- Hypermetropia is a condition in which distant objects are seen clearly, but nearby objects appear blurred.
- The near point shifts beyond 25 cm, making close-up tasks like reading difficult.
- The image of nearby objects forms behind the retina.
- Causes include reduced curvature of the lens or cornea and shortening of the eyeball.
- It is corrected using a convex lens of positive power, which converges light rays to focus the image on the retina.
Definitions [17]
Definition: Lens
A lens is a transparent refracting medium bounded by either two spherical surfaces, or one spherical surface and the other surface plane.
OR
A lens is a transparent medium bound by two surfaces.
OR
A lens is a transparent medium (such as glass) bounded by two curved surfaces or one curved and one plane surface.
Definition: Converging Lens or Convex Lens
A lens which bulges out in the middle, is a convex lens. A light beam converges on passing through such a lens, so it is also called a converging lens.
OR
The lens which has two spherical surfaces which are puffed up outwards is called a convex or double convex lens.
OR
The lenses which are thicker in the middle and thinner at the edges, are called 'convex lenses'.
Definition: Diverging Lens or Concave Lens
A lens which is bent inwards in the middle is a concave lens. Such a lens diverges the light rays incident on it, so it is also called a diverging lens.
OR
This lens is thicker near the centre as compared to the edges. The lens with both surfaces spherical on the inside is called a concave or double concave lens.
OR
The lenses which are thinner in the middle and thicker at the edges, are called 'concave lenses'.
Definition: Centre of Curvature
The centres of spheres whose parts form surfaces of the lenses are called centres of curvatures of the lenses.
Definition: Radius of Curvature
The radii (R1 and R2) of the spheres whose parts form surfaces of the lenses are called the radii of curvature of the lens.
Definition: Optical Centre
The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre of a lens.
OR
The point on the principal axis of a lens such that a ray of light directed towards it emerges from the lens in the same direction, without deviation.
Definition: Principal Focus
Principal focus (F) is the point on the principal axis at which light rays parallel to the principal axis converge after passing through a convex lens.
Definition: Focal Length
The distance between the optical centre and principal focus of a lens is called its focal length.
Definition: Principal Axis
The imaginary line passing through both centres of curvature is called the principal axis of the lens.
OR
The line joining the centres of curvature of the surfaces of the lens is called the 'principal axis' of the lens.
Define the power of a lens.
Power of a lens is defined as the ability of a lens to bend the rays of light. It is given by the reciprocal of focal length in metre.
The power of a lens is a measure of the deviation produced by it in the path of rays refracted through it.
Definition: Power of a Lens
The deviation of the incident light rays produced by a lens on refraction through it, is a measure of its power.
or
The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P.
OR
The power (P) of a thin lens is equal to the reciprocal of its focal length (f) measured in metres.
Define the following term:
Adaptation
Adaptation is the process by which the human eye adjusts to changes in light intensity.
- Light Adaptation: When a person moves from a dark environment to a brightly lit area (e.g., stepping out of a cinema hall in the afternoon), they initially experience a dazzling effect. After a few seconds, the eyes adjust to the brightness. This process is called light adaptation.
- Dark Adaptation: When a person enters a dark area from a brightly lit environment (e.g., entering a cinema hall), they initially struggle to see clearly. Gradually, their vision improves as the eyes adapt to the darkness. This process is called dark adaptation.
Definition: Simple Microscope
A convex lens with small focal length produces a virtual, erect and bigger image of an object as shown in the figure. Such a lens is called simple microscope or magnifying lens.
Definition: Compound Microscope
"A compound microscope is made of two convex lenses: objective and eyepiece."
Definition: Telescope
Telescope is used to see distant objects clearly in their magnified form.
Definition: Astronomical Telescopes
The telescopes used to observe astronomical sources like the stars and the planets are called astronomical telescopes.
Definition: Persistence of Vision
Persistence of vision is the phenomenon where an image stays on the retina for about 1/16th of a second even after the object is removed.
Formulae [4]
Formula: Lens Magnification
Magnification (m) = \[\frac{\text{Height of the Image}}{\text{Height of the object}}=\frac{h^{\prime}}{h}\]
Magnification in terms of object and image distances:
Magnification (m ) = \[\frac {h'}{h}\] = \[\frac {v}{u}\]
Formula: Lens Formula
\[\frac {1}{v}\] - \[\frac {1}{u}\] = \[\frac {1}{f}\]
Formula: Power of a Lens
Power of lens (in D) = \[\frac{1}{\text{focal length (in metre)}}\]
or
P = \[\frac {1}{f}\]
or
P = \[\frac {1}{f (m)}\]
Power of a Lens in a Medium:
P = (n2 - n1)\[\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)\] = \[\frac {n_1}{f}\]
Formula: Combination of Lenses
- If two lenses with focal lengths f1 and f2 are kept in contact with each other, the combination has an effective focal length given by
\[\frac {1}{f}\] = \[\frac {1}{f_1}\] + \[\frac {1}{f_2}\]
Key Points
Key Points: Concept of Lenses
- Lenses are widely used in daily life, such as in spectacles, peepholes, magnifiers, and telescopes.
- Light passing through a lens undergoes refraction twice: once on entering and once on exiting the lens.
- The shape of a lens affects the direction of light; convex lenses converge light, while concave lenses diverge it.
- Most lenses have surfaces that are parts of spheres, with common types including biconvex, biconcave, plano-convex, and meniscus lenses.
Key Points: Characteristics and Location of Images for a Convex Lens
| S. No. | Position of the Object | Position of the Image | Size of the Image | Nature of the Image |
|---|---|---|---|---|
| 1 | At infinity | At focus (F₂) | Point image | Real and inverted |
| 2 | Beyond (2F₁) | Between (F₂) and (2F₂) | Smaller | Real and inverted |
| 3 | At (2F₁) | At (2F₂) | Same size | Real and inverted |
| 4 | Between (F₁) and (2F₁) | Beyond (2F₂) | Larger | Real and inverted |
| 5 | At focus (F₁) | At infinity | Very large | Real and inverted |
| 6 | Between (F₁) and O | On the same side of the lens as the object | Very large | Virtual and erect |
Key Points: Characteristics and Location of Images for a Concave Lens
| S. No. | Position of the Object | Position of the Image | Nature of the Image | Size of the Image | Remarks / Applications |
|---|---|---|---|---|---|
| 1 | At infinity | At focus (F₂), on the same side as the object | Virtual and erect | Highly diminished | — |
| 2 | At any finite distance | Between focus (F) and optical centre, same side | Virtual and erect | Diminished | Image moves closer to optical centre as object nears |
| 3 | General behaviour of concave lens | Always on the same side of the object | Virtual and upright | Always smaller than object | Independent of object position |
| 4 | As object moves closer to the lens | Image shifts from F₂ towards optical centre | Virtual and erect | Gradually increases, still < object | — |
| 5 | — | — | Forms virtual, diminished image | — | Used in spectacles (myopia) and Galilean telescopes |
Key Points: Sign Convention
- Pole (mirror) or optical centre (lens) is the origin; principal axis is the X-axis.
- Distances to the right are positive, to the left are negative; heights above the axis are positive, below are negative.
- Concave mirror: and R are negative; Convex mirror: and R are positive.
- Real images: image distance and magnification are negative; Virtual images: both are positive.
- Lenses are always negative; they are positive for real images and negative for virtual images; they are positive for convex lenses and negative for concave lenses.
Key Points: Human Eye
- The human eye works like a camera, forming a real and inverted image on the retina, which is light-sensitive.
- The cornea allows light to enter the eye and performs most of the refraction, while the lens fine‑tunes the focus.
- The iris controls the pupil, regulating the amount of light entering the eye—contracting in bright light and widening in dim light.
- The power of accommodation is the ability of the eye lens to change its focal length by altering its curvature using the ciliary muscles.
- For a normal eye, the near point is 25 cm and the far point is at infinity.
Key Points: Myopia
- Myopia is a vision defect in which distant objects appear blurry, while near objects are seen clearly.
- This occurs because the image of distant objects forms on the retina.
- The far point is not at infinity but is shifted closer to the eye.
- Causes include increased curvature of the cornea/lens or elongation of the eyeball.
- Corrected using a concave lens of negative power, which diverges light rays to focus the image on the retina.
Key Points: Hypermetropia
Key Points: Presbyopia
- Presbyopia is an age-related vision defect where the eye’s ability to focus on nearby objects decreases.
- It is caused by weakened ciliary muscles and reduced flexibility of the eye lens.
- The near point shifts farther, making close-up vision difficult.
- Bifocal lenses are commonly used for correction—concave at the top (for myopia) and convex at the bottom (for hypermetropia).
- It can also be corrected with contact lenses or, in some cases, surgery.
Key Points: Use of Concave Lenses
- Laser devices such as scanners, CD players, and medical equipment use concave lenses to achieve proper focus.
- Door peepholes use concave lenses to provide a wider view of the outside through a small opening.
- Spectacles for myopia use concave lenses to correct nearsightedness.
- Torchlights use concave lenses to spread light from a small bulb over a wide area.
- Cameras, telescopes, and microscopes mainly use convex lenses, but may also use concave lenses to improve image quality.
key Points: Use of Convex Lenses
- A simple microscope uses a convex lens to show a bigger, erect image of nearby objects.
- A compound microscope uses two convex lenses to achieve high magnification of tiny objects, such as cells.
- In compound microscopes, the objective has a small focal length, and the eyepiece has a longer focal length.
- Telescopes use convex lenses to magnify distant objects, such as stars; the objective collects light, and the eyepiece magnifies it.
- Convex lenses are also used in cameras, projectors, and spectrographs for focusing and image formation.
Key Points: Persistence of Vision
- Rod cells in the retina detect light intensity and help in vision under dim light, while cone cells detect colour and function in bright light.
- Cone cells are sensitive to red, green, and blue light; the brain processes their signals to produce the perception of colour.
- Colour blindness occurs when certain cone cells are absent or non-functional, causing difficulty in distinguishing specific colours, though general eyesight remains normal.
Important Questions [18]
- An Object is Held 20 Cm Away from a Converging Lens of Focal Length 10 Cm. Find the Position of the Image Formed.
- Give a scientific reason. Simple microscope is used for watch repairs.
- Calculate the Focal Length of a Corrective Lens Having Power +2d.
- Calculate the Focal Length of a Corrective Lens Having Power +2.5 D.
- Kavita from 10th is Using Spectacles. the Power of the Lenses in Her Spectacles is –2.5 Dioptre. Answer the Following Questions:Which Lenses Are Used in Her Spectacles? and State the Defect of Vision Kavita is Suffering From.
- Surabhi from Std. X Uses Spectacle. the Powr of the Lenses in Her Spectacle is 0.5 D. Answer the Following Questions from the Given Information
- If focal length of a convex lens is 20 cm at what is the power of the lens?
- The power of convex lens of focal length 20 cm is ______.
- In simple microscope ______ lens is used.
- Observe the figure and answer the following questions: Name the defect of vision represented in the above figure. State the reasons for this defect.
- Rewrite the Following Table So as to Match Second and Third Column with First Column.
- Anuja Cannot See the Blackboard Writing but She Can See Nearby Things. (A) What is the Eye Defect She is Suffering From? (B) State the Possible Reason for Her Defect
- In a Std. X Class Out of 40 Students 10 Students Use Spectacles, 2 Students Have Positive Power and 8 Students Have Negative Power of Lenses in Their Spectacles.
- Given Below is a Diagram Showing a Defect of Human Eye. Study It And Answer the Following Questions.
- What is Myopia?
- Explain Two Possible Reasons of Myopia. How Can It Be Corrected? Explain with a Suitable Diagram.
- Complete the following table by observing the given figures: Figure → Points ↓ (a) Name of the defect (b) Position of the image (c) Lens used to correct the defect ______ ______ ______ ______ ______
- Anuja’S Father Cannot See Nearby Objects Clearly. (A) What is the Eye Defect He is Suffering From? (B) How is It Corrected?
Concepts [16]
- Concept of Lenses
- Images Formed by Convex Lenses
- Images Formed by Concave Lenses
- Sign Convention
- Lens Formula
- Magnification
- Power of a Lens
- Combination of Lenses
- The Human Eye
- Defects of Vision and Their Corrections > Myopia
- Defects of Vision and Their Corrections > Hypermetropia
- Defects of Vision and Their Corrections > Presbyopia
- Apparent Size of an Object
- Use of Concave Lenses
- Use of Convex Lenses
- Persistence of Vision
