Std 10th Science Part 1 – Chapter 7 : Lenses Exercise Questions And Answers Maharashtra Board

Q1. Match the columns in the following table and explain them.

Column 1	Column 2	Column 3
Farsightedness	Nearby object can be seen clearly	Bifocal lens
Presbyopia	Far away object can be seen clearly	Concave lens
Nearsightedness	Problem of old age	Convex lens

Answer –

Column 1	Column 2	Column 3	Explanation
Farsightedness	Nearby object can be seen clearly	Bifocal lens	In Farsightedness, far objects are clear. It’s corrected with a convex lens (positive power) to increase the overall converging power of the eye.
Presbyopia	Far away object can be seen clearly	Concave lens	This defect is due to the eye lens’s decreased focusing power with age. It requires a bifocal lens to correct both near and far vision.
Nearsightedness	Problem of old age	Convex lens	In Nearsightedness, near objects are clear. It’s corrected with a concave lens (negative power) to diverge light and decrease the eye’s converging power.

Q2. Draw a figure explaining various terms related to a lens.

Explanation of Terms:

Centre of Curvature (C): The centers of the spheres (C1, C2) whose parts form the surfaces of the lenses.
Radius of Curvature (R): The radii (R1, R2) of these spheres.
Principal Axis: The imaginary line passing through both centers of curvature.
Optical Centre (O): The point inside the lens on the principal axis through which light rays pass without changing direction.
Principal Focus (F): The point on the principal axis where rays parallel to the axis meet (convex lens) or appear to come from (concave lens) after refraction.

Focal Length (f): The distance between the optical centre (O) and the principal focus (F).

Q3. 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.

Position of Object: The object must be placed at 2F₁.
Position of Image: The image is formed at 2F₂.
Size of Image: Same size as the object.
Nature of Image: Real and inverted.

Q4. Give scientific reasons:

a. Simple microscope is used for watch repairs.

A simple microscope is a convex lens with a short focal length that is used as a magnifying lens.
When repairing watches, tiny parts need to be seen as a virtual, erect, and larger image.
The simple microscope provides this up to 20 times larger image, making the small parts easier to handle and inspect for defects.

b. One can sense colours only in bright light.

The retina of the eye has two types of light-sensitive cells: rod-like cells and conical cells.
Rod cells respond to the intensity (brightness/dimness) of light.
Conical cells respond to colour.

Conical cells do not respond to faint light, so colours are only perceived in bright light when these cells are sufficiently excited.

c. We cannot clearly see an object kept at a distance less than 25 cm from the eye.

For a normal human eye, the closest distance at which an object can be seen clearly without stress is the minimum distance of distinct vision, which is 25 cm.
If an object is brought closer than 25 cm, the ciliary muscles holding the lens cannot adjust the focal length enough to form a sharp image on the retina. The image will be blurry, and the eye will be under stress.

Q5. Explain the working of an astronomical telescope using refraction of light.

Principle : A refracting telescope uses two main convex lenses: an objective lens and an eyepiece.

Working:

The objective lens has a large diameter and a larger focal length to collect the maximum amount of light from a distant astronomical object.
The objective forms a real image of the distant object, which acts as the object for the eyepiece.
The eyepiece is a smaller convex lens with a smaller focal length. It magnifies the image formed by the objective.
The final image is a highly magnified image of the distant object. The lenses are fitted in a metallic tube to allow the distance between them to be changed for focusing.

Q6. Distinguish between:

a. Farsightedness and Nearsightedness

Farsightedness (Hypermetropia)	Nearsightedness (Myopia)
Clear vision for distant objects; fuzzy for nearby objects.	Clear vision for nearby objects; indistinct for distant objects.
Image forms behind the retina.	Image forms in front of the retina.
Caused by : Lens curvature too small (less converging power) or eyeball too short.	Caused by : Lens curvature too large (more converging power) or eyeball too long.
Defect can be corrected using a convex lens (positive power).	Defect can be corrected using concave lens (negative power).

b. Concave lens and Convex Lens

Concave Lens (Divergent)	Convex Lens (Convergent)
Thinner in the center and thicker at the edges.	Thicker in the center and thinner at the edges.
Light rays diverge after refraction; it is called a divergent lens.	Light rays converge after refraction; it is called a converging lens.
Forms only virtual, erect, and smaller images.	Can form both real/virtual and larger/smaller images, depending on object position.
Focal length (f) is negative.	Focal length (f) is positive.

Q7. What is the function of iris and the muscles connected to the lens in human eye?

Answer –

The Iris and the muscles connected to the eye lens have crucial functions that allow us to see clearly in different conditions.

1. Function of the Iris

The Iris is the dark, fleshy screen behind the cornea. It has a small hole at its center called the pupil.

The main function of the Iris is to control the amount of light entering the eye.
It does this by changing the diameter of the pupil.
- If the light falling on the eye is too bright, the pupil contracts.
- If the light is dim, the pupil widens.

2. Function of the Ciliary Muscles (Muscles connected to the lens)

The muscles near the lens (ciliary muscles) change the shape of the double convex transparent crystalline lens.

Their primary function is to adjust the focal length of the eye lens to focus the image clearly on the retina.
This capacity of the lens to change its focal length as needed is called the power of accommodation.
- When seeing distant objects, the lens becomes flat, and its focal length increases.

When seeing nearby objects, the lens becomes more rounded, and its focal length decreases.

Q8. Solve the following examples.

i. 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?

Formula: Power P = 1/f(m)
Given: P = +1.5 D
Focal Length (f):
f = 1 / P = 1 / +1.5 m ≈ +0.67 m
Lens Type: Since the power is positive, the lens is a convex lens.
Defect of Vision: A convex lens is used to correct farsightedness (hypermetropia).

ii. 5 cm high object is placed at a distance of 25 cm from a converging lens of focal length of 10 cm. Determine the position, size and type of the image.

Given:
- Converging lens: Convex lens.
- Object height (h₁) = 5 cm
- Object distance (u) = −25 cm (always negative )
- Focal length (f) = +10 cm (positive for convex lens )
1. Position of the Image (v):
- Lens Formula: 1/v − 1/u = 1/f
  1/v = 1/u + 1/f = 1/−25 + 1/10 = −2+5 / 50 = 3 / 50
  v = 50 / 3 ≈ +16.7 cm
2. Size of the Image (h₂):
- Magnification Formula: h₂/ h₁= v / u
3. Type of Image:
- Since v is positive and h₂ is negative, the image is Real and inverted.
Answer: Position: +16.7 cm (on the opposite side of the lens). Size: 3.3 cm (inverted). Type: Real and inverted.

iii. 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?

Formula: P = P₁ + P₂+ P₃ (Total power is the sum of individual powers )
Given: P₁= 2 D, P₂ = 2.5 D, P₃= 1.7 D
Calculation:
P = 2 D + 2.5 D + 1.7 D = 6.2 D

Answer: The total power of the lens combination is 6.2 D.

iv. 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?

Given:
- Object distance (u) =−60 cm (always negative )
- Virtual image is formed in front of the lens (on the left side, same side as the object), so v is negative : v = −20 cm.
1. Focal Length (f):

2. Type of Lens:
- Since the focal length (f) is negative, the lens is a diverging lens (concave lens).

Answer: Focal length: −30 cm. Type: Diverging lens (Concave).

Lenses Class 10th Chapter 7 : Science Part 1 Exercise Questions And Answers Maharashtra Board