Chapter 1 · ApplicationsLight at workExplore online

Applications & Explore Online പ്രകാശത്തിന്റെ അപവർത്തനം

Refraction and total internal reflection are not just textbook ideas. They carry the internet into Kerala homes through KFON cables, let a doctor look inside a stomach, make a cycle visible at night without a bulb, turn light round corners in periscopes and binoculars, make a diamond sparkle, paint “water” on a hot national highway and make a temple pond look dangerously shallow. Every lab below uses only the ideas of this chapter – and the computer works out every ray with the real law of refraction.

📖 The ideas behind every lab

All angles are measured from the normal (ലംബം), never from the surface.

1. Optical fibre internet – light in a glass thread

An optical fibre (ഒപ്റ്റിക്കൽ ഫൈബർ) is a glass thread thinner than a hair. Its centre, the core (ആന്തരകോർ), is wrapped in a cladding (ക്ലാഡിങ്) of slightly rarer glass and an outer jacket (ബാഹ്യകവചം) for protection. Light that enters one end meets the wall of the core at an angle greater than the critical angle, so it is totally reflected – again and again – and comes out only at the far end, even when the cable bends. Messages travel as quick pulses of light (light on = 1, light off = 0). One cable carries thousands of signals at once, at the speed of light in glass, over long distances and with little loss. The KFON (Kerala Fibre Optic Network) cables strung along KSEB electric posts carry the internet to homes, schools and offices this way.

💡 Fibre lab: total internal reflection along a bendable fibre

The core is drawn hugely enlarged. Each time the ray meets the wall, the computer finds the angle of incidence and compares it with the critical angle: greater → total internal reflection; smaller → the light refracts out and the signal is lost. Real fibres bend gently compared with their thickness, so the light stays in. The cladding values are example numbers to show the idea.

📝 In the exam this looks like: “What is the principle of an optical fibre? Why does light not come out through its sides?” → Total internal reflection. Light inside the core travels from a denser to a rarer medium and strikes the wall at an angle of incidence greater than the critical angle, so it is totally reflected each time and comes out only at the far end. Advantages for communication: thousands of signals at once, at the speed of light, over long distances, with very little loss of intensity.

2. Endoscopy – a doctor’s eye on a fibre

In endoscopy (എൻഡോസ്കോപ്പി) a doctor slides a thin, flexible tube through the mouth into the stomach. Inside the tube are two bundles of optical fibres. One bundle carries light from a lamp into the stomach. The other carries the picture back out: each fibre brings back the light from one tiny spot, so the image is built up like the dots of a mobile-phone screen. Because the light travels by total internal reflection, it follows the tube round every bend – no cutting needed.

🩺 Endoscope lab: one fibre = one dot of the picture

The picture on the screen has one dot for every fibre. More, thinner fibres give a sharper picture – real endoscopes have many thousands. The fibres must also stay in the same order at both ends; jumble them and the dots arrive in the wrong places.

📝 In the exam this looks like: “How are optical fibres used in the medical field?” → In endoscopy: a bundle of optical fibres carries light into the body (for example the stomach), and another bundle carries the image back out by total internal reflection, so the doctor can see inside without surgery. (Textbook Fig 1.25: collect information and present it in class.)

3. Cycle reflectors and night road safety

A cycle or lorry reflector (റിഫ്ലക്റ്റർ) has no bulb and no battery, yet it shines brightly in the headlights of the vehicle behind. It is made of many small right-angled prisms (പ്രിസം) (Fig 1.22). Light enters the front face PQ along the normal, so it is not refracted. Inside, it meets the slanted face at A at 45°, which is greater than 42°, the critical angle of glass – total internal reflection. It meets the face at B, again at 45° – total internal reflection again – and leaves through PQ parallel to the incoming light, going back towards the driver. A plane mirror would send the light back to the driver only if it faced the car exactly.

🚲 Reflector lab: plane mirror vs prism reflector

The prism is glass (n = 1.5). The computer traces each ray with the law of refraction and the critical angle. In this flat (2-D) prism, total internal reflection at both faces lasts until the beam is about 4.8° off; beyond that part of the light leaks out of one face (orange). Even then, the light that comes back always travels parallel to the incoming beam – straight back to the car. Real reflectors use tiny three-faced “corner cube” prisms, which work over a much wider range of directions.

⚠ At night, keep your cycle’s red rear reflector and the white front reflector clean and unbroken, wear light-coloured clothes or a reflective strip, and never ride without them – reflectors work only when someone’s headlights fall on them.

📝 In the exam this looks like: Let us assess Q8 – “Why are many small prisms used in motorcycle reflectors?” → Light from the headlights of a vehicle behind falls normally on the prisms, strikes the slanted faces at 45° (greater than the critical angle of glass, 42°), is totally internally reflected twice and goes back towards the driver. So the motorcycle is seen at night without any bulb; many prisms make it bright and visible from many directions.

4. Prism periscopes and binoculars

A periscope (പെരിസ്കോപ്പ്) lets you see over a wall or out of a submarine. The simple ones use two plane mirrors, but better ones use two right-angled glass prisms (Fig 1.23). Light enters a prism along the normal, meets the slanted face (the hypotenuse) at 45°, is totally reflected and turns through 90°; the second prism turns it again towards the eye. Total internal reflection sends back all the light, while a silvered mirror absorbs a little at each reflection and its front glass surface adds a faint second image – so the prism periscope gives a clearer, brighter view. Prism binoculars use the same trick: right-angled prisms fold the long path of the light inside a short body.

🔭 Periscope lab: what if the prism were made of something else?

The light always meets the slanted face at 45°. Total internal reflection needs 45° to be greater than the critical angle, so the prism must have a critical angle below 45° – a refractive index above about 1.41. Glass (42°) works; water (48.6°) does not – most of the light escapes through the slanted face and only a little is reflected.

📝 In the exam this looks like: “Why are right-angled prisms used in a periscope instead of plane mirrors?” → Light falls normally on a face of the prism and meets the hypotenuse at 45°, which is greater than the critical angle of glass (42°). It is totally internally reflected and turned through 90°. Total internal reflection reflects all the light, so the image is brighter and clearer than with mirrors (no faint double image). Draw the diagram with the two prisms as in Fig 1.23.

5. Why diamonds sparkle

Diamond has the highest refractive index in our table: 2.42. Light is very slow inside it (about 1.24 × 108 m/s), and its critical angle is only 24.4°, compared with 42° for glass. A jeweller cuts the bottom of a diamond into slanted faces. Light entering through the top meets those faces at angles greater than 24.4°, so it is totally internally reflected again and again and finally comes out through the top, towards your eye – the diamond sparkles. A glass copy of exactly the same shape lets much of the light leak out of its bottom faces, because 42° is a much bigger critical angle to beat.

💎 Sparkle lab: the same cut in diamond and in glass

A side view (cross-section) of a “brilliant” cut. Eight parallel rays fall on the top face; the computer follows every ray with the law of refraction, total internal reflection and the small part that is reflected at each surface. Thick, bright rays carry more light. The “sparkle” bar is the share of the light that comes back out through the top.

📝 In the exam this looks like: “The critical angle of diamond is 24.4° and that of glass is 42°. Which will sparkle more? Why?” → Diamond. Its critical angle is much smaller, so light inside it meets the faces at angles greater than the critical angle far more often; it is totally internally reflected many times and comes out through the top. Light is also slowest in diamond (v = c/n = 3 × 108/2.42 ≈ 1.24 × 108 m/s) – it is the optically densest medium in the table.

6. The highway mirage – “water” on a hot road

On a sunny afternoon on the NH, the road far ahead seems to have a pool of water that disappears as you reach it (Fig 1.21). Near the hot road the air is hotter and therefore optically rarer; higher up the air is cooler and denser. Light from the sky or a distant tree travelling down at a small angle passes from denser into rarer layers, so it bends away from the normal more and more. Near the road its angle of incidence becomes greater than the critical angle and it is totally reflected upwards into your eyes. You see the sky – and an upside-down tree – below the road surface, just like a reflection in water. This is a mirage (മരീചിക).

🛣️ Mirage lab: rays bending through layers of hot air

Heights are stretched about 28 times compared with distances, otherwise the rays would look flat. The air above is at 30 °C; the hottest layer lies just above the road. The refractive index of air changes only in the fifth decimal place, so only rays that dip less than about half a degree below the horizon are turned back up – that is why a mirage is always seen far ahead. The panel at the top left shows what the driver sees towards the tree and along the open road.

📝 In the exam this looks like: “Why does the road far ahead look wet on a hot summer day?” → The air near the hot road is hotter and optically rarer; the layers above are denser. Light from the sky and from distant objects passes from denser to rarer layers and bends away from the normal more and more; when the angle of incidence exceeds the critical angle it is totally internally reflected upwards. It reaches our eyes from below, and we see an inverted image, as if reflected from water – a mirage.

7. Pond and pool safety – deeper than it looks

Light from the bottom of a pond bends away from the normal as it leaves the water, and your eye traces it back along a straight line – so the bottom seems raised. Looking straight down, the bottom seems to be at about three-quarters of its real depth (real depth ÷ 1.33). From farther away, at a slant, the light leaves at a larger angle and bends even more, so the pond looks much shallower still (textbook p. 15). A temple pond or a quarry pool that looks waist-deep from the steps may be well over your head.

🏊 Pond lab: apparent depth and the viewing angle

The real ray from the stone to the eye is red; the eye “believes” the light came straight along the dashed line, so the stone appears at the faint position. The apparent depth is worked out from the law of refraction (water 1.33). The curve on the right shows how fast the pond seems to get shallower as you look from farther away.

⚠ Ponds, quarry pools, canals and rivers are deeper than they look. Never jump or wade in to “check” the depth; swim only where an adult and a lifeguard say it is safe.

📝 In the exam this looks like: “The bottom of a pond seems more raised when seen from far away than from close by. Why?” → Light from the bottom bends away from the normal as it passes from water (denser) into air (rarer). From far away the light reaching the eye leaves the water at a larger angle, so it bends more and the bottom appears more raised – the pond looks shallower than it really is. Also: “A fisherman aims below the fish he sees – why?” → the fish appears higher than it really is.

✅ Check yourself

🌐 Explore online – resources hub

Everything in this guide works offline. The links below are extras: simulations, short videos and articles from trusted sources (PhET and others), grouped by topic. They need internet and open in a new tab, so you will not lose your place here. Ask a parent or teacher before you go online. The PhET simulations (for example Bending Light, which the textbook mentions) run directly in your browser – nothing to install.

Bending of light (refraction)

🌐 Bending of light (refraction) needs internet · opens in a new tab

Speed of light and refractive index

🌐 Speed of light and refractive index needs internet · opens in a new tab

Everyday refraction: coins, pools and fishing

🌐 Everyday refraction: coins, pools and fishing needs internet · opens in a new tab

Atmospheric refraction: twinkling stars and sunsets

🌐 Atmospheric refraction: twinkling stars and sunsets needs internet · opens in a new tab

Total internal reflection and critical angle

🌐 Total internal reflection and critical angle needs internet · opens in a new tab

Optical fibres and endoscopy

🌐 Optical fibres and endoscopy needs internet · opens in a new tab

Mirages

🌐 Mirages needs internet · opens in a new tab

Reflectors, periscopes and prisms

🌐 Reflectors, periscopes and prisms needs internet · opens in a new tab

Malayalam explainers

🌐 Malayalam explainers needs internet · opens in a new tab

More to explore

🌐 More to explore needs internet · opens in a new tab
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Now pull the whole chapter together: Summary & Quick Quiz ▶