction of energy transfer.
- Every electromagnetic wave is transverse, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
- The changing electric and magnetic fields oscillate at right angles to the direction in which the wave transfers energy.
- Electromagnetic waves do not need particles, so they can travel through a vacuum.
- All electromagnetic waves travel through a vacuum at 3.0×108 m s−13.0\times10^8\,\text{m s}^{-1}3.0×108m s−1.
- The equation v=fλv=f\lambdav=fλ links wave speed vvv, frequency fff and wavelength λ\lambdaλ.
Example
Calculating wavelength
- For a radio wave with f=2.0×105 Hzf=2.0\times10^5\,\text{Hz}f=2.0×105Hz in a vacuum, use λ=vf\lambda=\dfrac{v}{f}λ=fv.
- λ=3.0×1082.0×105=1.5×103 m\lambda=\dfrac{3.0\times10^8}{2.0\times10^5}=1.5\times10^3\,\text{m}λ=2.0×1053.0×108=1.5×103m.
Energy moves from source to observer
Definition
Electromagnetic radiation
Energy transferred through space by oscillating electric and magnetic fields.
- A source produces electromagnetic radiation and an observer or detector absorbs some of the energy it carries.
- Visible light from a lamp transfers energy to the retina or a light sensor when it is absorbed.
- Infrared radiation from a heater transfers energy to skin and increases its thermal energy store.
- Radio waves from a transmitting aerial transfer energy to a receiving aerial and can produce electrical oscillations.
Exam technique
For a complete explanation, identify the source, name the electromagnetic radiation and identify the observer or detector that absorbs the energy.
Refraction changes a ray's direction
Definition
Refraction
The change in direction of a wave caused by a change in speed as it passes from one medium into another.
- Light slows down when it enters glass from air and bends towards the normal.
- Light speeds up when it leaves glass for air and bends away from the normal.
- A ray travelling along the normal has i=0∘i=0^\circi=0∘, so its speed changes without a change in direction.
- For a parallel-sided rectangular block, the emergent ray is parallel to the incident ray but is laterally displaced.
Common Mistake
- Measure the angles of incidence and refraction from the normal, not from the surface of the block.
- Do not explain a lower wave speed by saying the light has lost energy.
Practical
Investigating refraction in glass
- Apparatus: Use a ray box with a single slit or a low-power classroom laser, rectangular glass block, plain paper, sharp pencil, ruler and protractor.
- Independent variable: Change the angle of incidence iii, measured from the normal.
- Dependent variable: Measure the angle of refraction rrr, measured from the normal.
- Control variables: Keep the glass block, light colour, entry face, point of incidence and angle-measuring method the same.
- Method:
- Place the block on the paper and draw precisely around it.
- Draw a normal at 90∘90^\circ90∘ to the entry face at the chosen point of incidence.
- Use a protractor to draw an incident line at a selected value of iii.
- Direct one narrow ray along the line and mark two well-separated points on the incident ray and two on the emergent ray.
- Remove the block, join each pair of points with a ruler and connect the entry and exit points to reconstruct the ray inside the glass.
- Measure iii and rrr from the normal.
- Repeat with at least five angles spread across a suitable range, such as 20∘20^\circ20∘ to 70∘70^\circ70∘.
- Repeat each angle at least three times, identify anomalous readings and calculate a mean value of rrr.
- Processing: Record iii and mean rrr with degrees in a results table and plot mean rrr against iii if requested.
- Expected pattern: As iii increases, rrr increases, but for air to glass r<ir<ir<i because the ray slows and bends towards the normal.
- Uncertainty: A thick ray, broad pencil lines, block movement and parallax when using the protractor make the angles uncertain.
- Improvements: Use a single narrow ray, draw thin lines, keep marks far apart, hold the block fixed, align the protractor carefully and use repeat readings.
- Safety: Keep the beam below eye level, never look into it or direct it at anyone, and handle the glass block carefully.
Ray diagrams show the interaction
- Draw the normal as a dashed line perpendicular to the boundary.
- Show an arrow on each ray to indicate the direction in which the light travels.
- Label iii between the incident ray and the normal, and label rrr between the refracted ray and the normal.
Exam technique
- Link every observed bend to a change in speed at the boundary.
- When evaluating the method, name an improvement and state how it reduces uncertainty.
Self review
- What makes an electromagnetic wave transverse?
- What speed do electromagnetic waves have in a vacuum?
- How does electromagnetic radiation transfer energy from source to observer?
- Why does light bend towards the normal when it enters glass from air?
- How are iii and rrr measured in the refraction practical?
