WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.021 Unit S P 5: Light and the electromagnetic spectrum. 00:00:10.700 --> 00:00:13.365 All electromagnetic waves are transverse. 00:00:13.515 --> 00:00:24.713 They transfer energy, need no material to travel through (no medium), and travel at the same speed in a vacuum: C approximately equals 3 times ten to the power 8 metres per second. 00:00:27.367 --> 00:00:38.389 The order from longest wavelength or lowest frequency to shortest wavelength or highest frequency is radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays. 00:00:38.539 --> 00:00:42.594 The spectrum is continuous; these bands have no gaps between them. 00:00:47.267 --> 00:00:53.591 The spectrum is continuous; the named groups are useful divisions, not gaps where no radiation exists. 00:00:53.741 --> 00:00:56.460 Human eyes detect only the visible band. 00:00:59.133 --> 00:01:05.528 Visible colours from longer to shorter wavelength are red, orange, yellow, green, blue, indigo and violet. 00:01:05.678 --> 00:01:08.195 Violet has a higher frequency than red. 00:01:10.867 --> 00:01:15.147 For electromagnetic waves, C equals F times lambda in a vacuum. 00:01:15.297 --> 00:01:18.352 As frequency increases, wavelength decreases. 00:01:18.502 --> 00:01:21.806 Convert wavelength into metres before calculating. 00:01:24.467 --> 00:01:28.145 The energy of an individual photon increases with frequency. 00:01:28.295 --> 00:01:36.657 Total energy transferred by a beam also depends on intensity and exposure time; frequency alone does not determine total energy. 00:01:39.333 --> 00:01:45.861 Different wavelengths interact differently with materials: they may be absorbed, transmitted, reflected or refracted. 00:01:46.011 --> 00:01:51.470 A material transparent to visible light need not transmit ultraviolet or infrared. 00:01:54.133 --> 00:01:57.055 Light travels more slowly in glass than in air. 00:01:57.205 --> 00:02:01.361 A ray entering glass at an angle to the normal bends towards the normal. 00:02:01.511 --> 00:02:04.327 It bends away from the normal when it returns to air. 00:02:07.000 --> 00:02:10.621 Place a rectangular glass block on paper and draw around it. 00:02:10.771 --> 00:02:16.519 Shine a narrow ray from a ray box onto a face, marking the incoming and outgoing ray positions. 00:02:19.200 --> 00:02:22.557 Remove the block and join the marks to trace the ray through it. 00:02:22.707 --> 00:02:29.308 Draw normals perpendicular to the entry and exit faces; measure incident and refracted angles with a protractor. 00:02:31.967 --> 00:02:34.356 Repeat with different incident angles. 00:02:34.506 --> 00:02:42.719 Keep the block and ray alignment consistent; use sharp pencil marks and widely separated points to reduce direction-reading uncertainty. 00:02:45.400 --> 00:02:51.627 A ray entering along the normal (normal incidence) continues straight, although its speed and wavelength change. 00:02:51.777 --> 00:02:54.667 Its frequency stays the same at both boundaries. 00:02:57.333 --> 00:03:04.104 Through parallel-sided glass in air, the emerging ray is parallel to the incident ray but displaced sideways. 00:03:04.254 --> 00:03:07.687 The two refractions change direction in opposite senses. 00:03:07.837 --> 00:03:11.573 The ray slows on entering glass; the frequency stays fixed. 00:03:16.233 --> 00:03:19.449 Use a ray box, not a laser aimed towards eyes. 00:03:19.599 --> 00:03:23.509 A beam does not need to be intensely bright to produce clear measurements. 00:03:26.167 --> 00:03:32.545 Herschel separated sunlight with a prism and placed thermometers in different colours and just beyond the red end. 00:03:32.695 --> 00:03:37.207 Temperature increased in the visible spectrum and rose further beyond red. 00:03:37.357 --> 00:03:40.429 Thermometers also warmed within the visible spectrum. 00:03:45.100 --> 00:03:50.293 The warming beyond visible red showed there was invisible radiation there: infrared. 00:03:50.443 --> 00:03:53.598 It was not the case that visible colours caused no warming. 00:03:56.267 --> 00:04:02.443 Ritter investigated beyond violet using silver chloride, which darkens on exposure to radiation. 00:04:02.593 --> 00:04:07.968 Strong darkening beyond the visible violet region provided evidence of ultraviolet. 00:04:08.118 --> 00:04:11.929 A chemical detector revealed radiation eyes could not see. 00:04:16.600 --> 00:04:27.189 Compare detectors suited to the radiation: eyes detect visible light, thermometers detect heating, and chemical or photographic materials can record radiation effects. 00:04:29.867 --> 00:04:34.709 Radio waves are used for radio and television broadcasting and communication. 00:04:34.859 --> 00:04:40.075 Charges vibrating back and forth (oscillating) in a transmitting circuit produce them. 00:04:40.225 --> 00:04:44.959 When an aerial receives the waves, they produce a changing electrical signal in it. 00:04:47.633 --> 00:04:52.400 The transmitted frequency is linked to the oscillation frequency of the charges. 00:04:52.550 --> 00:04:56.355 Information can be carried by variations in the radio signal. 00:04:59.033 --> 00:05:08.301 Microwaves are used in communication, including satellite links and some mobile or Wi-Fi systems, and in microwave ovens where absorption heats food. 00:05:10.967 --> 00:05:15.490 Infrared is used for heating, cooking, remote controls and thermal imaging. 00:05:15.640 --> 00:05:22.903 A thermal camera detects emitted infrared to show temperature patterns; it does not make infrared visible to unaided eyes. 00:05:25.567 --> 00:05:27.721 Warm objects emit infrared. 00:05:27.871 --> 00:05:35.838 Passive infrared motion detectors respond to changing infrared patterns, for example from a person moving through a monitored area. 00:05:38.500 --> 00:05:45.561 The atmosphere transmits much visible light and some radio or microwave or infrared bands but absorbs others. 00:05:45.711 --> 00:05:55.036 Many ultraviolet, X-ray and gamma-ray observations require space-based telescopes; high, dry locations help some infrared observations. 00:05:57.700 --> 00:06:03.883 Visible light enables vision, photography and illumination; very intense light can damage the eyes. 00:06:04.033 --> 00:06:06.679 Never look into the Sun or powerful beams. 00:06:09.333 --> 00:06:15.370 Ultraviolet can cause fluorescence for security marks and banknote checks, and can disinfect water. 00:06:15.520 --> 00:06:21.673 UV exposure can damage skin and eyes and increase skin-cancer risk; tanning is not a risk-free use. 00:06:24.333 --> 00:06:29.933 X-rays form medical and security images because different materials absorb them differently. 00:06:30.083 --> 00:06:35.396 Dense bone absorbs more X-rays than soft tissue, creating contrast on a detector. 00:06:35.546 --> 00:06:41.102 Image brightness depends on detector processing; the absorption difference creates contrast. 00:06:45.767 --> 00:06:52.166 Gamma radiation can be used with radioactive tracers for imaging and to sterilise medical equipment. 00:06:52.316 --> 00:06:56.616 Penetration is useful but requires shielding and controlled exposure. 00:06:59.267 --> 00:07:06.370 Ionising radiation can remove electrons and damage cells or D N A; X-rays and gamma rays are ionising. 00:07:06.520 --> 00:07:12.440 UV can also damage D N A; hazard increases with radiation type, dose and exposure. 00:07:15.100 --> 00:07:24.037 Microwaves can heat internal tissues, infrared can cause burns and sufficiently intense radio-frequency radiation can also cause heating. 00:07:24.187 --> 00:07:27.673 Non-ionising does not mean harmless at every intensity. 00:07:30.333 --> 00:07:35.548 Reduce unnecessary exposure using appropriate shielding, distance and exposure time. 00:07:35.698 --> 00:07:41.531 Medical uses balance benefit against risk; the appropriate radiation depends on the task. 00:07:44.200 --> 00:07:49.415 For reflection, angle of incidence equals angle of reflection; measure both from the normal. 00:07:49.565 --> 00:07:55.239 A smooth surface reflects rays in an orderly way and can form an image (specular reflection). 00:07:55.389 --> 00:07:59.805 A rough surface scatters them in different directions (diffuse reflection). 00:07:59.955 --> 00:08:04.713 Each ray still obeys the same reflection rule at the point where it meets the surface. 00:08:07.367 --> 00:08:11.342 A coloured opaque surface absorbs some wavelengths and reflects others. 00:08:11.492 --> 00:08:18.382 A red surface under white light mainly reflects red; under blue light it may look dark because little blue is reflected. 00:08:21.033 --> 00:08:25.619 A colour filter transmits selected wavelengths and absorbs much of the rest. 00:08:25.769 --> 00:08:35.091 A red filter transmits red light; a red filter followed by a blue filter can give very little transmitted light if their transmission bands do not overlap. 00:08:37.767 --> 00:08:44.335 When light enters a material where it travels faster, for example from glass into air, it bends away from the normal. 00:08:44.485 --> 00:08:48.044 This is movement from a higher refractive index to a lower one. 00:08:48.194 --> 00:08:52.133 At the critical angle, the refracted ray travels along the boundary. 00:08:54.800 --> 00:09:00.601 Total internal reflection happens when light travels from a higher refractive index to a lower one, 00:09:00.751 --> 00:09:02.458 such as glass to air, 00:09:02.608 --> 00:09:05.912 and the angle of incidence is greater than the critical angle. 00:09:06.062 --> 00:09:09.558 All the light is reflected back into the original material. 00:09:09.708 --> 00:09:12.332 Optical fibres use this to guide light. 00:09:12.482 --> 00:09:16.707 The incident and reflected rays remain in the higher-index material. 00:09:21.367 --> 00:09:28.032 A converging lens is thicker in the middle and brings rays initially parallel to its principal axis towards a focus. 00:09:28.182 --> 00:09:34.709 A diverging lens is thinner in the middle and spreads them as though they came from a focus on the incident side. 00:09:37.367 --> 00:09:42.017 Lens power in dioptres equals 1 divided by focal length in metres. 00:09:42.167 --> 00:09:46.937 A shorter focal length means a more powerful lens: it bends light more strongly. 00:09:47.087 --> 00:09:52.037 Converging lenses have positive power; diverging lenses have negative power. 00:09:54.700 --> 00:10:04.746 In a ray diagram for a converging lens, a ray parallel to the axis passes through the far focus, and a ray through the optical centre continues approximately straight. 00:10:04.896 --> 00:10:07.456 Their intersection locates a real image. 00:10:07.606 --> 00:10:13.077 The intersection of the emerging rays gives the inverted real image; drawing is schematic. 00:10:17.733 --> 00:10:24.291 A converging lens forms a real, upside-down (inverted) image when the object is beyond the focal point. 00:10:24.441 --> 00:10:28.029 Beyond twice the focal length, the image is smaller (diminished). 00:10:28.179 --> 00:10:31.685 Between one and two focal lengths it is larger (magnified). 00:10:31.835 --> 00:10:35.700 At twice the focal length, it is the same size as the object. 00:10:38.367 --> 00:10:49.842 For an object inside a converging lens’s focal length, extend the emerging rays backwards: the image is virtual, upright and magnified on the object side, as in a magnifying glass. 00:10:52.500 --> 00:10:57.451 A diverging lens gives an upright, diminished virtual image for a real object. 00:10:57.601 --> 00:11:05.483 A virtual image is located by backward extensions of rays; it cannot be projected directly onto a screen as a real image can. 00:11:08.133 --> 00:11:15.051 Draw a labelled principal axis, lens, focus points and object, then use straight rays with arrow directions. 00:11:15.201 --> 00:11:21.023 Use dashed lines for backward extensions rather than treating them as real rays travelling backwards. 00:11:23.700 --> 00:11:26.715 All bodies emit electromagnetic radiation. 00:11:26.865 --> 00:11:36.368 As temperature increases, the emitted power generally rises and the distribution shifts towards shorter wavelengths; hot enough objects can emit visible light. 00:11:39.033 --> 00:11:46.030 (Higher tier) A body at constant temperature absorbs energy at the same average rate as it emits it (Higher tier). 00:11:46.180 --> 00:11:53.986 If absorbed power exceeds emitted power it warms; if emitted power exceeds absorbed power it cools, until the balance changes. 00:11:56.667 --> 00:12:04.761 (Higher tier) Earth’s temperature depends on the balance of incoming solar radiation, reflected radiation and outgoing radiation. 00:12:04.911 --> 00:12:14.649 Greenhouse gases absorb and re-emit some outgoing infrared, affecting the balance (Higher tier); reflected sunlight and absorbed energy are different parts of the balance. 00:12:17.300 --> 00:12:25.759 Dark, matt surfaces are generally better absorbers and emitters of thermal radiation than shiny, light surfaces under comparable conditions. 00:12:25.909 --> 00:12:29.256 Good emission and absorption tend to accompany one another. 00:12:31.933 --> 00:12:38.929 For the core practical, compare at least four otherwise similar containers with different colours or shiny or dull surfaces. 00:12:39.079 --> 00:12:46.565 Put equal volumes of hot water at the same initial temperature into them and use lids; record temperature at fixed intervals. 00:12:46.715 --> 00:12:51.329 Compare temperature changes with the same apparatus and exposure conditions. 00:12:56.000 --> 00:13:04.275 Keep container material and size, water volume, thermometer position and room conditions consistent; compare cooling curves and repeat. 00:13:04.425 --> 00:13:12.103 Faster cooling can indicate stronger radiation, but conduction and convection also affect the experiment and must be considered. 00:13:14.767 --> 00:13:21.250 To compare absorption, put otherwise identical surfaces the same distance from the same radiation source. 00:13:21.400 --> 00:13:24.461 Keep starting temperature and exposure time the same. 00:13:24.611 --> 00:13:31.563 Use the same material so a change caused by colour or texture is not confused with one caused by different materials. 00:13:34.233 --> 00:13:37.526 That completes Light and the electromagnetic spectrum. 00:13:37.676 --> 00:13:41.021 Revisit the notes and test yourself on the revision website.