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| The Human Eye and the Colourful World |
👁️✨ The Wonders of Light: Understanding the Human Eye and Optical Phenomena
Light is an essential part of our daily lives, allowing us to see the world around us. In our previous discussions, we explored refraction of light through lenses and how they form images. But did you know that the human eye also contains a lens that plays a crucial role in vision?
In this post, we will dive into the fascinating workings of the human eye, the function of its lens, and how spectacles correct vision defects. Additionally, we will explore some amazing optical phenomena in nature, such as:
🌈 Rainbow Formation – How sunlight and raindrops create a spectacular arc of colors.
🌍 Why is the Sky Blue? – The science behind the beautiful blue hue of our sky.
🌞 Splitting of White Light – How a prism reveals the hidden colors within light.
Stay tuned as we uncover the magic of light and its incredible role in both vision and nature! 🔍✨
Detailed Notes on The Human Eye
11.1 The Human Eye
Introduction
- The human eye is one of the most valuable and sensitive sense organs.
- It allows us to see the beautiful, colorful world around us.
- Other senses like smell, taste, sound, and touch help identify objects, but color perception is only possible through vision.
Structure of the Human Eye
- The human eye is similar to a camera as it has a lens system that forms images on a light-sensitive screen called the retina.
- It is approximately spherical in shape, with a diameter of about 2.3 cm.
Main Parts of the Human Eye
1️⃣ Cornea
- A thin, transparent membrane forming a bulge at the front of the eye.
- Responsible for most of the refraction (bending) of light that enters the eye.
2️⃣ Iris
- A dark, muscular diaphragm behind the cornea.
- Controls the size of the pupil, regulating the amount of light entering the eye.
- The color of the eye (black, brown, blue, etc.) depends on the pigmentation of the iris.
3️⃣ Pupil
- A small circular opening in the center of the iris.
- It regulates the amount of light entering the eye.
- Bright Light → Pupil contracts (reduces size) to allow less light.
- Dim Light → Pupil expands (increases size) to allow more light.
4️⃣ Eye Lens
- A convex, flexible, and transparent structure.
- Helps in focusing light on the retina.
- Adjusts focal length for focusing on near or far objects (accommodation).
5️⃣ Retina
- The light-sensitive layer at the back of the eye.
- Contains millions of light-sensitive cells:
- Rods → Help in vision in dim light (night vision).
- Cones → Responsible for color vision and bright light vision.
- Converts light into electrical signals, which are sent to the brain.
6️⃣ Optic Nerve
- Transmits electrical signals from the retina to the brain.
- The brain processes these signals to create the final image perception.
7️⃣ Aqueous Humour and Vitreous Humour
- Aqueous Humour: A clear fluid between the cornea and the lens, providing nutrients and maintaining pressure.
- Vitreous Humour: A gel-like substance inside the eyeball, maintaining the eye's shape and optical properties.
Working of the Human Eye
- Light enters the eye through the cornea and passes through the pupil.
- The lens focuses light on the retina, forming an inverted, real image.
- The retina converts light into electrical impulses, which travel via the optic nerve to the brain.
- The brain processes the signals and allows us to see objects in their correct orientation.
Adaptation to Light Intensity
- The pupil acts like a variable aperture controlled by the iris.
- In bright light, the pupil contracts to reduce the amount of light entering the eye.
- In dim light, the pupil expands to allow more light, helping us see in darkness.
- This adjustment takes a few seconds and is called adaptation.
Importance of the Human Eye
✅ Helps in perceiving shapes, colors, and movements.
✅ Allows us to navigate the environment safely.
✅ Enables us to enjoy the beauty of nature, art, and colors.
✅ Works in coordination with the brain to process visual information.
Fun Fact
- The human eye can differentiate between approximately 10 million colors!
Detailed Notes on Power of Accommodation
11.1.1 Power of Accommodation
Definition
- The power of accommodation is the ability of the eye lens to adjust its focal length to focus on objects at different distances.
Structure and Function of the Eye Lens
- The eye lens is made of a fibrous, jelly-like material.
- Its curvature is modified by the ciliary muscles, allowing focal length adjustments.
How the Eye Focuses on Objects?
1️⃣ Distant Objects
- Ciliary muscles relax → Lens becomes thin → Focal length increases → Distant objects appear clear.
2️⃣ Nearby Objects
- Ciliary muscles contract → Lens becomes thick → Focal length decreases → Nearby objects appear clear.
Least Distance of Distinct Vision
- The minimum distance at which an object can be seen clearly without strain is called the least distance of distinct vision.
- Also known as the near point of the eye.
- For a young adult with normal vision, it is about 25 cm.
Far Point of the Eye
- The farthest point at which objects can be seen clearly is called the far point of the eye.
- For a normal eye, it is infinity.
- A normal human eye can see objects clearly between 25 cm and infinity.
Cataract: A Common Eye Problem
- With aging, the eye lens may become milky and cloudy.
- This condition is known as cataract and leads to partial or complete loss of vision.
- Cataract surgery can help restore vision by replacing the cloudy lens with an artificial lens.
Why Do We Have Two Eyes Instead of One?
Advantages of Having Two Eyes
1️⃣ Wider Field of View
- One eye provides a horizontal field of view of about 150°.
- Two eyes increase it to about 180°, helping in better peripheral vision.
2️⃣ Better Detection of Faint Objects
- Two eyes allow the brain to process more light and details, making it easier to detect faint objects.
3️⃣ Depth Perception (3D Vision)
- Two eyes provide stereopsis, allowing us to perceive depth and distance.
- When one eye is closed, the world appears flat (2D).
- With both eyes open, we can judge how far or close objects are.
Eye Position in Animals
- Predator animals (like humans, lions, and owls) have both eyes in the front for better depth perception.
- Prey animals (like rabbits and deer) have eyes on the sides for a wider field of view, helping them detect predators.
Detailed Notes on Defects of Vision and Their Correction
11.2 Defects of Vision and Their Correction
- The human eye sometimes loses its power of accommodation, leading to blurred vision.
- This occurs due to refractive defects in the eye.
- There are three common refractive defects of vision:
1️⃣ Myopia (Near-sightedness)
2️⃣ Hypermetropia (Far-sightedness)
3️⃣ Presbyopia - These defects can be corrected using suitable spherical lenses.
(a) Myopia (Near-Sightedness)
🔹 Definition:
- A person with myopia can see nearby objects clearly but distant objects appear blurred.
- The far point is closer than infinity (normal far point).
🔹 Causes:
1️⃣ Excessive curvature of the eye lens → The lens becomes too convex, bending light too much.
2️⃣ Elongation of the eyeball → The retina moves farther back, causing the image to form in front of the retina instead of on the retina.
🔹 Image Formation:
- The image of a distant object is formed in front of the retina instead of on the retina.
🔹 Correction:
- Concave lenses (diverging lenses) are used to correct myopia.
- The concave lens diverges the incoming light rays, making them focus on the retina instead of in front of it.
📌 Example: People who have difficulty seeing distant objects clearly, like a blackboard from the last bench, often have myopia.
(b) Hypermetropia (Far-Sightedness)
🔹 Definition:
- A person with hypermetropia can see distant objects clearly but nearby objects appear blurred.
- The near point is farther than the normal 25 cm.
🔹 Causes:
1️⃣ Increase in focal length of the eye lens → The lens becomes too flat, reducing its converging power.
2️⃣ Eyeball is too small → The retina moves forward, causing the image to form behind the retina instead of on it.
🔹 Image Formation:
- The image of a nearby object is formed behind the retina instead of on the retina.
🔹 Correction:
- Convex lenses (converging lenses) are used to correct hypermetropia.
- The convex lens converges the incoming light rays, making them focus on the retina instead of behind it.
📌 Example: People who have difficulty reading books or newspapers up close often have hypermetropia.
(c) Presbyopia
🔹 Definition:
- Age-related vision defect where people find it difficult to see nearby objects clearly.
- The near point recedes further away from 25 cm.
🔹 Causes:
1️⃣ Weakening of ciliary muscles → The eye lens loses its ability to adjust its curvature.
2️⃣ Reduced flexibility of the eye lens → The lens becomes stiff, making accommodation difficult.
🔹 Correction:
- Convex lenses are used to correct presbyopia.
- If a person has both myopia and hypermetropia, they require bifocal lenses:
- The upper part is concave (for distant vision).
- The lower part is convex (for near vision).
🔹 Modern Solutions:
- Contact lenses
- Surgical interventions like LASIK
📌 Example: Many older adults use bifocal lenses to read books while also seeing distant objects clearly.
Summary Table of Vision Defects and Their Correction
| Defect | Can See Clearly | Cannot See Clearly | Cause | Image Formation | Correction |
|---|---|---|---|---|---|
| Myopia (Near-Sightedness) | Near objects | Distant objects | Excessive curvature of the lens OR elongated eyeball | In front of the retina | Concave lens (diverging lens) |
| Hypermetropia (Far-Sightedness) | Distant objects | Near objects | Increased focal length OR small eyeball | Behind the retina | Convex lens (converging lens) |
| Presbyopia (Age-Related) | None properly without glasses | Nearby objects (sometimes distant objects too) | Weakening of ciliary muscles & stiff lens | Behind the retina (like hypermetropia) | Bifocal lenses (convex & concave), contact lenses, surgery |
Q U E S T I O N S & A N S W E R S
1. What is meant by power of accommodation of the eye?
✅ Answer:
- The power of accommodation is the ability of the eye lens to adjust its focal length to focus on objects at different distances.
- The ciliary muscles control the curvature of the lens, making it thicker for nearby objects and thinner for distant objects.
2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
✅ Answer:
- The person is suffering from myopia (near-sightedness).
- This defect is corrected using a concave (diverging) lens of suitable power.
- A concave lens diverges the incoming light rays, allowing the image to form on the retina instead of in front of it.
3. What is the far point and near point of the human eye with normal vision?
✅ Answer:
- The far point of a normal human eye is infinity (∞).
- The near point (least distance of distinct vision) is 25 cm for a young adult with normal vision.
4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
✅ Answer:
- The student is likely suffering from myopia (near-sightedness).
- Myopia causes difficulty in seeing distant objects clearly, such as a blackboard from the last row.
- This defect can be corrected using concave (diverging) lenses, which help focus the image on the retina.
Eye Donation: A Gift of Sight 👁️✨
🔹 35 million people worldwide are blind; 4.5 million suffer from corneal blindness, mostly children.
🔹 Eye donation restores vision through corneal transplants.
Who Can Donate?
✅ Any age/gender, even those with glasses, cataract surgery, diabetes, or hypertension.
🚫 Not eligible: AIDS, Hepatitis B/C, rabies, leukemia, meningitis patients.
Key Facts
⚡ Eyes must be donated within 4-6 hours after death—call an eye bank.
🏥 Quick, simple, and no disfigurement.
👁️ One pair of eyes can help FOUR people!
Pledge today—give the gift of vision! 🌟
Detailed Notes on Refraction of Light Through a Prism
11.3 Refraction of Light Through a Prism
Introduction
- Refraction is the bending of light when it passes from one medium to another.
- In a rectangular glass slab, the emergent ray is parallel to the incident ray but is slightly displaced.
- In a triangular glass prism, light undergoes two refractions at inclined surfaces, causing the emergent ray to bend at an angle to the incident ray.
Structure of a Prism
- A triangular glass prism has:
- Two triangular bases
- Three rectangular lateral surfaces (inclined to each other)
- The angle between two lateral faces is called the angle of the prism (A).
Activity: Observing Refraction in a Prism
Steps to perform the experiment:
1️⃣ Place a glass prism on a white sheet and trace its outline.
2️⃣ Draw an incident ray PE, inclined to one refracting surface (AB).
3️⃣ Fix two pins (P, Q) along PE and observe their images through the other surface (AC).
4️⃣ Fix two more pins (R, S) so that they align with the images of P and Q.
5️⃣ Remove the prism and pins, and draw the paths of the rays.
6️⃣ Mark:
- Incident ray (PE)
- Refracted ray (EF) inside the prism
- Emergent ray (FS) after exiting the prism
- Angles of incidence (∠i), refraction (∠r), emergence (∠e), and deviation (∠D)
Understanding Refraction in a Prism
- At the first surface (AB), light enters from air to glass and bends towards the normal.
- At the second surface (AC), light exits from glass to air and bends away from the normal.
- Unlike a glass slab, in a prism, the emergent ray is not parallel to the incident ray.
- The bending of the emergent ray creates an angle of deviation (∠D), which is the angle between the incident ray and the emergent ray.
Key Observations
✅ The emergent ray deviates from its original path due to the inclined surfaces.
✅ The angle of deviation (∠D) depends on:
- The angle of the prism (A)
- The angle of incidence (∠i)
- The material of the prism (refractive index)
✅ A prism can split white light into its constituent colors (dispersion).
This experiment demonstrates how a prism alters the path of light, unlike a glass slab, and introduces the concept of angle of deviation. 📐🌈
Detailed Notes on Dispersion of White Light by a Glass Prism
11.4 Dispersion of White Light by a Glass Prism
Introduction
- A rainbow is a beautiful natural phenomenon that displays seven colors.
- This occurs due to the dispersion of sunlight by tiny water droplets in the atmosphere.
- To understand this, we must first study how light refracts through a prism.
Activity: Observing Dispersion Through a Prism
Steps:
1️⃣ Make a small slit in a thick sheet of cardboard.
2️⃣ Allow sunlight to pass through the slit, forming a narrow beam of white light.
3️⃣ Place a glass prism in the path of the light beam.
4️⃣ Slowly rotate the prism until light emerges onto a screen.
5️⃣ Observe the formation of a band of colors.
What is Dispersion?
- The splitting of white light into its component colors is called dispersion.
- The band of colors obtained is called a spectrum.
- The seven colors of the spectrum are:
Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
🔹 Why do different colors appear?
- Different colors of light bend (refract) at different angles when passing through a prism.
- Red bends the least, and Violet bends the most.
- This separation of colors forms a spectrum.
Newton’s Experiment on White Light
- Isaac Newton first demonstrated that sunlight is composed of seven colors.
- He used two identical prisms:
1️⃣ The first prism split white light into its seven colors.
2️⃣ The second prism (inverted position) recombined the spectrum into white light. - This proved that white light is a mixture of different colors.
Rainbow: A Natural Example of Dispersion
🌈 Formation of a Rainbow
- A rainbow forms due to the dispersion, refraction, and internal reflection of sunlight in water droplets after rainfall.
- The water droplets act like tiny prisms, bending and splitting sunlight into VIBGYOR colors.
- A rainbow is always seen opposite to the Sun.
📌 Other Places to See Rainbows:
- In waterfalls or fountains on a sunny day, when the Sun is behind you.
Key Takeaways
✅ Dispersion: Splitting of white light into its colors.
✅ Prism: Causes different bending for different colors, forming a spectrum.
✅ Newton’s Experiment: Proved white light is made of seven colors.
✅ Rainbow Formation: Due to dispersion, refraction, and internal reflection in water droplets.
This section explains how prisms, Newton’s experiment, and natural rainbows reveal the true composition of light. 🌈✨
Detailed Notes on Atmospheric Refraction
11.5 Atmospheric Refraction
Introduction
- Atmospheric refraction is the bending of light due to changes in the density and refractive index of the Earth’s atmosphere.
- It causes various optical effects like twinkling of stars, advance sunrise, and delayed sunset.
Observing Atmospheric Refraction
- You might have noticed the wavering or flickering of objects when looking through hot air above a fire or a radiator.
- This happens because:
🔹 Hot air is less dense → Lower refractive index
🔹 Cool air is denser → Higher refractive index - Since air is not uniform, the light from objects fluctuates, causing a shimmering effect.
Twinkling of Stars ✨
🌟 Why do stars twinkle?
- Starlight enters Earth’s uneven atmosphere and undergoes continuous refraction as it passes through layers of varying density.
- The light bends towards the normal, making the star appear slightly higher than its actual position.
- The apparent position fluctuates, and the amount of starlight reaching our eyes changes constantly.
- This makes the star appear brighter at times and dimmer at others, causing the twinkling effect.
🌍 Why don’t planets twinkle?
- Planets are much closer and appear as extended sources (not point-sized).
- Light from different parts of the planet’s surface averages out the variations, nullifying the twinkling effect.
Advance Sunrise and Delayed Sunset 🌅🌇
- The Sun appears 2 minutes earlier than the actual sunrise and 2 minutes later than the actual sunset.
- Why?
🔹 Light from the Sun bends due to atmospheric refraction.
🔹 The Sun appears higher than its actual position when near the horizon.
🔹 Even when the Sun is below the horizon, its light refracts, making it visible to us.
🔹 This creates the illusion of a longer day.
📌 Apparent Flattening of the Sun
- At sunrise and sunset, the Sun appears oval or flattened due to greater refraction at the horizon than at the top.
Key Takeaways
✅ Twinkling of stars → Due to atmospheric refraction and fluctuating light intensity.
✅ Planets don’t twinkle → Their extended size cancels out fluctuations.
✅ Advance sunrise & delayed sunset → Due to the bending of sunlight in Earth’s atmosphere.
✅ Flattening of the Sun → Caused by unequal refraction at different parts of the Sun’s disc.
This section explains how the Earth’s atmosphere bends light, creating beautiful natural effects like twinkling stars, longer days, and optical illusions! 🌞✨
Detailed Notes on Scattering of Light
11.6 Scattering of Light
- Scattering of light occurs when light rays interact with small particles in a medium and deviate from their original path.
- It is responsible for various natural optical phenomena, such as:
- 🌌 Blue color of the sky
- 🌊 Color of deep-sea water
- 🌅 Red color of the Sun at sunrise and sunset
When light passes through a medium containing small particles (such as dust, water droplets, or air molecules), these particles scatter light in different directions. The amount of scattering depends on the size of the particles and the wavelength of light.
11.6.1 Tyndall Effect
- The Tyndall effect is the phenomenon of light scattering by colloidal particles in a medium.
- This effect makes the path of a light beam visible when it passes through a colloidal solution or a medium containing fine particles.
Examples of Tyndall Effect:
🔹 When a beam of sunlight enters a room through a small hole, its path becomes visible due to dust particles scattering the light.
🔹 In a dense forest, sunlight passing through water droplets in mist appears as scattered beams of light.
Effect of Particle Size on Scattering:
- The color of the scattered light depends on the size of the scattering particles:
- Very fine particles → Scatter mainly blue light (shorter wavelengths).
- Larger particles → Scatter longer wavelengths (red, orange, yellow).
- Very large particles → Scatter all wavelengths equally, making the scattered light appear white.
11.6.2 Why is the Sky Blue?
- The molecules of air and tiny particles in the atmosphere have a size smaller than the wavelength of visible light.
- These particles scatter shorter-wavelength light (blue, violet) more than longer-wavelength light (red, orange).
Scientific Explanation:
- Sunlight (white light) consists of seven colors (VIBGYOR) with different wavelengths.
- When sunlight enters the atmosphere, it interacts with air molecules and dust particles.
- Shorter wavelengths (blue, violet) are scattered more than longer wavelengths (red, orange).
- However, since our eyes are more sensitive to blue light than violet, we see the sky as blue.
Why Does the Sky Appear Dark in Space?
- If Earth had no atmosphere, there would be no scattering of light, and the sky would appear black.
- Astronauts in space and passengers in high-altitude planes see a black sky because there are fewer particles to scatter light.
Why Are Danger Signals Red?
- Red light has the longest wavelength, so it is least scattered by fog, smoke, or dust.
- This makes red signals clearly visible from long distances, which is why they are used for traffic lights, stop signals, and warning signs.
11.6.3 Color of the Sun at Sunrise and Sunset
- At sunrise and sunset, the Sun appears reddish because of greater scattering of shorter wavelengths (blue, violet).
- During these times, sunlight travels a longer distance through the atmosphere, so:
- Most of the blue light gets scattered away, and
- Only the longer wavelengths (red, orange) reach our eyes.
Scientific Explanation:
- At noon, the Sun is directly overhead, so sunlight travels a shorter distance through the atmosphere.
- Less scattering occurs, and all colors reach our eyes, making the Sun appear white.
- Near the horizon (during sunrise and sunset), sunlight passes through a thicker layer of atmosphere.
- Blue and violet light are scattered out, leaving behind red and orange light, which we see.
Activity to Demonstrate Scattering of Light
Steps:
1️⃣ Place a strong white light source (S) at the focus of a converging lens (L1) to get a parallel beam of light.
2️⃣ Allow the light to pass through a transparent glass tank (T) filled with clear water.
3️⃣ Pass the beam through a circular hole (C) in a cardboard.
4️⃣ Observe the sharp image of the hole on a screen (MN) using a second converging lens (L2).
5️⃣ Add 200 g of sodium thiosulphate (hypo) and 1-2 mL of concentrated sulfuric acid to the water.
6️⃣ Wait for 2-3 minutes as microscopic sulfur particles form, making the water a colloidal solution.
Observations:
🔹 From the sides of the tank, blue light is seen, due to the scattering of short wavelengths (blue) by small sulfur particles.
🔹 From the circular hole (screen side), an orange-red color appears, similar to sunrise and sunset.
✅ This experiment demonstrates why the sky is blue and why the Sun appears red at sunrise and sunset.
Key Takeaways
✅ Tyndall Effect → Light scattering by colloidal particles makes the light beam visible.
✅ Blue Sky → Shorter wavelengths (blue) are scattered more by air molecules.
✅ Sunrise & Sunset Appear Red → Blue light is scattered out, leaving longer red wavelengths.
✅ Red Signals for Warnings → Red light is least scattered and visible over long distances.
✅ Sky Appears Dark in Space → No atmosphere means no scattering of light.
These concepts explain how scattering of light creates beautiful natural phenomena like the blue sky, red sunsets, and Tyndall effect. 🌅🌌✨
🌟 E X E R C I S E S & A N S W E R S
1️⃣ The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to:
🔘 (a) Presbyopia
🔘 (b) Accommodation ✅
🔘 (c) Near-sightedness
🔘 (d) Far-sightedness
✅ Answer: (b) Accommodation
🔹 The ciliary muscles 🤓 adjust the curvature of the eye lens, allowing it to focus on objects at different distances.
🔹 This ability is called accommodation.
2️⃣ The human eye forms the image of an object at its:
🔘 (a) Cornea
🔘 (b) Iris
🔘 (c) Pupil
🔘 (d) Retina ✅
✅ Answer: (d) Retina
🔹 The retina 👁️ is a light-sensitive layer at the back of the eye.
🔹 It contains rods & cones 🏮🔵 that help in vision and send signals to the brain 🧠 via the optic nerve.
3️⃣ The least distance of distinct vision for a young adult with normal vision is about:
🔘 (a) 25 m
🔘 (b) 2.5 cm
🔘 (c) 25 cm ✅
🔘 (d) 2.5 m
✅ Answer: (c) 25 cm
🔹 The least distance of distinct vision (or near point) is the minimum distance at which an object can be seen clearly without strain.
4️⃣ The change in focal length of an eye lens is caused by the action of the:
🔘 (a) Pupil
🔘 (b) Retina
🔘 (c) Ciliary muscles ✅
🔘 (d) Iris
✅ Answer: (c) Ciliary muscles
🔹 Ciliary muscles 💪 control the thickness of the eye lens.
🔹 When they contract, the lens becomes thicker to focus on near objects.
🔹 When they relax, the lens becomes thinner to focus on distant objects.
5️⃣ A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 dioptres. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
✅ Answer:
The formula for focal length (f) is:
📌 f = 1 / P (where P is the power in dioptres, and f is in meters)
🔹 For distant vision:
P = –5.5 D
📌 f = 1 / (–5.5) = –0.182 m (or –18.2 cm)
🔹 For near vision:
P = +1.5 D
📌 f = 1 / (+1.5) = 0.667 m (or 66.7 cm)
🔹 The negative sign indicates that a concave lens is used for distant vision, while a convex lens is used for near vision.
6️⃣ The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
✅ Answer:
- A myopic person (near-sighted) can see nearby objects clearly but distant objects appear blurry.
- The far point is 80 cm (0.8 m), so the lens needed should focus light at this distance.
📌 Power (P) = 1 / f
📌 P = 1 / (–0.8) = –1.25 D
🔹 A concave lens 👓 (diverging lens) of –1.25 dioptres is needed to correct myopia.
7️⃣ Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
✅ Answer:
🔹 Hypermetropia (Far-sightedness) is a defect where a person cannot see nearby objects clearly.
🔹 The near point is 1 m instead of the normal 25 cm.
📌 Using the lens formula:
1/f = 1/v – 1/u
where:
- v = 25 cm = 0.25 m (normal near point)
- u = 1 m (defective near point)
📌 1/f = 1/0.25 – 1/1
📌 1/f = 4 – 1 = 3
📌 f = 1/3 = 0.333 m (or 33.3 cm)
🔹 Power (P) = 1/f = 1/0.333 = +3 D
🔹 A convex lens 🔍 of +3 dioptres is required.
8️⃣ Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
✅ Answer:
- The ciliary muscles cannot increase the curvature of the lens beyond a certain limit.
- If an object is closer than 25 cm, the eye lens cannot focus it properly on the retina, causing blurriness.
9️⃣ What happens to the image distance in the eye when we increase the distance of an object from the eye?
✅ Answer:
- The image distance in the eye remains constant because the eye lens adjusts its focal length to keep the image focused on the retina.
- As the object moves farther, the lens becomes thinner, increasing the focal length to maintain a sharp image.
🔟 Why do stars twinkle?
✅ Answer:
- Stars twinkle due to atmospheric refraction.
- As starlight passes through Earth's atmosphere, it bends due to varying air densities.
- This causes the star's apparent position to change constantly, making it appear brighter at times and dimmer at others → creating the twinkling effect.
1️⃣1️⃣ Explain why the planets do not twinkle.
✅ Answer:
- Planets are much closer to Earth and appear as extended sources of light (not point-sized).
- Light from different parts of the planet's surface undergoes different refractions, and these fluctuations cancel each other out.
- Hence, planets do not twinkle like stars.
1️⃣2️⃣ Why does the Sun appear reddish early in the morning?
✅ Answer:
- During sunrise and sunset, sunlight travels a longer distance through the atmosphere.
- Shorter wavelengths (blue, violet) are scattered away, and only longer wavelengths (red, orange) reach our eyes.
- This makes the Sun appear reddish at these times.
1️⃣3️⃣ Why does the sky appear dark instead of blue to an astronaut?
✅ Answer:
- The blue color of the sky is due to scattering of sunlight by the atmosphere.
- In space, there is no atmosphere to scatter light.
- Since there is no scattering, the sky appears black instead of blue to astronauts. 🚀🌌
