5.2.1 Electromagnetic waves and refraction in glass (core practical)

Electromagnetic waves transfer energy

1. Every electromagnetic wave is transverse, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
2. The changing electric and magnetic fields oscillate at right angles to the direction in which the wave transfers energy.
3. Electromagnetic waves do not need particles, so they can travel through a vacuum.
4. All electromagnetic waves travel through a vacuum at $3.0\times 10^8{m\; s}^{-1}$.
5. The equation $v=f\lambda$ links wave speed $v$, frequency $f$ and wavelength $\lambda$.

Energy moves from source to observer

1. A source produces electromagnetic radiation and an observer or detector absorbs some of the energy it carries.
2. Visible light from a lamp transfers energy to the retina or a light sensor when it is absorbed.
3. Infrared radiation from a heater transfers energy to skin and increases its thermal energy store.
4. Radio waves from a transmitting aerial transfer energy to a receiving aerial and can produce electrical oscillations.

Refraction changes a ray's direction

1. Light slows down when it enters glass from air and bends towards the normal.
2. Light speeds up when it leaves glass for air and bends away from the normal.
3. A ray travelling along the normal has $i=0^{\circ }$, so its speed changes without a change in direction.
4. For a parallel-sided rectangular block, the emergent ray is parallel to the incident ray but is laterally displaced.

Ray diagrams show the interaction

1. Draw the normal as a dashed line perpendicular to the boundary.
2. Show an arrow on each ray to indicate the direction in which the light travels.
3. Label $i$ between the incident ray and the normal, and label $r$ between the refracted ray and the normal.

5.2.2 Groups of the electromagnetic spectrum

The spectrum has seven groups

1. From longest wavelength and lowest frequency to shortest wavelength and highest frequency, the order is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
2. The groups meet without gaps, so the spectrum is continuous even though named groups make it easier to describe.
3. Moving from radio waves to gamma rays, wavelength decreases while frequency increases.
4. Because $v=f\lambda$ and every group has the same speed in a vacuum, a higher frequency corresponds to a shorter wavelength.

Visible light is a narrow range

1. Within visible light, the order from longest to shortest wavelength is red, orange, yellow, green, blue, indigo and violet.
2. Red light has the longest wavelength and lowest frequency in the visible range.
3. Violet light has the shortest wavelength and highest frequency in the visible range.
4. Infrared lies beyond the red end and ultraviolet lies beyond the violet end, but neither can be detected by the eye.

Ordering answers need both trends

1. An order based on decreasing wavelength runs from radio waves to gamma rays.
2. An order based on increasing frequency also runs from radio waves to gamma rays.
3. If the direction is reversed, both trends reverse, so gamma rays to radio waves means increasing wavelength and decreasing frequency.

5.2.3 Wavelength-dependent behaviour of EM waves

A material's response depends on wavelength

1. A substance can absorb, transmit, reflect or refract electromagnetic waves, and its response can change with wavelength.
2. Absorbed radiation transfers energy to the material and may increase its thermal energy store or cause changes in its particles.
3. Transmitted radiation passes through the material, although its speed and wavelength may differ from their values in a vacuum.
4. Reflected radiation changes direction at the surface and returns into the original substance.
5. Refracted radiation changes direction because its speed changes at a boundary.

The same material treats wavelengths differently

1. Ordinary window glass transmits much of the visible range, which is why objects can be seen through it.
2. The same glass absorbs much of the ultraviolet reaching it, so transmission cannot be predicted from transparency to visible light alone.
3. A red filter transmits a band of red wavelengths and absorbs much of the other visible wavelengths.
4. A shiny metal surface reflects a large proportion of visible and infrared radiation but may respond differently at other wavelengths.
5. Water can transmit visible light over short distances while absorbing parts of the infrared range strongly.

Evidence must identify the interaction

1. A lower detector reading after a wave meets a material shows that less radiation was transmitted.
2. The missing transmitted energy may have been absorbed or reflected, so transmission data alone cannot separate those two processes.
3. A temperature rise in the material provides evidence that some radiation was absorbed.
4. A detector placed on the incident side can provide evidence for reflection.

5.2.4 Velocities of EM waves in different substances

Wave speed changes at a boundary

1. Electromagnetic waves travel at $3.0\times 10^8{m\; s}^{-1}$ in a vacuum but usually travel more slowly in transparent substances.
2. When a wave crosses a boundary, its frequency remains constant because the source fixes the frequency.
3. The relationship $v=f\lambda$ means that a lower speed at constant frequency produces a shorter wavelength.
4. A higher speed at constant frequency produces a longer wavelength.

Speed determines the direction of bending

1. A ray entering a slower substance at an angle bends towards the normal.
2. A ray entering a faster substance at an angle bends away from the normal.
3. A ray crossing along the normal changes speed and wavelength but does not change direction.
4. The greater the change in speed between two substances, the greater the change in direction for the same angle of incidence.

Frequency stays fixed across the boundary

1. Wave crests cannot accumulate or disappear at a boundary, so the number of cycles crossing each second is unchanged.
2. For light entering glass from air, speed decreases, frequency stays constant and wavelength decreases.
3. For light leaving glass for air, speed increases, frequency stays constant and wavelength increases.

Ray explanations need a complete chain

1. First state whether the wave speed increases or decreases on entering the new substance.
2. Then state that the frequency is unchanged.
3. Use $v=f\lambda$ to determine whether wavelength increases or decreases.
4. Finally state whether the ray bends towards or away from the normal when it meets the boundary at an angle.

Calculations use one consistent set of units

1. Use wave speed in ${m\; s}^{-1}$, frequency in $Hz$ and wavelength in $m$.
2. Convert nanometres using $1nm=1\times 10^{-9}m$ before substituting into $v=f\lambda$.
3. A correct numerical answer should include a unit and an appropriate number of significant figures.