Chapter 13: Light

1. What helps us see objects?

Light reflected from or emitted by the object entering our eyes.(Vastu se reflect hui ya nikli hui light ka aankhon mein aana).

2. What is reflection of light?

Bouncing back of light from a surface.(Prakash ka kisi satah se takrakar vaapas lautna).

3. State the first law of reflection.

Angle of incidence equals angle of reflection (∠i = ∠r).

4. What is the angle between the incident ray and the normal called?

Angle of incidence (∠i).

5. What kind of reflection forms a clear image?

Regular reflection.

6. Give an example of a surface causing diffused reflection.

Paper, Wall, Wood.(Kagaz, Deewar, Lakdi).

7. Can reflected light be reflected again?

Yes.

8. What phenomenon causes multiple images with angled mirrors?

Multiple reflections.

9. How many colours make up white sunlight?

Seven (VIBGYOR).

10. What is dispersion of light?

Splitting of white light into its constituent colours.(Safed prakash ka apne rangon mein bant jaana).

11. Name the transparent front part of the eye.

Cornea.

12. What controls the amount of light entering the eye?

Iris (by adjusting pupil size).

13. Where is the image formed inside the eye?

Retina.

14. Which cells on the retina are sensitive to bright light and colour?

Cones.

15. Which vitamin deficiency can cause night blindness?

Vitamin A.

16. What is the Braille system used for?

Enabling visually impaired people to read and write.(Drishti badhit logon ko padhne likhne mein madad karna).

17. Who developed the Braille system?

Louis Braille.

1. How are we able to see non-luminous objects?

• Non-luminous objects do not produce their own light.
• We see them when light from a luminous source (like sun ☀️ or bulb 💡) falls on them.
• This light gets reflected (usually diffused reflection) from the object’s surface.
• When this reflected light enters our eyes 👁️, we are able to see the object. (Jab dusre source se light un par gir kar reflect hoti hai aur hamari aankhon mein aati hai).

2. State the two laws of reflection.

• First Law: The angle of incidence (∠i) is equal to the angle of reflection (∠r). (Aapatan kon = Pravartan kon).
• Second Law: The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane. (Incident ray, reflected ray aur normal ek hi plane mein hote hain).

3. Differentiate between regular and diffused reflection.

• Regular Reflection: Occurs on smooth/polished surfaces (mirror). Parallel incident rays remain parallel after reflection. Forms clear images. (Chikni satah par hota hai. Parallel rays reflect hokar parallel rehti hain. Saaf image banti hai).
• Diffused Reflection: Occurs on rough/irregular surfaces (wall, paper 🧱📄). Parallel incident rays scatter in different directions after reflection. Does not form clear images, makes object visible. (Khurduri satah par. Parallel rays alag-alag disha mein phail jaati hain. Saaf image nahi banti, par vastu dikhti hai).

4. What makes sunlight appear white even though it consists of seven colours?

• Sunlight is indeed a mixture of seven colours (VIBGYOR 🌈).
• When all these colours are mixed together in the right proportion, the resulting light appears white to our eyes. (Jab yeh saare rang sahi anupaat mein milte hain, toh roshni safed dikhai deti hai).
• A prism can separate these colours (dispersion), and combining them again (e.g., with another inverted prism or a spinning Newton’s disc) reforms white light.

5. Describe the function of Iris and Pupil in the human eye.

• Iris: The coloured, muscular diaphragm in the eye. Its main function is to control the size of the pupil. (Aankh ka rangeen hissa jo pupil ka size control karta hai).
• Pupil: The small opening in the centre of the iris. It allows light to enter the eye. (Iris ke beech ka chhota chhed jisse light andar aati hai).
• How they work together: In bright light ☀️, the iris contracts, making the pupil smaller to limit light entry. In dim light 🌃, the iris expands, making the pupil larger to allow more light in. (Iris pupil ko chhota/bada karke aankh mein jaane wali light ko regulate karta hai).

6. What are rods and cones? Where are they located and what are their functions?

• Rods and Cones are two types of light-sensitive nerve cells located in the **retina** at the back of the eye. (Yeh retina mein sthit prakash-samvedansheel koshikayein hain).
• Rods: Sensitive to dim light. Help us see in low light conditions (night vision). Do not sense colour. 🌃 (Kam roshni ke liye sensitive hain. Raat mein dekhne mein madad karte hain. Rang nahi pehchaan sakte).
• Cones: Sensitive to bright light and colours 🌈. Help us see colours and sharp details in good light. ☀️ (Tez roshni aur rangon ke liye sensitive hain. Rang aur detail dekhne mein madad karte hain).

7. Why is it important to take care of our eyes? Mention two ways.

• Importance: Eyes are precious organs allowing us to see the world. Damage can lead to vision impairment or blindness.(Aankhein anmol hain jo humein duniya dekhne deti hain. Nuksaan se drishti kam ya khatam ho sakti hai).
• Way 1: Balanced Diet: Eat Vitamin A rich food (carrots 🥕, leafy greens) to prevent night blindness.(Vitamin A wala khana khayein).
• Way 2: Proper Reading Habits: Read at normal distance (~25 cm) with sufficient light. Avoid reading in too dim/bright light or in moving vehicles.(Sahi doori aur paryapt roshni mein padhein).
• (Also: Wash eyes, don’t rub, use specs if needed, avoid looking at sun directly).

8. What is the Braille system? How does it work?

• Braille is a tactile system for reading/writing used by visually impaired people.(Yeh drishti badhit logon ke liye chhoo kar padhne/likhne ki pranali hai).
• Developed by Louis Braille.
• It uses patterns of **raised dots** ⠿ arranged in rectangular cells (6 dots max per cell).
• Each unique dot pattern represents a letter, number, punctuation mark, or symbol.
• Visually impaired persons read by feeling these raised dot patterns with their fingertips ✋.

9. How does a Kaleidoscope work?

• A kaleidoscope is a toy creating beautiful patterns.(Yeh sundar pattern banane wala khilona hai).
• It typically contains three rectangular mirror strips joined together to form a prism, placed inside a tube.
• At one end, there are loose, coloured pieces (like glass beads).
• When you look through the other end and rotate the tube, the coloured pieces form multiple images due to repeated reflections between the mirrors. 🔄✨
• These multiple images combine to create intricate, symmetrical patterns.

10. What causes the formation of a rainbow?

• A rainbow 🌈 is formed due to the **dispersion** of sunlight by tiny water droplets suspended in the air, usually after rainfall.(Indradhanush baarish ke baad hawa mein latki paani ki boondon dwara surya ke prakash ke **vikshepan** ke karan banta hai).
• These water droplets act like small prisms 💎.
• When sunlight enters a droplet, it refracts (bends), splits into its seven constituent colours (VIBGYOR), reflects internally, and refracts again as it comes out.
• This separation of colours creates the beautiful band of colours we see as a rainbow in the sky, usually opposite to the Sun.

11. What is the Blind Spot in the eye?

• It is the point on the retina where the Optic Nerve leaves the eye to go to the brain 🧠.(Retina par woh bindu jahan se Optic Nerve aankh se bahar dimaag tak jaati hai).
• At this specific spot, there are **no photoreceptor cells** (no rods or cones).(Is khaas jagah par koi Rods ya Cones nahi hote).
• Since there are no sensory cells, any light falling on the blind spot cannot be detected.
• Therefore, no vision is possible at this location.(Isliye, is jagah par koi drishti sambhav nahi hai).

1. Explain the Laws of Reflection with the help of an activity description or diagram concept.

Reflection is the bouncing back of light. It follows two laws:

Laws of Reflection:

1. First Law: Angle of incidence (∠i) = Angle of reflection (∠r).
2. Second Law: Incident ray, reflected ray, and normal (at point of incidence) all lie in the same plane.

Activity Concept (To Verify):

• Fix a white sheet of paper on a board. Draw a line MM’ representing a mirror.
• Draw a perpendicular line ON (Normal) at point O on MM’.
• Draw an incident ray AO making an angle ∠i with the normal ON. Fix two pins P1, P2 on AO.
• Place a plane mirror strip vertically on MM’.
• Look at the reflection of pins P1, P2 from the other side of the normal.
• Fix two more pins P3, P4 such that they appear to be in a straight line with the images of P1, P2.
• Remove the mirror and pins. Draw the reflected ray OB passing through P3, P4.
• Measure the angle between OB and ON. This is the angle of reflection (∠r).
• You will find that ∠i = ∠r (verifying 1st law).
• Also, the incident ray (AO), reflected ray (OB), and normal (ON) are all drawn on the sheet of paper, which represents one plane (verifying 2nd law).

2. Explain the difference between Regular and Diffused Reflection using diagrams (or detailed description).

Reflection Types:

• Regular Reflection:
  • Occurs from smooth, highly polished surfaces (like plane mirror 🪞, still water surface).
  • Parallel incident light rays ➡️➡️ striking the surface are reflected back as **parallel rays** in a specific direction.
  • Follows laws of reflection precisely at each point.
  • Forms **clear, sharp images**.
  • *(Description without diagram: Imagine parallel arrows hitting a flat line and bouncing off as parallel arrows)*.
  Chikni satah se. Parallel aane wali kiranen reflect hokar parallel hi jaati hain. Saaf image banti hai.
• Diffused (Irregular) Reflection:
  • Occurs from **rough or uneven surfaces** (like paper 📄, wood 🪵, wall 🧱).
  • Surface irregularities cause the normal at each point to be different.
  • Parallel incident light rays striking the surface are reflected back in **many different directions** (scattered).
  • Each individual ray still obeys the laws of reflection at its point of incidence.
  • Does **not form clear images**, but makes the rough surface itself visible to us from different angles.
  • *(Description without diagram: Imagine parallel arrows hitting a jagged line and bouncing off in various random directions)*.
  Khurduri satah se. Parallel aane wali kiranen reflect hokar alag-alag dishaon mein phail jaati hain. Saaf image nahi banti, par vastu dikhti hai.

3. Describe the construction and working of a Kaleidoscope.

A kaleidoscope is an optical toy that produces beautiful, symmetrical patterns.

Construction:

• Typically consists of a tube (cardboard usually). (Ek tube hota hai).
• Inside, three rectangular plane mirror strips are joined together along their lengths to form a triangular prism shape. 🪞🔼 (Andar, teen aaytakar darpan patliyan judkar ek prism jaisa shape banati hain).
• One end of the tube is closed with a cardboard disc having a central hole to look through (eyepiece). (Ek sira band hota hai jisme dekhne ke liye chhed hota hai).
• The other end is closed by two glass discs (one transparent, one ground/frosted) with a small gap between them.
• Small, colorful objects (like pieces of broken bangles, beads 🌈✨) are placed loosely in the gap between the glass discs. (Doosre sire par do glass disc ke beech rangeen tukde (jaise choodi ke tukde) rakhe jaate hain).

Working:

• When you look through the eyepiece and rotate the tube, the coloured pieces tumble around.
• Light entering from the far end reflects off these pieces.
• This reflected light then undergoes **multiple reflections** between the three mirrors arranged inside. 🔄 (Andar lage teeno sheeshon ke beech is roshni ka **multiple reflection** hota hai).
• These multiple images combine to form intricate, beautiful, symmetrical patterns that change as the tube is rotated. (Yeh multiple images milkar khoobsurat patterns banate hain jo tube ghumane par badalte rehte hain).

4. What is dispersion of light? How can you demonstrate it?

Dispersion: The phenomenon of splitting of white light into its constituent seven colours (VIBGYOR) when it passes through a transparent medium like a prism.

• Sunlight, which appears white, is actually a mixture of these seven colours.
• Each colour of light bends by a slightly different amount when passing through the prism (violet bends most, red bends least). This causes the colours to separate out.

Demonstration with a Prism 💎:

• Take a glass prism. Allow a narrow beam of sunlight to enter a dark room through a small hole.
• Place the prism in the path of this light beam.
• Place a white screen or wall on the other side of the prism.
• You will observe a beautiful band of seven colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red – VIBGYOR) on the screen. This band is called the spectrum of white light.

Natural Example:

• A **Rainbow 🌈** is formed by dispersion of sunlight by water droplets in the atmosphere after rain.

5. Explain the structure and function of the main parts of the human eye.

The human eye 👁️ is a complex sensory organ. Main parts include:

• Cornea: Transparent outer layer at the front. It refracts (bends) most of the light entering the eye. (Paardarshi bahari parat jo light ko bend karti hai).
• Iris: Coloured muscular diaphragm behind cornea. Controls pupil size. (Rangeen hissa jo pupil ka size control karta hai).
• Pupil: Opening in the center of iris. Regulates light entry (small in bright, large in dim light). (Iris ke beech ka chhed jo light entry control karta hai).
• Lens: Convex lens located behind the pupil. Focuses light onto the retina by adjusting its shape (accommodation). (Pupil ke peeche convex lens jo retina par light focus karta hai).
• Retina: Light-sensitive screen at the back of the eye. Contains Rods (for dim light vision) and Cones (for bright light and colour vision). Converts light into electrical signals. (Peeche ki screen jispar image banti hai. Rods aur Cones hote hain. Light ko electrical signals mein badalta hai).
• Optic Nerve 🧠: Bundle of nerve fibres carrying electrical signals from retina to the brain. (Signals retina se dimaag tak le jaane wali nerves).
• (Also: Ciliary muscles adjust lens focus; Vitreous/Aqueous humour maintain shape).

The eye works like a camera, forming an inverted real image on the retina, which the brain interprets correctly.

6. How should we take care of our eyes?

Eye care is very important:

• Proper Light for Reading: Avoid reading in too little or too much light to prevent eyestrain/damage. (Padhne ke liye sahi roshni istemal karein).
• Maintain Reading Distance: Keep book at normal distance (~25 cm), not too close or far. (Kitab ko sahi doori par rakhein).
• Wash Eyes: Wash eyes frequently with clean water 💧. Don’t rub if something enters, wash gently. (Aankhein saaf paani se dhoyein, ragdein nahi).
• Balanced Diet: Eat foods rich in Vitamin A (carrots 🥕, leafy greens) to prevent night blindness. (Vitamin A wala bhojan khayein).
• Limit Screen Time: Reduce exposure to screens (TV, computer, mobile) and take breaks.
• Avoid Direct Sunlight/Strong Light: Never look directly at the sun ☀️ or very powerful lights.
• Use Spectacles if Advised 👓: If you have vision problems, use corrective glasses as prescribed.
• Regular Check-ups: Get eyes checked regularly by an eye specialist.

7. What are visually impaired persons? How can they read and write?

Visually impaired persons are those who have difficulty seeing, ranging from partial sight loss to complete blindness.

How they manage:

• They often develop their other senses like touch ✋ and hearing 👂 more keenly.
• Various **aids** help them:
  • Non-optical aids: Use sense of touch or hearing. Examples: Braille system for reading/writing, auditory aids like audio books/tapes 🎧, talking calculators/computers 💻.
  • Optical aids: For those with partial vision. Examples: Magnifiers 🔍, telescopic aids, specific lenses (bifocal, contact, tinted).
• The most popular resource for visually challenged persons is the Braille System.

8. Explain the Braille system in detail.

Braille is a tactile system enabling visually impaired individuals to read and write.

• Developer: Louis Braille (who lost his sight in childhood).
• Basis: Uses patterns of raised dots ⠿ felt by fingertips ✋.
• Braille Cell: Each character (letter, number, punctuation) is represented by a unique pattern of dots within a rectangular ‘cell’ comprising 6 dot positions (2 columns of 3 dots).
• Dot Patterns: Different combinations of raised dots within these 6 positions form distinct characters. For example, ‘A’ might be dot 1 raised, ‘B’ dots 1 & 2 raised, etc.
• Reading/Writing: Reading is done by moving fingertips across lines of Braille dots. Writing can be done using a slate and stylus or a Braille typewriter/printer.
• Application: Braille has been adapted for many languages worldwide, including Indian languages. Codes exist for mathematics and science too.
• Importance: Empowers visually impaired people with literacy, enabling education and communication.

9. What is persistence of vision? How is it used?

Persistence of Vision: It is the phenomenon where the impression (or sensation) of an image continues on the retina for a short period even after the object is removed from view.

• This persistence lasts for about **1/16th of a second**.
• Application in Motion Pictures 🎬🎞️: Movies are actually a sequence of still pictures (frames).
• These frames are projected onto the screen at a rate typically faster than 16 frames per second (usually 24 frames/sec).
• Because the image of one frame persists briefly on our retina when the next frame appears, our brain blends these slightly different images together, creating the illusion of smooth movement.
• If the projection rate were slower than 16 frames/sec, we would perceive the movie as a series of jerky, separate pictures rather than continuous motion.

10. How is light reflected multiple times? Give two examples where this phenomenon is applied.

Light follows the laws of reflection. When reflected light from one surface strikes another reflecting surface, it bounces off again. This process can repeat multiple times, leading to **multiple reflections**.

Applications:

• 1. Periscope: Used in submarines 🚢 or trenches to see objects that are not in the direct line of sight. Simple periscopes use two plane mirrors set parallel to each other at 45° angle inside a tube. Light from the object reflects off the top mirror, travels down, reflects off the bottom mirror, and enters the observer’s eye. (Objects ko dekhne ke liye jo seedhi nazar mein nahi hain. Do parallel mirrors 45° par lage hote hain).
• 2. Kaleidoscope ✨: A toy using three plane mirrors joined at an angle (usually 60°) inside a tube. Loose coloured objects at one end create beautiful, symmetrical patterns due to multiple reflections between the mirrors as the tube is rotated. (Teen angled mirrors ke beech multiple reflections se sundar patterns banane wala khilona).
• 3. Hairdresser’s Mirrors 💇🪞: Using two mirrors (front and back) allows customer to see the back of their head through multiple reflections.