Lenses Class 10th Solutions | Lenses SSC Class 10 Questions And Answers
.webp "Lenses Class 10th Solutions | Lenses SSC Class 10 Questions And Answers")
Match the columns in the following table and explain them.
| Column 1 | Column 2 | Column 3 |
|---|---|---|
| Farsightedness | Far away object can be seen clearly | Convex lens |
| Presbyopia | Problem of old age | Bifocal lens |
| Nearsightedness | Nearby object can be seen clearly | Concave lens |
Solution 1: Scientific and Written Exam Answer
Vision defects occur due to improper focusing of light on the retina. The correction for each defect is done using suitable lenses.
- Farsightedness (Hypermetropia): A person with farsightedness can see distant objects clearly but has difficulty seeing nearby objects. This occurs when the eyeball is too short or the lens is too flat. It is corrected using a convex lens, which converges the light rays before they enter the eye.
- Presbyopia: This is an age-related vision defect where the flexibility of the eye lens decreases, making it difficult to focus on nearby and distant objects. It is corrected using bifocal lenses, which have both convex and concave sections.
- Nearsightedness (Myopia): A person with nearsightedness can see nearby objects clearly but has difficulty seeing distant objects. This occurs when the eyeball is too long or the lens is too curved. It is corrected using a concave lens, which diverges the light rays before they enter the eye.
Solution 2: Simple and Understandable Answer
Our eyes sometimes have trouble seeing things clearly because of different vision problems. Here’s how they work:
- Farsightedness (Hypermetropia): You can see things far away but not up close. A convex lens helps fix this.
- Presbyopia: As people get older, their eyes lose flexibility, making it hard to see things at different distances. A bifocal lens (which has two types of lenses) helps.
- Nearsightedness (Myopia): You can see things up close but not far away. A concave lens helps fix this.
Example: If your grandparents struggle to read a book but can see far objects, they likely have presbyopia and need bifocal glasses.
Draw a figure explaining various terms related to a lens.
Solution 1: Scientific and Written Exam Answer
A lens is a transparent optical component with curved surfaces that refract light to form images. Lenses are classified into two types:
- Convex Lens (Converging Lens): This lens converges parallel light rays to a focal point. It is commonly used in magnifying glasses, cameras, and the human eye.
- Concave Lens (Diverging Lens): This lens diverges parallel light rays, making them appear to come from a focal point. It is used in spectacles to correct nearsightedness.
1. Principal Axis:
The principal axis is an imaginary straight line passing through the optical center and the center of curvature of the lens. Light rays traveling along this axis do not bend.
2. Optical Center (O):
The optical center is the central point of the lens where light rays pass without any deviation. It is located at the midpoint of the lens.
3. Focal Point (F):
The focal point is where parallel light rays converge (convex lens) or appear to diverge from (concave lens). The distance from the optical center to this point is called the focal length (f).
Formula: The focal length of a lens is related to its curvature by the lens maker’s formula:
$$ \frac{1}{f} = (n-1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) $$
4. Center of Curvature (C):
The center of curvature is the center of the sphere from which the lens surface is a part. A lens has two centers of curvature, one for each curved surface.
5. Radius of Curvature (R):
The radius of curvature is the distance between the center of curvature and the optical center of the lens.
6. Principal Focus:
The principal focus is a specific point on the principal axis where parallel light rays converge (for a convex lens) or appear to diverge from (for a concave lens).
Diagram:
Solution 2: Simple and Understandable Answer
A lens is a transparent object that bends light to form images. There are two types of lenses:
- Convex Lens: This lens makes light rays come together. It is used in magnifying glasses.
- Concave Lens: This lens spreads light rays apart. It is used in glasses for nearsightedness.
1. Principal Axis:
The principal axis is an imaginary line that runs through the center of the lens. It helps us understand how light moves through the lens.
2. Optical Center:
The optical center is the middle point of the lens. Light passing through this point does not change direction.
3. Focal Point:
The focal point is where the light rays meet (in a convex lens) or appear to come from (in a concave lens). The distance to this point is the focal length.
4. Center of Curvature:
The center of curvature is the center of the curved surface of the lens. A lens has two such points.
5. Radius of Curvature:
The radius of curvature is the distance from the center of curvature to the center of the lens. It tells us how curved the lens is.
6. Principal Focus:
The principal focus is the point where parallel light rays either meet (for a convex lens) or appear to come from (for a concave lens).
Example:
A convex lens is used in a magnifying glass to make things look bigger. A concave lens is used in glasses to help people see faraway objects clearly.
At which position will you keep an object in front of a convex lens so as to get a real image of the same size as the object? Draw a figure.
Solution 1: Scientific and Written Exam Answer
To obtain a real image of the same size as the object using a convex lens, the object should be placed at twice the focal length (2F) from the lens.
Concept:
A convex lens forms different types of images based on the object's position:
- If the object is placed at 2F, the image is real, inverted, and of the same size at 2F on the other side of the lens.
- If the object is beyond 2F, the image is smaller and located between F and 2F.
- If the object is between F and 2F, the image is magnified and located beyond 2F.
- If the object is at F, no real image is formed (image is at infinity).
Formula:
The position of the image is given by the lens formula:
$$ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} $$
Where:
- f = focal length of the convex lens
- u = object distance
- v = image distance
Conclusion:
When the object is placed at 2F, the image is real, inverted, and of the same size at 2F on the opposite side.
Diagram:
Solution 2: Simple and Understandable Answer
To get a real image of the same size as the object using a convex lens, place the object at twice the focal length (2F) from the lens.
How It Works:
- If you place an object at 2F, the image will appear at 2F on the other side.
- The image will be real, inverted, and of the same size.
Example:
This principle is used in cameras and projectors to form clear, real images.
Diagram:
Give scientific reason: Simple microscope is used for watch repairs.
Solution 1: Scientific and Written Exam Answer
A simple microscope is used in watch repairs because it provides a magnified image of small components, allowing precision in assembling and repairing delicate watch parts.
Concept:
- A simple microscope consists of a convex lens that produces a magnified, erect, and virtual image of the object placed within its focal length.
- It increases the visibility of minute components, making it easier for watchmakers to examine and fix intricate parts.
Formula:
The magnification power of a simple microscope is given by:
$$ M = 1 + \frac{D}{f} $$
Where:
- M = magnification
- D = least distance of distinct vision (25 cm for the human eye)
- f = focal length of the convex lens
Conclusion:
The use of a simple microscope helps watchmakers magnify tiny watch parts, ensuring accuracy and efficiency in repairs.
Solution 2: Simple and Understandable Answer
A simple microscope helps in watch repairs because it makes tiny watch parts look bigger, making it easier for the watchmaker to work with small screws, gears, and other delicate components.
How It Works:
- It uses a convex lens to make objects appear larger and clearer.
- This helps in seeing fine details that are not visible to the naked eye.
Example:
Just like a magnifying glass helps in reading small text, a simple microscope helps watchmakers repair small parts with accuracy.
Give scientific reason: One can sense colours only in bright light.
Solution 1: Scientific and Written Exam Answer
The human eye can sense colours only in bright light because of the presence of cone cells in the retina, which are responsible for colour vision.
Concept:
- The retina of the human eye contains two types of photoreceptor cells – rod cells and cone cells.
- Rod cells help in dim light and night vision, but they cannot detect colours.
- Cone cells are sensitive to bright light and allow us to perceive different colours.
Conclusion:
Since cone cells work only in adequate light, we can sense colours only in bright light, while in low light, everything appears in shades of grey.
Solution 2: Simple and Understandable Answer
We can see colours clearly only when there is enough light because our eyes need special cells called cone cells to detect colours.
How It Works:
- In bright light, cone cells are active and help us see different colours.
- In dim light, only rod cells work, and they do not sense colours, making everything appear black and white.
Example:
When we enter a dark room, colours look faint or disappear, but when we switch on the light, we can see all colours properly.
Give scientific reason: We cannot clearly see an object kept at a distance less than 25 cm from the eye.
Solution 1: Scientific and Written Exam Answer
The human eye has a minimum distance, known as the near point (least distance of distinct vision), which is 25 cm for a normal eye. If an object is placed closer than 25 cm, the eye lens cannot adjust properly to form a clear image.
Concept:
- The ciliary muscles in the eye control the curvature of the eye lens to adjust focus.
- When an object is closer than 25 cm, the lens cannot become more convex beyond its limit.
- As a result, the image does not form properly on the retina, causing a blurred vision.
Conclusion:
Since the eye lens has a focusing limit, objects placed too close appear unclear and blurred.
Solution 2: Simple and Understandable Answer
Our eyes can clearly see objects only if they are at least 25 cm away. If we bring an object too close, it looks blurry because our eye lens cannot adjust beyond a certain limit.
How It Works:
- The eye lens changes shape to focus on objects at different distances.
- If an object is closer than 25 cm, the lens cannot adjust properly to form a clear image.
Example:
Try bringing a book very close to your eyes. You will notice that the words become unclear and blurry.
Explain the working of an astronomical telescope using refraction of light.
Solution 1: Scientific and Written Exam Answer
An astronomical telescope is an optical instrument used to observe distant celestial objects like stars and planets. It works on the principle of refraction of light through a combination of two convex lenses.
Construction:
- It consists of two convex lenses – the objective lens (large aperture) and the eyepiece lens (small aperture).
- The objective lens gathers light from a distant object and forms a real, inverted, and smaller image at its focal plane.
- The eyepiece lens acts as a magnifying glass, enlarging the image for detailed viewing.
Working Principle:
- Light from a distant object enters the objective lens and converges to form an inverted image at its focus.
- The eyepiece lens then magnifies this image before it reaches the eye.
- The final image formed is magnified, inverted, and virtual.
Formula for Magnification:
The magnification of an astronomical telescope is given by:
$$ M = \frac{F_o}{F_e} $$
Where:
- M = Magnification
- F_o = Focal length of the objective lens
- F_e = Focal length of the eyepiece lens
Conclusion:
The astronomical telescope allows us to see faraway celestial objects with greater clarity by using the principle of refraction and magnification.
Solution 2: Simple and Understandable Answer
An astronomical telescope helps us see stars, planets, and galaxies by using two convex lenses.
How It Works:
- The big front lens (objective lens) collects light from a faraway object.
- It forms a small, upside-down image inside the telescope.
- The small back lens (eyepiece lens) magnifies this image so that we can see it clearly.
Example:
When you use a telescope to look at the moon, you see a bigger, clearer image because the lenses bend the light to zoom in on the object.
Distinguish between Farsightedness and Nearsightedness.
Solution 1: Scientific and Written Exam Answer
| Feature | Farsightedness (Hypermetropia) | Nearsightedness (Myopia) |
|---|---|---|
| Definition | A condition where distant objects are seen clearly, but nearby objects appear blurry. | A condition where nearby objects are seen clearly, but distant objects appear blurry. |
| Cause | Occurs due to a shorter eyeball or weaker eye lens, causing light to focus behind the retina. | Occurs due to a longer eyeball or stronger eye lens, causing light to focus in front of the retina. |
| Correction | Corrected using a convex lens to bring the image onto the retina. | Corrected using a concave lens to spread the light rays before they reach the retina. |
| Example | Older adults often develop hypermetropia, making it difficult to read small print. | Common in school-going children who have trouble seeing the blackboard clearly. |
Solution 2: Simple and Understandable Answer
What is Farsightedness?
Farsightedness (Hypermetropia) means you can see far objects clearly, but close objects look blurry. It happens because light focuses behind the retina instead of on it.
What is Nearsightedness?
Nearsightedness (Myopia) means you can see close objects clearly, but far objects look blurry. It happens because light focuses in front of the retina instead of on it.
How to Correct It?
- Farsightedness is corrected using a convex lens (curved outward).
- Nearsightedness is corrected using a concave lens (curved inward).
Example:
Grandparents often need reading glasses because they struggle to see small text (Farsightedness).
Students who sit at the back of a classroom might not see the board clearly and need glasses (Nearsightedness).
Distinguish between Concave Lens and Convex Lens
Solution 1: Scientific and Written Exam Answer
| Feature | Concave Lens | Convex Lens |
|---|---|---|
| Definition | A lens that is thinner in the center and thicker at the edges, causing light rays to diverge. | A lens that is thicker in the center and thinner at the edges, causing light rays to converge. |
| Image Formation | Forms a virtual, erect, and smaller image always. | Forms a real or virtual image, depending on the object's position. |
| Nature of Lens | Also called a diverging lens because it spreads light rays apart. | Also called a converging lens because it brings light rays together. |
| Use in Vision Correction | Used to correct nearsightedness (myopia). | Used to correct farsightedness (hypermetropia). |
| Example | Used in peepholes of doors and some glasses. | Used in magnifying glasses, cameras, and microscopes. |
Solution 2: Simple and Understandable Answer
What is a Concave Lens?
A concave lens is thinner in the middle and thicker at the edges. It spreads light rays apart and forms a small, upright, and virtual image.
What is a Convex Lens?
A convex lens is thicker in the middle and thinner at the edges. It brings light rays together and can form both real and virtual images, depending on the object's position.
How Are They Used?
- Concave lenses are used in glasses for nearsighted people and peepholes in doors.
- Convex lenses are used in magnifying glasses, microscopes, and cameras.
Example:
When you look through a peephole in a door, it makes things look smaller – this is a concave lens.
When you use a magnifying glass, it makes objects look bigger – this is a convex lens.
What is the function of the iris and the muscles connected to the lens in the human eye?
Solution 1: Scientific and Written Exam Answer
Function of the Iris:
The iris is the colored part of the eye that controls the amount of light entering the eye. It contains muscle fibers that adjust the size of the pupil depending on the intensity of light.
- In bright light, the circular muscles of the iris contract, making the pupil smaller to reduce light entry.
- In dim light, the radial muscles of the iris contract, making the pupil larger to allow more light inside.
Function of the Muscles Connected to the Lens:
The ciliary muscles are responsible for changing the shape of the lens to help focus light on the retina. This process is called accommodation.
- When focusing on a near object, the ciliary muscles contract, making the lens thicker for better focus.
- When focusing on a distant object, the ciliary muscles relax, making the lens thinner for clear vision.
Solution 2: Simple and Understandable Answer
What is the Iris?
The iris is the colored part of our eye. It helps control the amount of light that enters the eye by adjusting the pupil size.
How Does the Iris Work?
- In bright light, the pupil becomes small to reduce glare.
- In dim light, the pupil becomes big to let in more light.
What Do the Eye Muscles Do?
The ciliary muscles help change the shape of the lens so we can see things clearly.
How Do the Eye Muscles Work?
- To see near objects, the lens becomes thicker.
- To see far objects, the lens becomes thinner.
Example:
When reading a book, the ciliary muscles contract to make the letters clear. When looking at the sky, the muscles relax to focus on distant objects.
Solve the following example:
A doctor has prescribed a lens having power +1.5 D. What will be the focal length of the lens? What is the type of the lens and what must be the defect of vision?
Solution 1: Scientific and Written Exam Answer
Step 1: Formula to Find Focal Length
The formula to calculate the focal length (f) of a lens is:
$$ P = \frac{1}{f} $$
Where:
- P = Power of the lens (in diopters, D)
- f = Focal length (in meters, m)
Step 2: Substituting the Given Values
Given power of the lens, P = +1.5 D
Using the formula:
$$ f = \frac{1}{P} = \frac{1}{1.5} $$
$$ f = 0.666 \text{ m} = 66.6 \text{ cm} $$
Step 3: Identifying the Type of Lens
Since the power of the lens is positive (+), the lens is a convex lens.
Step 4: Identifying the Defect of Vision
A convex lens is used to correct farsightedness (hypermetropia), a condition where a person can see distant objects clearly but has difficulty seeing nearby objects.
Solution 2: Simple and Understandable Answer
How to Find Focal Length?
The formula to find focal length is:
$$ f = \frac{1}{P} $$
By putting P = +1.5, we get:
$$ f = 0.666 \text{ m} = 66.6 \text{ cm} $$
What Type of Lens Is It?
The power +1.5 D means the lens is convex (thicker in the middle).
What Eye Problem Does It Solve?
A convex lens is used to correct farsightedness (hypermetropia). People with this condition can see far objects clearly but near objects appear blurry.
Example:
If a person has trouble reading a book but can see distant road signs clearly, they might need a convex lens with a positive power.
Solve the following example:
A 5 cm high object is placed at a distance of 25 cm from a converging lens with a focal length of 10 cm. Determine the position, size, and type of the image.
Solution 1: Scientific and Written Exam Answer
Step 1: Given Data
- Object height (ho) = 5 cm
- Object distance (u) = -25 cm (negative as per sign convention)
- Focal length (f) = 10 cm
Step 2: Using the Lens Formula
The lens formula is:
$$ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} $$
Step 3: Substituting Values
$$ \frac{1}{10} = \frac{1}{v} - \frac{1}{-25} $$
$$ \frac{1}{v} = \frac{1}{10} + \frac{1}{25} $$
$$ \frac{1}{v} = \frac{5 + 2}{50} = \frac{7}{50} $$
$$ v = \frac{50}{7} \approx 7.14 \text{ cm} $$
Step 4: Finding Image Size
The magnification formula is:
$$ m = \frac{h_i}{h_o} = \frac{-v}{u} $$
$$ m = \frac{-7.14}{-25} = 0.285 $$
$$ h_i = m \times h_o = 0.285 \times 5 = 1.43 \text{ cm} $$
Step 5: Image Characteristics
- Image distance v ≈ 7.14 cm (on the same side as the image)
- Image height hi ≈ 1.43 cm (smaller than object)
- Since v is positive, the image is real and inverted.
Solution 2: Simple and Understandable Answer
Step 1: Understanding the Problem
A 5 cm object is placed 25 cm away from a convex lens (a converging lens) with a focal length of 10 cm.
Step 2: What Happens to the Image?
- Using the lens formula, we find that the image distance (v) is about 7.14 cm.
- The image height is 1.43 cm, which means it is smaller than the object.
Step 3: What Type of Image is Formed?
- Since v is positive, the image is real and inverted.
- The image is diminished (smaller) in size.
- The image is formed on the same side as the outgoing rays.
Example for Better Understanding:
When you use a magnifying glass at a certain distance, the image appears upside down and smaller. This is similar to how a convex lens works when an object is placed at a certain distance.
Solve the following example:
Three lenses having power 2 D, 2.5 D, and 1.7 D are kept touching in a row. What is the total power of the lens combination?
Solution 1: Scientific and Written Exam Answer
Step 1: Understanding the Concept
When multiple thin lenses are placed in contact, the total power of the combination is given by the formula:
$$ P_{total} = P_1 + P_2 + P_3 $$
Step 2: Substituting the Given Values
$$ P_{total} = 2 + 2.5 + 1.7 $$
$$ P_{total} = 6.2 D $$
Step 3: Interpretation of the Result
- The total power of the lens combination is +6.2 D.
- Since the power is positive, the lens combination acts as a convex lens.
- This means the combination will converge light rays and can be used for farsightedness correction or magnification.
Solution 2: Simple and Understandable Answer
Step 1: What is Given?
We have three lenses with power values 2 D, 2.5 D, and 1.7 D placed in a row.
Step 2: How to Find Total Power?
- When lenses are placed together, their powers simply add up.
- So, 2 + 2.5 + 1.7 = 6.2 D.
Step 3: What Does the Answer Mean?
- The final power of +6.2 D means the combination acts as a convex lens.
- A convex lens helps to focus light and is used in magnifying glasses, cameras, and correcting farsightedness.
Example for Better Understanding:
If you stack three magnifying glasses, their ability to magnify increases. Similarly, adding lens powers increases their ability to focus light.
Solve the following example:
An object kept 60 cm from a lens gives a virtual image 20 cm in front of the lens. What is the focal length of the lens? Is it a converging lens or diverging lens?
Solution 1: Scientific and Written Exam Answer
Step 1: Given Data
- Object distance (u) = -60 cm (since the object is placed on the left of the lens)
- Image distance (v) = -20 cm (virtual image is formed on the same side as the object)
- Focal length (f) = ?
Step 2: Use the Lens Formula
The lens formula is:
$$ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} $$
Step 3: Substituting the Values
$$ \frac{1}{f} = \frac{1}{-20} - \frac{1}{-60} $$
$$ \frac{1}{f} = -\frac{1}{20} + \frac{1}{60} $$
$$ \frac{1}{f} = \frac{-3 + 1}{60} $$
$$ \frac{1}{f} = \frac{-2}{60} $$
$$ \frac{1}{f} = \frac{-1}{30} $$
Step 4: Finding the Focal Length
$$ f = -30 cm $$
Step 5: Identifying the Type of Lens
- The focal length is negative, which means it is a concave lens (diverging lens).
- A concave lens always forms a virtual, upright, and diminished image.
Solution 2: Simple and Understandable Answer
Step 1: What is Given?
- An object is placed 60 cm away from a lens.
- The image is formed 20 cm in front of the lens and is virtual.
Step 2: How to Find the Focal Length?
- Using the lens formula, we calculate the focal length and get f = -30 cm.
Step 3: What Does the Answer Mean?
- The negative focal length tells us that the lens is a concave lens.
- A concave lens always forms a virtual, smaller, and upright image.
Example for Better Understanding:
Think about a spectacle lens used for nearsightedness—it is a concave lens because it spreads out light rays.