HCI Worksheet 4 EM waves (answers)

Hwa Chong Institution Sec 3 (SMTP) 1 Name: __________________________________________ Class: __________ Date: ___________ Sec 3 Physics Worksheet 4: EM waves Conceptual Questions 1. What colour of the visible spectrum has the longest wavelength? Which has the shortest wavelength? Red has the longest wavelength, and violet the shortest. 2. In a boutique store with only fluorescent lighting, a customer insists on taking dresses into the daylight at the doorway to check their colour. Is she reasonable? Explain. Yes. The profile (or distribution) of wavelengths ar e quite different. Take a look at the distribution of light given out by a standard fluorescent light (left) and a natural sunshine fluorescent (right). We can see the absence of several wavelengths. For example, in the normal florescent, a piece of navy-blue clothing can easily be mistaken as black. 3. Fire engines used to be red. Yellow-green is now the preferred colour. Why? Under broad daylight, the difference is small. However, in the darkness of night, red is not so easily seen b y the eye. Our eye is most sensitive to yellow -green colour, which is the dominant wavelength that the Sun produces. Therefore, painting fire engines yellow -green make them more easily distinguishable at night. (Singapore has yet to adopt this new colour scheme)

Hwa Chong Institution Sec 3 (SMTP) 2 4. Which EM waves are associated with high-energies? Which ones are the low-energies? Short wavelengths, such as gamma rays, X-rays and UV, are considered as high energy. Radio waves, microwaves and infrared are considered as low energy. 5. Does a microwave photon have a lot of energy? How can our food be "cooked" by microwaves? The complete description of the workings of a microwave ove n is beyond the scope of the syllabus. However, while a single photon of microwave does not have a lot of energy, we can still get a lot of microwave energy if we concentr ate a lot of microwave energy at a particular location. This high output of microwave energy is what the microwave oven does. A typical microwave oven uses about 1000 W during its operation. (1 W is 1 J of energy per second) 6. Can radar cook birds which happen to fly by during its operation? There are often allegations that an intense pulse of radar (radio waves) can cook unfortunate birds that fly past the radio dish. To check this claim, Mythbusters used a microwave transmitter to show that a cold turkey cannot be heated up significantly by merely putting it in front of a radiating dish. https://www.youtube.com/watch?v=-czRE9rdHMA&hl=en-GB&gl=SG. However, in the televised Japanese drama series Galileo 2 , the physicist (actor) showed an intense microwave beam directed at him that could burn a train ticket (thermal paper). The question about the birds cooked by radar remains an open one. 7. Human beings have a body temperature of around 37 oC. What is the dominant EM radiation that our bodies emit? The dominant wavelength produced by the human body is mostly in the infrared. That is why infrared thermometers are used to measure body temperatures. (Right): Infrared imaging. The hotter parts are shown as red , and the cooler parts are blue. In the show Galileo Season 2 episode 1, Professor Yukawa showed that a beam of intense microwave directed at a person can cause harm. He wore a special eyewear to protect his eyes from the microwaves.

Hwa Chong Institution Sec 3 (SMTP) 3 8. Can the human body produce EM waves other than infrared? Yes. However, it is very, very little in quantity compared to infrared. The larger the energy difference between that radiation and infrared, the lesser the radiation's quantity. However, we can do things to enhance other forms of radiation in the human body. For example, in a positron emission tomography (PET) scan, a radioactive isotope is injected into the body to induce gamma rays' emission. 9. Does light travel infinitely fast , or does it have a finite speed? The speed of light (or EM waves) is finite and is 299 792 458 m s-1. In practice, we say it is . 813 00 10 m s − . According to the theory of relativity, nothing travels faster than light in a vacuum. 10. Can we look into the past? We are always looking into the past. Since light takes time to travel from the object to our eyes, the light emitted from an object must have been emitted some time ago. The light from the person next to you was emitted a nanosecond earlier. The light from the Sun was emitted 8.5 minutes ago. The light from the nearest star was emitted more than four years ago, and the farthest objects in the Universe emitted their rays 13 billion years ago. Very ancient light indeed. 11. Is Earth a bright object in the solar system? That depends on which part of the EM spectrum. Since th e 1950s, there has been increasing use of radio waves on Earth (for telecommunication). Earth is , therefore, quite bright in radio. There are distinct differences between the radio from the Sun. For example, the Sun emits radio in a continuous spectrum whi le Earth's radio is emitted at specific frequencies. Do we have to be wary about aliens noticing our radio waves? The human-made radio waves that Earth emitted ha ve reached about 70 light -years, so we do not have to worry that some extra - terrestrials will soon visit us. 12. There is a rule (Wien's displacement law) that tells us the type of radiation an object emits depends on its temperature. Cool objects emit radio waves mostly, while very hot objects (e.g. plasma of about 10 million K) emit X-rays primarily. Therefore, by looking at the colour of stars, we can tell how hot the star surfaces are. There are red stars, orange stars, yellow stars, blue stars and white stars. Why are there no green stars? In the visible light spectrum, green is somewhere in the middle, sandwiched between blue and yellow. In other words, if a star is emitting in green dominantly, it is also emitting significant amounts of blue and yellow. The combination of the various wavelengths makes the star appear white. A PET/CT scan system

Hwa Chong Institution Sec 3 (SMTP) 4 SECTION A: MULTIPLE CHOICE QUESTIONS 1 B 2 C 3 A 4 C 5 D 6 D 7 C 8 C 9 B 10 B 11 D 12 A 13 C 14 A 15 B SECTION B: STRUCTURED QUESTIONS 1 Complete the gaps in the diagram of the electromagnetic spectrum. [1] RADIOWAVES Microwave Infrared Visible light Ultra-violet X-rays GAMMA b) State two properties of ultra-violet radiation, which differ from those of infra-red radiation. UV radiation has a higher frequency and can cause more harm to live tissues and organisms. c) State two properties that are common to all forms of electromagnetic radiation. Any two appropriate properties 1. They are all transverse waves. 2. They travel at a . 813 0 10 m s − in a vacuum but slow down in other media. 3. They do not require any material medium to spread from one point to another. 4. They transfer energy from one place to another. 5. They obey the laws of refraction and reflection. 2 Very short-wavelength radio waves can be used to determine the Moon's distance from the Earth by measuring the time taken for radio waves to travel from the Earth to the Moon and back again. Calculate the delay between the transmission and receptio n of the signal when the Moon is . 83 9 10 m from the Earth, given that the speed of electromagnetic waves is . 813 0 10 m s − . . .. 8 8 3 9 102 2 2 6 s 3 0 10 dt v  = = =    3a) Red on top, violet below b) Refractive index of violet is more than 1.5. The r efractive index is inversely proportional to speed which is directly proportional to wavelength. Violet has a shorter wavelength and hence a larger refractive index