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Chapter 11 Light: Shadows And Refl Ections
Sources of Light: Luminous and Non-Luminous
The ability to see the world around us depends entirely on the presence of light. Light is a form of energy that travels in straight lines and interacts with objects differently depending on their nature. To understand these interactions, we first classify objects based on their ability to emit light.
1. Luminous Objects
Objects that give out or emit their own light are known as Luminous Objects. These sources are responsible for illuminating the darkness and making non-luminous things visible.
A. Natural Sources of Light
- The Sun: The primary and most powerful natural source of light for our planet.
- Stars: Distant celestial bodies that emit light through nuclear reactions.
- Lightning: A natural electrical discharge in the atmosphere during thunderstorms.
- Bioluminescence: Certain animals like Fireflies (Jugnu) emit light to communicate. In India, these are commonly seen in the Western Ghats during the summer.
B. Artificial (Man-made) Sources of Light
- Incandescent Bulbs: Traditional glass bulbs that use a glowing filament.
- Fluorescent Tubes: Common in Indian households and offices for wider light dispersion.
- LED Lamps: Light Emitting Diodes, which are highly efficient modern sources.
- Flame-based lighting: Candles, oil lamps (Diyas), and gas lanterns used extensively in ancient times.
2. Non-Luminous Objects
Objects that do not produce their own light are termed Non-Luminous Objects. We can only see them when light from a luminous source falls on them and reflects towards our eyes.
- The Moon: Although it appears bright at night, the Moon is a non-luminous body. It simply reflects the light of the Sun.
- Planetary Bodies: Earth, Mars, and Venus reflect sunlight to appear visible in space.
- Daily Life Objects: Most things around us like tables, chairs, trees, pustak (books), and buildings are non-luminous.
- Mirrors: While they appear very "bright," they are actually non-luminous and are designed to highly reflect external light.
Comparison of Luminous and Non-Luminous Objects
| Feature | Luminous Objects | Non-Luminous Objects |
|---|---|---|
| Light Emission | Emit their own light. | Do not emit own light. |
| Visibility | Visible due to their own energy. | Visible due to reflection. |
| Examples | Sun, Stars, Torch, LED. | Moon, Earth, Wood, Plastic. |
Advancement in Lighting: The LED Revolution
In the Indian Perspective, there has been a massive shift toward using LED (Light Emitting Diode) technology through government initiatives like the UJALA scheme. This advancement is critical for several reasons:
- Energy Efficiency: LEDs consume significantly less power than traditional incandescent bulbs.
- Durability: They have a much longer lifespan, often lasting up to $15,000$ to $25,000$ hours.
- Cost-Effectiveness: Despite a higher initial purchase price, the long-term savings on electricity bills make them cheaper.
- Environmental Impact: They generate less heat and are easier to recycle, making them better for the Paryavaran (environment).
Example 1. If a traditional bulb costs $\text{₹} \ 20 \ \text{}$ and lasts for 1,000 hours, while an LED bulb costs $\text{₹} \ 80 \ \text{}$ but lasts for 15,000 hours, calculate the total cost of bulbs needed to cover 15,000 hours using traditional bulbs. Which is more economical?
Answer:
To determine the most economical option, we calculate the total expenditure for both types of bulbs over the same duration of $15,000$ hours.
Step 1: Calculate the number of traditional bulbs required ($N$):
$N = \frac{\text{Total Duration}}{\text{Life of one bulb}}$
$N = \frac{15,000 \text{ hours}}{1,000 \text{ hours/bulb}} = 15 \text{ bulbs}$
Step 2: Calculate the total cost for traditional bulbs ($C_{trad}$):
$C_{trad} = N \times \text{Price per bulb}$
$C_{trad} = 15 \times \text{₹} \ 20 = \text{₹} \ 300 \ \text{}$
Step 3: Compare with the cost of one LED bulb ($C_{LED}$):
$C_{LED} = \text{₹} \ 80 \ \text{}$ (since one bulb covers the entire $15,000$ hours).
Mathematical Derivation of Savings ($S$):
$S = C_{trad} - C_{LED}$
$S = 300 - 80 = \text{₹} \ 220 \ \text{}$
Conclusion: The LED bulb is significantly more economical, saving a total of $\text{₹} \ 220 \ \text{}$ over its lifetime compared to traditional bulbs.
Rectilinear Propagation: Does Light Travel in a Straight Line?
One of the most striking observations in nature is that light always appears to follow a fixed, unbending path. Whether it is the beam of a torch in a dark room or the headlights of a bus winding through the Western Ghats, light does not curve on its own.
Experimental Evidence
To confirm this property, we can perform simple scientific investigations:
1. The Matchbox Investigation
- Setup: Take three matchboxes and make a hole in each inner tray at the exact same height.
- Alignment: Place them in a row so all holes are perfectly aligned.
- Observation: When a torch is shone through the first hole, a bright spot appears on the screen at the other end.
- Result: If even one matchbox is moved slightly out of line, the spot of light disappears. This proves light cannot "dodge" the obstacle.
2. The Flexible Pipe Test
- Look at a burning candle flame through a straight hollow pipe. The flame is clearly visible.
- Now, bend the pipe and try to look at the flame again.
- Inference: You cannot see the flame through the bent pipe because light travels in a straight line and cannot curve around the bend of the pipe.
The Laser Beam Observation
In a science laboratory, a low-power laser pointer provides excellent visual proof of light's path:
- When passed through a beaker of clear water, the beam is often invisible.
- By adding a drop of milk to the water, the laser beam becomes a distinct straight line.
- The milk particles scatter the light, allowing us to see that the beam follows a perfectly straight path inside the liquid.
Mathematical Derivation: Sunlight Journey
Even though light travels in a straight line, the distance from the Sun to Earth is so vast that it takes time to reach us. We can derive the time taken using the formula:
Formula for Time
$\text{Time } (t) = \frac{\text{Distance } (d)}{\text{Speed of Light } (c)}$
Given Values:
- Average distance ($d$) $\approx 1.5 \times 10^{11} \text{ metres}$ (or $15$ crore kilometres).
- Speed of light ($c$) $\approx 3 \times 10^{8} \text{ m/s}$ (or $3$ lakh kilometres per second).
Calculation:
$t = \frac{1.5 \times 10^{11}}{3 \times 10^8} = 500 \text{ seconds}$
$\text{In minutes: } \frac{500}{60} = 8 \text{ minutes and } 20 \text{ seconds}$
Conclusion: If the Sun were to stop emitting light, we would only find out after $8 \text{ minutes and } 20 \text{ seconds}$.
Classification of Materials based on Light Transmission
When light encounters an object, it interacts with the material in different ways. Based on how much light is allowed to pass through, materials are categorized into three distinct groups.
Summary of Material Types
| Category | Light Transmission | Shadow Formation | Common Examples |
|---|---|---|---|
| Transparent | Passes completely | No shadow (or very faint) | Clear Glass, Air, Pure Water. |
| Translucent | Passes partially | Lighter/Faint shadows | Tracing paper, Frosted glass, Butter paper. |
| Opaque | Does not pass at all | Dark shadows | Wood, Metal, Cardboard, Human Body. |
Detailed Analysis
A. Transparent Materials
- These materials allow almost all light to pass through them.
- We can see through them clearly.
- Example: The windshield of the bus Keshav was riding in allows the driver to see the road clearly at night.
B. Translucent Materials
- They allow some light to pass, but the light is scattered.
- Objects on the other side appear blurred or hazy.
- Example: Tracing paper used in Indian schools for maps or a thin cloth.
C. Opaque Materials
- These materials block light completely.
- We cannot see through them at all.
- The blocked light results in the formation of a Shadow on the opposite side.
- Example: A polished steel plate or a thick textbook.
Example 1. A student wants to buy materials for a science project. A sheet of clear glass costs $\text{₹} \ 120 \ \text{}$, a sheet of plywood costs $\text{₹} \ 85 \ \text{}$, and a roll of tracing paper costs $\text{₹} \ 25 \ \text{}$. Classify these based on light transmission and calculate the total cost.
Answer:
1. Classification:
- Clear Glass: Transparent (allows light fully).
- Plywood: Opaque (blocks light completely).
- Tracing Paper: Translucent (allows light partially).
2. Total Cost Calculation:
$\text{Total Cost} = \text{Cost of Glass} + \text{Cost of Plywood} + \text{Cost of Tracing Paper}$
$\text{Total Cost} = \text{₹} \ 120 + \text{₹} \ 85 + \text{₹} \ 25 = \text{₹} \ 230 \ \text{}$
The total expenditure for the project materials is $\text{₹} \ 230 \ \text{}$.
Shadow Formation and Characteristics
A shadow is a space or region of darkness formed when an opaque object intercepts the path of light. Since light travels in a straight line, it cannot bend around the object, resulting in a dark patch on the opposite side.
Essentials for Shadow Observation
To produce and observe a shadow, the following three components are strictly required:
- A Source of Light: This can be a natural source like the Sun or an artificial source like a torch or electric bulb.
- An Opaque Object: This is the "blocker." Materials like wood, metal, or the human body that do not allow light to pass through are best for forming distinct shadows.
- A Screen: A surface where the shadow is cast. In daily life, the ground, a wall, or a piece of cardboard acts as a screen.
Defining Characteristics of Shadows
Shadows possess specific traits that distinguish them from images:
- Darkness: Regardless of the color of the object (e.g., a red ball or a green leaf), the shadow is always dark or black.
- Shape Information: A shadow gives information about the general outline or shape of the object but does not show details like eyes, patterns, or colors.
- Size Variation: The size of a shadow is not fixed. It changes based on the distance between the light source, the object, and the screen.
Mathematical Logic of Shadow Size
If the distance between the source and the object decreases, the shadow becomes larger. Conversely, if the object is moved closer to the screen, the shadow becomes smaller and sharper.
Indian Cultural Heritage: Shadow Puppetry
India has a rich tradition of Shadow Play, where stories from epics like the Ramayana and Mahabharata are told using light and shadows. Key regional styles include:
- Tholu Bommalata: A vibrant shadow puppet theatre from Andhra Pradesh.
- Tholpavakoothu: A traditional art form dedicated to temple rituals in Kerala.
- Ravana Chhaya: A unique style from Odisha using non-jointed deer-skin puppets.
- Togalu Gombeyaata: The shadow puppet tradition of Karnataka.
Reflection of Light and Plane Mirrors
When light falls on a shiny or polished surface, it "bounces back" into the same medium. This phenomenon is called Reflection. A plane mirror (a flat, smooth reflecting surface) is the most common tool used to study this property.
Characteristics of Images in a Plane Mirror
The image we see in a mirror is different from a shadow. Its characteristics are derived as follows:
- Erect Nature: The image is upright. If you stand straight, your image also stands straight (top stays at the top).
- Equal Size: The size of the image is exactly equal to the size of the object.
- Object-Image Distance: The image is formed behind the mirror. The distance of the image from the mirror ($d_i$) is equal to the distance of the object from the mirror ($d_o$).
Mathematically: $d_o = d_i$
- Virtual Image: The image formed by a plane mirror cannot be obtained on a screen placed behind or in front of the mirror.
Lateral Inversion
This is the most unique property of a plane mirror. When you stand in front of a mirror:
- Your Left appears as the Right of the image.
- Your Right appears as the Left of the image.
The "Ambulance" Application
In India, the word $\text{AMBULANCE}$ is written in reverse (Laterally Inverted) as $\text{ECNALUBMA}$ on the front of the vehicle. This is done so that a driver looking in their rear-view mirror can read it correctly as AMBULANCE and give way immediately.
Traditional Indian Mirrors: Aranmula Kannadi
While standard mirrors are made of glass with a silver coating at the back, India produces a world-famous mirror in Kerala:
- Material: It is a handmade metal-alloy mirror, not made of glass.
- Technique: The exact composition of the metals is a family secret maintained by craftsmen in Aranmula.
- Scientific Edge: Unlike glass mirrors where reflection happens from the back (causing secondary reflections), the Aranmula Kannadi reflects from the front surface, providing a distortion-free and ultra-clear image.
- Value: These mirrors are often expensive, ranging from $\text{₹} \ 2,500 \ \text{}$ to over $\text{₹} \ 20,000 \ \text{}$, and are considered symbols of prosperity.
Comparison: Object vs. Image in Plane Mirror
| Feature | Object | Image in Plane Mirror |
|---|---|---|
| Direction | Original | Erect (Upright) |
| Sides | Normal | Laterally Inverted (Left $\leftrightarrow$ Right) |
| Distance from Mirror | $x \text{ cm}$ | $x \text{ cm}$ (Equal) |
| Size | $h \text{ cm}$ | $h \text{ cm}$ (Equal) |
Example 1. Prerna is standing $2 \text{ metres}$ away from a large plane mirror in her school. She moves $0.5 \text{ metres}$ closer to the mirror. What is the new distance between Prerna and her image?
Answer:
- Initial state: Distance from mirror ($d_o$) = $2 \text{ m}$. Image distance ($d_i$) = $2 \text{ m}$.
- New state: Prerna moves $0.5 \text{ m}$ closer. New $d_o = 2 - 0.5 = 1.5 \text{ m}$.
- Property: Since $d_o = d_i$, the new image distance ($d_i$) is also $1.5 \text{ m}$.
- Calculation: The distance between Prerna and her image is $d_o + d_i$.
$\text{Distance} = 1.5 + 1.5 = 3 \text{ metres}$.
The total distance between Prerna and her image is now $3 \text{ metres}$.
Optical Devices: Pinhole Camera, Periscope, and Kaleidoscope
By applying the scientific principles of rectilinear propagation (light traveling in a straight line) and reflection, we can construct various optical devices that expand our ability to observe the world.
1. The Pinhole Camera
A pinhole camera is a simple imaging device that does not use a lens. It provides the most fundamental proof that light travels in a straight line.
A. Mechanism and Principle
- It works on the principle of Rectilinear Propagation of Light.
- Light rays from the top and bottom of an object pass through a tiny hole (pinhole).
- These rays cross each other at the hole, resulting in an image on the screen.
B. Characteristics of the Pinhole Image
- Inverted: The image formed is always upside down because of the crossing of light rays.
- Colored: Unlike a shadow, the image shows the actual colors of the object.
- Real: The image is formed on a screen (like tracing paper).
- Size: The size of the image changes based on the distance between the object and the pinhole.
2. The Periscope
A periscope is an optical instrument that allows an observer to see objects that are not in the direct line of sight, such as looking over a wall or around a corner.
A. Construction and Principle
- It works on the principle of Reflection of Light.
- It consists of a Z-shaped tube with two plane mirrors fixed inside.
- The mirrors are placed parallel to each other and inclined at an angle of $45^\circ$ to the path of light.
B. Practical Applications
- Military Use: Used by soldiers in bunkers or trenches to observe enemy movements without being seen.
- Navy: Essential in submarines to see objects above the water's surface while remaining submerged.
- General Use: Helpful for viewing over the heads of tall crowds at parades or sports events.
3. The Kaleidoscope
A kaleidoscope is an optical toy that creates beautiful, symmetrical patterns through the use of multiple reflections.
A. Structure and Working
- It typically uses three rectangular plane mirror strips joined together to form a triangle.
- One end of the tube contains colorful objects, such as broken glass pieces or colored bangles (Churiyan).
- Multiple reflections between the mirrors create endless, repeating patterns.
B. Unique Features
- Infinite Variety: You will rarely see the same pattern twice when you rotate or shake the device.
- Professional Use: Used by textile designers and artists to get new ideas for fabrics, wallpapers, and carpet designs.
Comparison of Optical Devices
| Device | Primary Principle | Image Type | Main Component |
|---|---|---|---|
| Pinhole Camera | Rectilinear Propagation | Inverted & Colored | Pinhole & Screen |
| Periscope | Reflection | Erect (Upright) | Two Parallel Mirrors |
| Kaleidoscope | Multiple Reflection | Symmetrical Patterns | Three Mirror Strips |
Example 1. A student wants to build a Kaleidoscope using materials from a local Indian market. He purchases 3 mirror strips for $\text{₹} \ 15 \ \text{}$ each, a piece of chart paper for $\text{₹} \ 10 \ \text{}$, and a packet of colorful glass beads for $\text{₹} \ 12 \ \text{}$. Calculate the total expenditure for the project.
Answer:
To find the total expenditure ($E$), we sum the individual costs of all materials:
- Cost of Mirrors: $3 \times 15 = \text{₹} \ 45 \ \text{}$
- Cost of Chart Paper: $\text{₹} \ 10 \ \text{}$
- Cost of Glass Beads: $\text{₹} \ 12 \ \text{}$
$\text{Total Cost } (E) = 45 + 10 + 12$
$E = \text{₹} \ 67 \ \text{}$
The total material cost for the kaleidoscope is $\text{₹} \ 67 \ \text{}$.
Let us enhance our learning
Question 1. Which of the following are luminous objects?
Mars, Moon, Pole Star, Sun, Venus, Mirror
Answer:
Question 2. Match the items in Column A with those in Column B.
| Column A | Column B |
|---|---|
| Pinhole camera | Blocks light completely |
| Opaque object | The dark region formed behind the object |
| Transparent object | Forms an inverted image |
| Shadow | Light passes almost completely through it |
Answer:
Question 3. Sahil, Rekha, Patrick, and Qasima are trying to observe the candle fl ame through the pipe as shown in Fig. 11.16. Who can see the fl ame?
Answer:
Question 4. Look at the images shown in Fig. 11.17 and select the correct image showing the shadow formation of the boy.
Answer:
Question 5. The shadow of a ball is formed on a wall by placing the ball in front of a fi xed torch as shown in Fig. 11.18. In scenario (i) the ball is closer to the torch, while in scenario (ii) the ball is closer to the wall. Choose the most accurate representation of the shadows formed in both scenarios from the options provided (a and b).
Answer:
Question 6. Based on Fig. 11.18, match the position of the torch in Column A with the characteristics of the ball’s shadow in Column B.
| Column A | Column B |
|---|---|
| If the torch is close to the ball | The shadow would be smaller |
| If the torch is far away | The shadow would be larger |
| If the ball is removed from the set-up | Two shadows would appear on the screen |
| If two torches are present in the set-up on the left side of the ball | A bright spot would appear on the screen |
Answer:
Question 7. Suppose you view the tree shown in Fig. 11.19 through a pinhole camera. Sketch the outline of the image of the tree formed in the pinhole camera.
Answer:
Question 8. Write your name on a piece of paper and hold it in front of a plane mirror such that the paper is parallel to the mirror. Sketch the image. What diff erence do you notice? Explain the reason for the diff erence.
Answer:
Question 9. Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations:
(i) At which of the given times is your shadow the shortest?
(ii) Why do you think this happens?
Answer:
Question 10. On the basis of following statements, choose the correct option.
Statement A: Image formed by a plane mirror is laterally inverted.
Statement B: Images of alphabets T and O appear identical to themselves in a plane mirror.
(i) Both statements are true
(ii) Both statements are false
(iii) Statement A is true, but statement B is false
(iv) Statement A is false, but statement B is true
Answer:
Question 11. Suppose you are given a tube of the shape shown in the Fig. 11.20 and two plane mirrors smaller than the diameter of the tube. Can this tube be used to make a periscope? If yes, mark where you will fi x the plane mirrors.
Answer:
Question 12. We do not see the shadow on the ground of a bird fl ying high in the sky. However, the shadow is seen on the ground when the bird swoops near the ground. Think and explain why it is so.
Answer: