Grade 10 Q&A: Chapter 7: Lenses

Concept Questions

Q1: What is a 'lens'?

Answer: A lens is a transparent optical device made of glass or plastic, bounded by two spherical surfaces or one spherical and one plane surface, which refracts light to form images.

Q2: Name the two main types of lenses.

Answer: The two main types of lenses are convex lenses (converging lenses) and concave lenses (diverging lenses).

Q3: What is the 'optical center' of a lens?

Answer: The optical center (O) of a lens is a point on the principal axis of the lens through which a ray of light passes undeviated.

Q4: What is the 'principal axis' of a lens?

Answer: The principal axis of a lens is an imaginary line passing through the optical center and perpendicular to both surfaces of the lens.

Q5: How many principal foci does a lens have?

Answer: A lens has two principal foci, F₁ and F₂.

Q6: Define 'focal length' of a lens.

Answer: The focal length (f) of a lens is the distance between the optical center and the principal focus of the lens.

Q7: Which type of lens is also known as a 'converging lens'?

Answer: A convex lens is also known as a 'converging lens' because it converges parallel rays of light after refraction.

Q8: Which type of lens is also known as a 'diverging lens'?

Answer: A concave lens is also known as a 'diverging lens' because it diverges parallel rays of light after refraction.

Q9: What is the 'power of a lens'?

Answer: The power of a lens (P) is a measure of its ability to converge or diverge light rays. It is defined as the reciprocal of its focal length in meters (P = 1/f).

Q10: What is the SI unit of the power of a lens?

Answer: The SI unit of the power of a lens is the dioptre (D).

Q11: What is 'myopia' (nearsightedness)?

Answer: Myopia (nearsightedness) is a defect of vision where a person can see nearby objects clearly but cannot see distant objects distinctly. The image forms in front of the retina.

Q12: What type of lens is used to correct myopia?

Answer: A concave lens is used to correct myopia.

Q13: What is 'hypermetropia' (farsightedness)?

Answer: Hypermetropia (farsightedness) is a defect of vision where a person can see distant objects clearly but cannot see nearby objects distinctly. The image forms behind the retina.

Q14: What type of lens is used to correct hypermetropia?

Answer: A convex lens is used to correct hypermetropia.

Q15: What is 'presbyopia'?

Answer: Presbyopia is an age-related eye defect where the eye loses its ability to focus on nearby objects due to the weakening of ciliary muscles and reduced flexibility of the eye lens.

Application-Based Questions

Q16: Draw a ray diagram to show image formation by a convex lens when the object is placed beyond 2F₁. State the nature of the image.

Answer: (Description of ray diagram, as drawing is not possible in text) Rays: 1. A ray parallel to the principal axis passes through F₂ after refraction. 2. A ray passing through the optical center goes undeviated. 3. A ray passing through F₁ becomes parallel to the principal axis after refraction. Nature of Image: The image formed is real, inverted, and diminished, located between F₂ and 2F₂ on the other side of the lens.

Q17: Draw a ray diagram to show image formation by a concave lens for any object position. State the nature of the image.

Answer: (Description of ray diagram, as drawing is not possible in text) Rays: 1. A ray parallel to the principal axis appears to diverge from F₁ after refraction. 2. A ray passing through the optical center goes undeviated. Nature of Image: The image formed is always virtual, erect, and diminished, located between the optical center and F₁ on the same side as the object.

Q18: A person uses a lens of power +2.5 D. What type of lens is it, and what vision defect does it correct?

Answer: * Since the power of the lens is positive (+2.5 D), it is a convex lens. * A convex lens is used to correct hypermetropia (farsightedness).

Q19: Why is a convex lens used as a magnifying glass?

Answer: A convex lens is used as a magnifying glass because when an object is placed between its optical center (O) and principal focus (F₁), it forms a virtual, erect, and magnified image on the same side as the object. This enlarged image allows us to see small objects in greater detail.

Q20: Explain why the focal length of a concave lens is considered negative.

Answer: According to the New Cartesian Sign Convention, distances measured against the direction of incident light are taken as negative. For a concave lens, the principal focus (F₁) from which the refracted rays appear to diverge is located on the same side as the incident light, to the left of the optical center. Therefore, its focal length is considered negative.

Higher-Order Thinking Questions

Q21: A student has difficulty reading the blackboard from the back row but can read a book clearly. What vision defect does the student have, and how can it be corrected? Explain with a ray diagram.

Answer: The student has myopia (nearsightedness). This occurs because the eye lens converges light rays too strongly, or the eyeball is too long, causing the image of distant objects to form in front of the retina. Correction: Myopia is corrected using a concave lens of appropriate power. A concave lens diverges the incoming parallel rays from distant objects slightly before they enter the eye, ensuring that the image forms precisely on the retina, allowing the person to see distant objects clearly. (Description of ray diagram: Parallel rays from a distant object converge in front of the retina in a myopic eye. A concave lens placed in front of the eye diverges these rays slightly so they converge exactly on the retina.)

Q22: Discuss the working principle of a simple microscope (magnifying glass).

Answer: A simple microscope, or magnifying glass, consists of a single convex lens. Its working principle is based on the formation of a virtual, erect, and magnified image. When a small object is placed within the focal length of the convex lens (between F₁ and O), the rays of light from the object, after passing through the lens, appear to diverge from a point on the same side as the object. When these diverging rays are extended backward, they appear to meet at a point, forming a virtual, erect, and magnified image. This enlarged image allows the observer to see fine details of the object that would otherwise be invisible to the naked eye.

Q23: How does the 'lens formula' relate the object distance, image distance, and focal length of a lens?

Answer: The lens formula is a mathematical relationship that connects the object distance (u), image distance (v), and focal length (f) of a lens. It is given by: 1/v - 1/u = 1/f Where: * v is the distance of the image from the optical center. * u is the distance of the object from the optical center. * f is the focal length of the lens. This formula is applicable for both convex and concave lenses, provided the New Cartesian Sign Convention is strictly followed for the values of u, v, and f.

Q24: Explain why a person with presbyopia often needs 'bifocal lenses'.

Answer: Presbyopia is an age-related eye defect where the eye loses its ability to focus on both near and distant objects clearly. This is because the ciliary muscles weaken, and the eye lens loses its flexibility, affecting both accommodation for near vision and sometimes clarity for distant vision. To correct both issues simultaneously, a person with presbyopia often needs bifocal lenses. These lenses have two distinct optical powers: the upper part is a concave lens (or has the power for distant vision) to correct distant vision, and the lower part is a convex lens (or has the power for near vision) to correct near vision, allowing the person to see clearly at different distances by simply looking through different parts of the lens.

Q25: Discuss the factors that affect the focal length of a lens.

Answer: The focal length of a lens is affected by several factors: 1. Refractive Index of the Lens Material: A higher refractive index of the lens material (relative to the surrounding medium) results in a shorter focal length because the light bends more. 2. Refractive Index of the Surrounding Medium: If the lens is placed in a medium with a different refractive index (e.g., water instead of air), its focal length will change. If the refractive index of the medium is greater than that of the lens, the lens can even behave as a diverging lens if it was converging in air, and vice-versa. 3. Radii of Curvature of the Lens Surfaces: The curvature of the lens surfaces directly influences how much light is refracted. Lenses with more curved surfaces (smaller radii of curvature) generally have shorter focal lengths. 4. Wavelength of Light: Due to dispersion, different wavelengths (colors) of light are refracted to different extents, meaning the focal length is slightly different for different colors of light (chromatic aberration).

Q26: What is 'magnification' produced by a lens, and how is it calculated?

Answer: Magnification (m) produced by a lens is the ratio of the height of the image (h') to the height of the object (h). It also equals the ratio of the image distance (v) to the object distance (u). Formula: m = h'/h = v/u * If m > 1, the image is magnified. * If m < 1, the image is diminished. * If m = 1, the image is the same size as the object. * A positive magnification indicates an erect (virtual) image, while a negative magnification indicates an inverted (real) image.

Q27: How are lenses used in a camera to capture images?

Answer: Lenses are crucial in a camera to capture images. A camera uses a convex lens (or a system of lenses) to focus light from the object onto a light-sensitive sensor (like a CCD or CMOS sensor in digital cameras, or film in traditional cameras). The lens converges the light rays to form a real, inverted, and diminished image on the sensor. The focal length of the lens determines the field of view and magnification, while the aperture controls the amount of light entering, and the shutter speed controls the exposure time, all working together to produce a sharp photograph.

Q28: Explain the working principle of a compound microscope.

Answer: A compound microscope uses two convex lenses: an objective lens (short focal length) and an eyepiece lens (longer focal length). 1. The object is placed just beyond the focal length of the objective lens. The objective lens forms a real, inverted, and magnified image. 2. This intermediate image then acts as the object for the eyepiece lens. The eyepiece lens is adjusted so that this intermediate image falls within its focal length. 3. The eyepiece then acts as a simple microscope, forming a final virtual, inverted (relative to the original object), and highly magnified image. The total magnification is the product of the magnifications of the objective and eyepiece lenses, allowing for observation of extremely small objects.

Q29: What is 'chromatic aberration' in lenses, and how is it minimized?

Answer: Chromatic aberration is an optical defect in lenses where different wavelengths (colors) of light are refracted by different amounts, causing them to focus at different points. This results in colored fringes or halos around images, making them appear blurred or distorted. It is minimized by using an achromatic doublet, which is a combination of two lenses (typically a convex lens made of crown glass and a concave lens made of flint glass) cemented together. These lenses have different dispersive powers, and when combined, they can correct for chromatic aberration at two specific wavelengths, significantly reducing the effect.

References

1. MSBSHSE Class 10 Science and Technology Textbook Part 1 (2021-22 English) - Chapter 7: Lenses
2. Maharashtra State Board 10th Standard Science Syllabus 2025-26
3. Balbharati Science and Technology Part 1 Textbook
4. Shaalaa.com Balbharati solutions for Science and Technology 1