Goyal Brothers Solutions for ICSE Class 10 Physics Chapter 6 Electromagnetic Waves And Spectrum

Exercise - 1

Question 1. What is the range of wavelength of visible light?
Answer: The band of visible light corresponds to wavelengths spanning from \( 4000\ \text{Å} \) up to \( 8000\ \text{Å} \) (which can also be written as \( 400\ \text{nm} \) to \( 800\ \text{nm} \)).
In simple words: Visible light is the small part of the light spectrum that our eyes can actually see, and its wavelengths run from 4000 to 8000 Angstroms.

Exam Tip: Always state the unit (Angstrom, Å or nanometer, nm) clearly when writing down the wavelength range of visible light, as examiners look for correct units.

Question 2. What do you understand by the term colour ?
Answer: Colour represents a sensory perception produced within our brain. It is not a physical, material object, but rather a subjective response triggered when electromagnetic waves of a specific wavelength stimulate the retina in our eyes.
In simple words: Colour is not a physical object you can touch. It is just the way our brain interprets different wavelengths of light when they hit our eyes.

Exam Tip: To secure full marks, define colour as a physiological sensation produced in the brain when the retina is excited by specific wavelengths of electromagnetic waves.

Question 3. (a) Define :
1. monochromatic light,
2. polychromatic light.
(b) Is white light polychromatic or monochromatic ? Explain your answer.
Answer: (a)
1. **Monochromatic light:** This term refers to light that is composed of only a single wavelength or a single distinct color.
2. **Polychromatic light:** This type of light consists of a blend of several different wavelengths or color bands.
(b) White light is polychromatic because it is not made of a single wavelength; instead, it is a composite mixture containing all seven major spectral color bands (from red to violet).
In simple words: Monochromatic light has only one color or wavelength, while polychromatic light is a mix of many colors. White light is a mix of all seven rainbow colors, so it is polychromatic.

Exam Tip: Use the prefix meanings: 'mono' means single, and 'poly' means many. Always mention 'single wavelength' and 'mixture of wavelengths' to score maximum points.

Question 4. Define or explain the following terms :
1. dispersion of light
2. spectrum
Answer:
1. **Dispersion of light:** This is the optical phenomenon in which a beam of white light splits into its individual constituent colors when it passes through a refracting medium like a glass prism.
2. **Spectrum:** This is the visible band of colors projected onto a screen or surface when a composite, polychromatic light undergoes dispersion.
In simple words: Dispersion is the process of splitting white light into a rainbow, and the rainbow band of colors that you see on a screen is called the spectrum.

Exam Tip: Ensure you describe the prism as the dispersing medium for dispersion, and a screen as the surface where the spectrum is captured.

Question 5. Draw a neat diagram for the dispersion of white light by a prism and label all the colours of the spectrum.
Answer: The schematic diagram below illustrates a narrow beam of white light entering a triangular glass prism, undergoing refraction and dispersing into its seven constituent colors (VIBGYOR) on a screen:

In simple words: When white light enters a glass prism, different colors bend by different amounts. Violet bends the most, and red bends the least, creating a beautiful rainbow spectrum.

Exam Tip: In your exams, always label the rays correctly in the sequence VIBGYOR from bottom to top on the screen, showing violet bending the most and red the least.

Question 6. Is the spectrum obtained by an equilateral glass prism pure or impure ? Explain your answer.
Answer: The spectrum formed by a single equilateral glass prism is an impure spectrum. This occurs because the different color bands spread out but overlap with their neighboring colors, meaning there are no sharp boundaries between them.
In simple words: The spectrum from a simple prism is impure because the colors blend and overlap into each other instead of staying in neat, separate bands.

Exam Tip: Use the term 'overlapping of colors' when explaining why a spectrum is impure, as this is the primary keyword evaluated by examiners.

Question 7. Draw a diagram showing that white light can be dispersed and then recombined by using two similar equilateral prisms.
Answer: The diagram below displays the dispersion of white light into its component rays by the first prism and its subsequent recombination back into a single white beam through an inverted second prism:

In simple words: If you place a second identical prism upside down behind the first one, it reverses the effect. The first prism splits the light, and the second one merges it back into white light.

Exam Tip: Make sure you draw the second prism completely inverted (upside down) relative to the first one. This inverted position is crucial for the recombination of colors.

Question 8. (a) What is ultraviolet spectrum ? What is the range of its wavelength in A° ?
(b) State four important properties of ultraviolet rays.
(c) State four uses of ultraviolet rays.
Answer: **(a)** The ultraviolet spectrum is the band of electromagnetic radiation lying just beyond the high-frequency violet end of the visible spectrum. Its wavelength spans from approximately \( 100\ \text{Å} \) to \( 4000\ \text{Å} \) (or \( 10\ \text{nm} \) to \( 400\ \text{nm} \)).
**(b)** Four key properties of ultraviolet rays are: 1. As electromagnetic waves, they carry no electric charge and are completely unaffected by electric or magnetic fields. 2. They conform to the standard optical laws of reflection and refraction. 3. They propagate through a vacuum with the speed of light, which is \( 3 \times 10^8\ \text{m/s} \) (or \( 3 \times 10^5\ \text{km/s} \)). 4. They exhibit strong chemical reactivity and can decompose silver salts, making them photographically active.
**(c)** Four practical uses of ultraviolet rays are: 1. They help in detecting food adulteration, such as identifying impurities in ghee, due to their ability to produce fluorescence. 2. They are widely employed to sterilize medical and surgical instruments as well as purify drinking water by killing microbes. 3. When absorbed by human skin, they stimulate the production of Vitamin D, which is essential for healthy bone development. 4. They are used in gemology to distinguish genuine diamonds from synthetic or imitation ones.
In simple words: Ultraviolet light is invisible light with wavelengths shorter than violet. It is used to kill germs, help our bodies make Vitamin D, find fake diamonds, and check if food has been mixed with cheap ingredients.

Exam Tip: Always write the speed in standard SI units (\( 3 \times 10^8\ \text{m/s} \)) or make sure to specify 'km/s' if writing \( 3 \times 10^5 \). Mixing up the units is a very common error.

Question 9. (a) What are infrared rays ? What is the range of wavelengths of infrared rays in A° ?
(b) State four important properties and four uses of infrared rays ?
Answer: **(a)** Infrared rays are electromagnetic waves located just beyond the long-wavelength red end of the visible spectrum. Their wavelengths range from about \( 8000\ \text{Å} \) to \( 10^7\ \text{Å} \) (or \( 800\ \text{nm} \) to \( 1\ \text{mm} \)).
**(b)** **Four key properties of infrared rays are:** 1. Due to their relatively long wavelengths, they undergo minimal scattering when passing through mist, fog, or dust particles. 2. They have a pronounced heating effect and can be detected using devices like a thermopile or a thermometer with a blackened bulb. 3. They travel in a vacuum at the speed of light, which is \( 3 \times 10^8\ \text{m/s} \). 4. Being electromagnetic wave packages, they are not deflected by magnetic or electric fields.
**Four key uses of infrared rays are:** 1. They are used in agricultural crop monitoring to detect variations in heat patterns, helping as an early warning system. 2. They are used for therapeutic purposes to relieve muscle pain and treat patients suffering from swollen joints. 3. They are utilized in specialized night vision binoculars and cameras. 4. They are employed in heat-seeking guidance systems for tracking and intercepting targets.
In simple words: Infrared rays are invisible heat rays with wavelengths longer than red light. They are useful because they can pass through fog, soothe sore muscles, help us see in the dark, and monitor the health of farm crops.

Exam Tip: When asked about infrared rays, always highlight their heating effect and their ability to penetrate fog and mist due to their longer wavelength.

Question 10. Write the names of all the components of electromagnetic spectrum in the order of increasing frequency.
Answer: The constituents of the electromagnetic spectrum arranged in ascending order of their frequency are:
1. Radio waves (\( \approx 3 \times 10^7\ \text{Hz} \))
2. Microwaves (\( \approx 3 \times 10^{11}\ \text{Hz} \))
3. Infrared waves (\( \approx 10^{12}\ \text{Hz} \))
4. Visible light (\( \approx 4 \times 10^{14}\ \text{Hz} \) to \( 8 \times 10^{14}\ \text{Hz} \))
5. Ultraviolet rays (\( \approx 10^{15}\ \text{Hz} \) to \( 10^{16}\ \text{Hz} \))
6. X-rays (\( \approx 3 \times 10^{16}\ \text{Hz} \) to \( 3 \times 10^{19}\ \text{Hz} \))
7. Gamma rays (\( > 10^{19}\ \text{Hz} \))
In simple words: To list them from lowest frequency (longest waves) to highest frequency (shortest waves): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and finally gamma rays.

Exam Tip: Increasing frequency is the same as decreasing wavelength. Make sure you read the question carefully so you do not write the order in reverse.

Question 11. What do you understand by the term “scattering of light”? Which colour of white light is scattered the least and why?
Answer: Scattering of light occurs when an incoming beam of light encounters tiny particles or molecules in its path, which absorb the light energy and subsequently re-emit it in various random directions.
Among the different components of white light, the **red color** is scattered the least. According to Rayleigh's scattering law, the intensity of scattered light is inversely proportional to the fourth power of its wavelength (\( I \propto \frac{1}{\lambda^4} \)). Because red light has the longest wavelength in the visible spectrum, it undergoes the least scattering.
In simple words: Scattering happens when light hits dust or air molecules, gets absorbed, and then bounces off in all directions. Red light bounces around the least because its waves are long and can easily pass by small particles.

Exam Tip: To earn maximum points, always state Rayleigh's law of scattering formula, \( I \propto \frac{1}{\lambda^4} \), explaining that longer wavelengths scatter less.

Question 12. Explain the following :
(a) Why does the sun appear red during sunrise or sunset?
(b) Why does the sky appear blue on the earth, but black on the moon ?
(c) Why are orange or yellow lights used as fog lights in automobiles ?
(d) Why do the distant hills appear blue ?
(e) Why does the sky appear blue, instead of violet ?
Answer: **(a)** At sunrise and sunset, sunlight has to travel through a much thicker layer of the Earth's atmosphere to reach our eyes. During this long journey, most of the shorter wavelengths (like blue and violet) are heavily scattered out of the direct line of sight. The longer wavelengths (such as red and orange) pass through with minimal scattering, making the sun look red.

**(b)** On Earth, the atmosphere contains gas molecules and fine dust particles that scatter sunlight. Since shorter blue wavelengths scatter more easily than red, the sky appears blue. On the Moon, there is no atmosphere to scatter light, so sunlight travels in straight lines without bouncing, leaving the sky looking completely black.
**(c)** Yellow or orange lights have longer wavelengths compared to green, blue, or violet. Because of this, they are scattered far less by fog, mist, and water droplets in the air. These colors can penetrate through foggy conditions much more effectively, improving visibility for drivers.
**(d)** Distant hills appear blue because the vast amount of air between us and the hills scatters the shorter wavelengths of blue light towards our eyes, masking the actual colors of the terrain.
**(e)** Although violet has an even shorter wavelength than blue and scatters more, the sky still looks blue to us. This happens for two main reasons: first, solar radiation contains far more blue light than violet; second, human eyes are much more sensitive to blue wavelengths than to violet.
In simple words: The sky is blue because air scatters blue light more than other colors. At sunset, the light travels through so much air that all the blue is scattered away, leaving only red and orange. Since the moon has no air, its sky is pitch black.

Exam Tip: In questions about color appearances in nature, always attribute the color change to 'scattering of light' and relate the amount of scattering to the wavelength of the light involved.

Multiple Choice Questions

Question 1. Visible light has electromagnetic waves between :
(a) 8000 Å to 6000 Å
(b) 8000 Å to 1000 Å
(c) 8000 Å to 4000 Å
(d) 8000 Å to 2000 Å
Answer: (c) 8000 Å to 4000 Å
In simple words: The light we can see has wavelengths ranging from 4000 to 8000 Angstroms.

Exam Tip: Be sure to memorize the visible spectrum limits in both Angstroms (\( 4000\ \text{Å} - 8000\ \text{Å} \)) and nanometers (\( 400\ \text{nm} - 800\ \text{nm} \)).

Question 2. 1 Angstrom unit (A°) is equal to :
(a) \( 10^{-10}\ \text{m} \)
(b) \( 10^{-12}\ \text{m} \)
(c) \( 10^{-8}\ \text{m} \)
(d) \( 10^6\ \text{m} \)
Answer: (a) \( 10^{-10}\ \text{m} \)
In simple words: One Angstrom is an extremely tiny measurement of length, equal to one ten-billionth of a meter.

Exam Tip: Keep in mind that \( 1\ \text{Å} = 10^{-10}\ \text{m} = 10^{-8}\ \text{cm} \). Knowing both forms is extremely helpful for multiple-choice questions.

Question 3. The most visible colour of visible spectrum is :
(a) blue
(b) red
(c) green
(d) orange
Answer: (b) red
In simple words: Red light is highly visible over long distances because its long waves do not scatter easily in the air.

Exam Tip: Red is the most visible over long distances under scattering conditions, which is why it is used as a safety/danger signal.

Question 4. The. band of seven colours formed, when white light passes through an equilateral prism is called :
(a) dispersion
(b) colour band
(c) spectrum
(d) none of the options
Answer: (c) spectrum
In simple words: A spectrum is the rainbow pattern of colors created when white light splits as it passes through a prism.

Exam Tip: Do not confuse the process (dispersion) with the product (spectrum). Dispersion is the phenomenon, while the spectrum is the actual band of colors obtained.

Question 5. Violet, green, red and yellow are the four colours formed on a white screen when a polychromatic light is dispersed. The arrangement of colours in the decreasing order of wave lengths is :
(a) green, red, yellow and violet
(b) red, yellow, green and violet
(c) violet, green, yellow and red
(d) violet, green, red and yellow
Answer: (b) red, yellow, green and violet
In simple words: When listing these colors from longest wavelength to shortest, the correct order is red, yellow, green, and then violet.

Exam Tip: Write down the VIBGYOR sequence first. Wavelength decreases from left to right (Red to Violet), so decreasing order means going from R to V.

Question 6. The angle of deviation is maximum for ……. when the dispersion of polychromatic light takes place
(a) red
(b) violet
(c) green
(d) blue
Answer: (b) violet
In simple words: Violet light bends or deviates the most when it travels through a glass prism.

Exam Tip: Remember that deviation is inversely proportional to wavelength. Shorter wavelengths like violet deviate the most, whereas longer wavelengths like red deviate the least.

Question 7. Which is not a source of ultraviolet radiations ?
(a) Sun
(b) Arc lamp
(c) Electric heater
(d) Star
Answer: (c) Electric heater
In simple words: Electric heaters produce heat, which is infrared radiation, not ultraviolet radiation.

Exam Tip: Remember that hot, incandescent bodies like electric heaters emit infrared (heat) radiation, not UV radiation.

Question 8. The electromagnetic radiation which are useful in night photography is:
(a) infrared rays
(b) ultraviolet rays
(c) visible light
(d) X-rays
Answer: (a) infrared rays
In simple words: Infrared rays can detect heat emitted by objects, making them very useful for taking photos in pitch-black darkness.

Exam Tip: For any question regarding photography in dark, foggy, or misty conditions, infrared radiation is always the correct choice.

Question 9. The electromagnetic wave associated with high energy is:
(a) ultraviolet ray
(b) X-ray
(c) gamma ray
(d) infrared ray
Answer: (c) gamma ray
In simple words: Gamma rays have the highest frequency and shortest wavelength, giving them the highest energy of any electromagnetic wave.

Exam Tip: Energy of an electromagnetic wave is directly proportional to its frequency (\( E = hf \)). Gamma rays have the highest frequency, hence the highest energy.

Question 10. During the Sunset the colour which scatters closest to the eye of observer is:
(a) yellow
(b) red
(c) blue
(d) violet
Answer: (b) red
In simple words: At sunset, most of the other colors get scattered away by the air, leaving red light to reach directly to our eyes.

Exam Tip: At sunrise/sunset, red light reaches our eyes directly with minimal scattering, while blue light scatters away from our line of sight.

Question 11. Infrared rays with very large wavelength are easily reflected by
(a) air
(b) glass
(c) carbon dioxide molecules
(d) both (b) and (c)
Answer: (c) carbon dioxide molecules
In simple words: Carbon dioxide molecules in our atmosphere are very good at absorbing and reflecting long infrared heat waves.

Exam Tip: This reflection and absorption of infrared rays by carbon dioxide molecules is the key mechanism behind the greenhouse effect.

Question 12. X-rays are produced by
(a) oscillating circuits in special vacuum tubes
(b) excitation of outermost electronic shell of an atom
(c) excitation of inner electron of atom and is sudden annihilation of high energy free electrons.
(d) nucleus of an atom, by the destruction of high energy particle.
Answer: (c) excitation of inner electron of atom and is sudden annihilation of high energy free electrons.
In simple words: X-rays are made when fast-moving electrons crash into a heavy metal target, displacing inner electrons of atoms.

Exam Tip: Remember that transition of inner shell electrons inside heavy atoms produces highly energetic X-rays, whereas nuclear changes produce gamma rays.

Questions from ICSE Examination Papers

2005

Question 1. Name any two electromagnetic waves which have a frequency higher than that of violet light. State one use of each.
Answer: Two electromagnetic radiations that possess a higher frequency than visible violet light are:
1. **X-rays:** These are extensively utilized in medicine to take images of the skeletal system to detect fractures or bone defects.
2. **Gamma rays:** These highly energetic rays are used in radiation therapy to target and destroy malignant cancer cells.
In simple words: X-rays and gamma rays have higher frequencies than violet light. X-rays help doctors see broken bones, and gamma rays are used to treat cancer.

Exam Tip: Always pair the radiation with its specific, primary application (such as medical imaging for X-rays) to ensure you get full credit.

Question 2. A glass slab is placed over a page on which the word vibgyor is printed wit!, each letter in its corresponding colour.
(a) Will the image of all the letters be in the same place ?
(b) If not, which letter will be raised to the maximum. Give a reason for your answer.
Answer: (a) No, the images of the printed letters will appear at different depths and will not lie in the same plane.
(b) The letter **V (Violet)** will be raised the most. This occurs because the refractive index of glass is highest for violet light (as refractive index is inversely proportional to wavelength). Since the apparent upward shift of an image increases with a higher refractive index (\( \text{Shift} = t(1 - \frac{1}{\mu}) \)), violet light experiences the greatest shift.
In simple words: No, they will not look like they are at the same depth. Violet is raised the most because glass bends violet light more than any other color, making it look closer to the surface.

Exam Tip: Remember that apparent shift is directly proportional to the refractive index of the medium, which is inversely proportional to wavelength. Use this step-by-step chain of logic to write your answer.

2006

Question 3. Give one use each of the following electromagnetic radiations given below
(a) Microwaves
(b) Ultraviolet radiation,
(c) Infrared radiation.
Answer: **(a) Microwaves:** These waves are utilized for rapid heating and cooking of food in microwave ovens, as well as in radar transmissions.
**(b) Ultraviolet radiation:** This radiation is used in water purifiers to sterilize drinking water and in clinics to disinfect surgical tools.
**(c) Infrared radiation:** These waves are commonly used in television remote controllers and wireless communication systems.
In simple words: Microwaves are used to heat food quickly, ultraviolet rays are used to kill germs in water, and infrared rays are used in TV remotes.

Exam Tip: When asked for 'one use each', list the most common everyday application (such as TV remotes for infrared) as it is the easiest to remember and highly recognized in marking schemes.

2007

Question 4. Why are infrared radiations preferred over ordinary visible light for taking photographs in fog ?
Answer: Infrared radiations possess much longer wavelengths than ordinary visible light. Because of this, they undergo minimal scattering when traveling through small particles of dust, fog, or smoke. Consequently, they can penetrate foggy air and capture clear images using specialized infrared-sensitive photographic plates.
In simple words: Infrared waves are long enough to pass through fog without bouncing off the water droplets, making them perfect for taking clear pictures in bad weather.

Exam Tip: Focus your explanation on the direct relationship between longer wavelengths and reduced scattering to satisfy the grading criteria.

Question 5. (a) A particular type of high energy invisible electromagnetic rays help us to study the structure of crystals. Name these rays and give another important use of these rays,
(b) How does the speed of light in glass change on increasing the wavelength of light ?
Answer: **(a)** The rays used to analyze crystal structures are **X-rays**. Another prominent application of X-rays is in medical radiography to locate fractured bones or detect foreign objects inside the body.
**(b)** The velocity of light in a glass medium increases as its wavelength increases. Because red light has a longer wavelength than violet light, it travels faster through glass.
In simple words: X-rays are used to study crystals and find swallowed objects or broken bones. In glass, colors with longer wavelengths (like red) travel faster than colors with shorter wavelengths.

Exam Tip: For part (b), remember the relation \( v = f\lambda \). In a refracting medium, light of a longer wavelength moves faster because the refractive index for it is lower.

2009

Question 6. (a) Why is white light considered to be polychromatic in nature?
(b) Give the range of the wavelength of those electromagnetic waves which are visible to us.
Answer: **(a)** White light is polychromatic because it is a blend of seven distinct color bands, represented by the acronym VIBGYOR. Although they travel at the same speed in a vacuum, they separate when passing through a dispersive medium.
**(b)** The wavelength band of visible electromagnetic waves is approximately \( 4000\ \text{Å} \) to \( 8000\ \text{Å} \).
In simple words: White light is called polychromatic because it is actually a mixture of seven different colors. The wavelengths we can see are between 4000 and 8000 Angstroms.

Exam Tip: Always write VIBGYOR in capital letters and define polychromatic explicitly as 'containing multiple wavelengths' for clarity.

2010

Question 7. Two parallel rays of Red and Violet travelling through air, meet the air-glass boundary as shown in the figure:
(a) Will their paths inside the glass be parallel ? Give a reason for your answer.
(b) Compare the speeds of the two rays inside the glass.
Answer: **(a)** No, their paths inside the glass will no longer be parallel. Glass has a higher refractive index for violet light than for red light (\( \mu_v > \mu_r \)). Consequently, the violet ray bends (deviates) more sharply than the red ray, making their angles of refraction different and causing their paths to diverge.
**(b)** Inside the glass slab, the red ray travels faster than the violet ray because the speed of light in a medium is inversely proportional to its refractive index (\( v = \frac{c}{\mu} \)).
In simple words: No, they won't be parallel inside the glass because glass bends violet light more than red light. Red light also travels faster through the glass than violet light does.

Exam Tip: Remember to write the inequality \( \mu_v > \mu_r \) to clearly show the examiner that you understand how refractive index varies with color.

2011

Question 8. (a) Suggest one way, in each case, by which we can detect the presence of:
1. Infrared radiations.
2. Ultraviolet radiations.
(b) Give one use of Infrared radiations.
Answer: **(a)** 1. **Infrared radiations:** We can detect them by moving a thermometer with a blackened bulb across the spectrum beyond the red end. A rapid, distinct rise in temperature indicates the presence of invisible heat-carrying infrared rays. 2. **Ultraviolet radiations:** We can detect them by placing silver chloride paper in the invisible region beyond the violet end. The paper turns violet, then dark brown, and eventually black due to chemical decomposition.
**(b)** **Use of infrared radiation:** These rays are widely used in handheld remote controls for televisions and media players.
In simple words: You can detect infrared using a thermometer with a blackened bulb (it will show a temperature jump), and ultraviolet using silver chloride paper (it will turn dark). Infrared is used in TV remotes.

Exam Tip: Specifically mention a 'blackened' bulb thermometer for detecting infrared, as black surfaces absorb heat radiation most efficiently.

2012

Question 9. (a) What is meant by ‘Dispersion of light’ ?
(b) In the atmosphere which colour of light gets scattered the least ?
Answer: **(a)** Dispersion of light is the process where a composite beam of white light splits up into its individual constituent colors when it passes through a transparent refracting medium like a prism.
**(b)** In our atmosphere, the **red** color of light undergoes the least amount of scattering.
In simple words: Dispersion is when white light splits into its rainbow colors inside a prism. Red is the color that scatters the least in our air.

Exam Tip: Ensure you define dispersion as 'splitting of white light' rather than just 'bending of light' to get full credit.

2013

Question 10. Name the radiations :
(a) that are used for photography at night
(b) used for detection of fracture in bones.
(c) whose wavelength range is from 100 A to 4000 A (or 10 nm to 400 nm).
Answer: **(a)** Infrared radiation is used for capturing photographs in the dark.
**(b)** X-rays are used to identify fractures or breaks in bones.
**(c)** Ultraviolet radiation has a wavelength range spanning from \( 100\ \text{Å} \) to \( 4000\ \text{Å} \).
In simple words: Night photos use infrared, finding bone breaks uses X-rays, and the waves from 100 to 4000 Angstroms are ultraviolet.

Exam Tip: Memorize the exact wavelength and frequency ranges of UV, visible, and infrared waves as they are highly tested in matching and fill-in-the-blank questions.

2014

Question 11. (a) Name the prism required for obtaining a spectrum of Ultraviolet light.
(b) Name the radiations which can be detected by a thermopile.
Answer: **(a)** A **quartz prism** is required because ordinary glass absorbs ultraviolet light, whereas quartz is completely transparent to it.
**(b)** A thermopile is highly sensitive to heat and is used to detect **infrared radiations**.
In simple words: You need a quartz prism to see ultraviolet light because regular glass blocks it. A thermopile is used to detect heat-carrying infrared rays.

Exam Tip: Always explain 'why' a quartz prism is used (glass absorbs UV light) even if the question only asks to name the prism, to show a complete understanding.

Question 12. Why is red colour used as a sign of danger?
Answer: Red light has the longest wavelength among all the visible colors. Because of this, it is scattered the least by dust, mist, and air molecules in the atmosphere. This allows red light to travel long distances without losing its intensity, making it clearly visible from far away.
In simple words: Red light has very long waves, so it doesn't get scattered by dust or fog. This means it can travel very far and be seen easily from a long distance, which is perfect for danger signs.

Exam Tip: Link the choice of red as a danger signal directly to its high wavelength and minimal atmospheric scattering.

2015

Question 13. (a) Why does the Sun appear red at sunrise?
(b) Name the subjective property of light related to its wave-length.
Answer: **(a)** During sunrise, sunlight must traverse a much greater thickness of the Earth's atmosphere to reach our eyes. During this long travel, the shorter wavelengths of light (like blue and violet) are scattered away in all directions by gas molecules. The longer wavelength red light remains largely unscattered and reaches our eyes directly, making the Sun appear red.
**(b)** The subjective sensation of light that corresponds directly to its physical wavelength is **colour**.
In simple words: At sunrise, sunlight travels through a lot of air, which scatters away the blue light. Only the long red waves reach our eyes. Our brain subjectively perceives different wavelengths of light as different colors.

Exam Tip: Be careful with the terms 'subjective' and 'objective'. Color is a subjective perception of the brain, while frequency and wavelength are objective, measurable physical properties.

Question 14. (a) Name the high energetic invisible electromagnetic waves which help in the study of the structure of crystals.
(b) State an additional use of the waves mentioned in part (a).
Answer: **(a)** The highly energetic, invisible electromagnetic waves utilized to study the arrangements of atoms in crystals are **X-rays** (via X-ray diffraction).
**(b)** An additional important use of X-rays is in medical diagnostics to locate fractures in bones and identify cavities in teeth.
In simple words: The high-energy waves used to study crystal structures are X-rays. Doctors also use X-rays to check for broken bones and dental issues.

Exam Tip: When writing about uses of X-rays, mention 'medical radiography' or 'detection of fractures' as highly standard textbook examples.

2016

Question 15. What do you understand by the term “Scattering of light”? Which colour of white light is scattered least and why ?
Answer: Scattering of light is the optical phenomenon where light waves strike tiny particles (such as gas molecules or dust) in the atmosphere, get absorbed, and are then re-radiated in various random directions.
The **red** component of white light is scattered the least. According to Rayleigh's law of scattering, the intensity of scattered light is inversely proportional to the fourth power of its wavelength (\( I \propto \frac{1}{\lambda^4} \)). Since red light has the longest wavelength in the visible spectrum, it experiences the lowest amount of scattering.
In simple words: Scattering is when light hits tiny particles in the air, gets absorbed, and then shoots out in all directions. Red light scatters the least because its waves are the longest.

Exam Tip: In questions about scattering, always mention Rayleigh's scattering formula \( I \propto \frac{1}{\lambda^4} \) to demonstrate a strong technical grasp of the topic.