WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.031 Unit C 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 --> 00:07:47.493 That completes Light and the electromagnetic spectrum. 00:07:47.643 --> 00:07:50.988 Revisit the notes and test yourself on the revision website.