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Q.- 1. Explanation of the Power of a Lens
The ability of a lens to bend (converge or diverge) light rays is referred to as its power or capacity.
- Definition: The power of a lens is equal to the inverse of its focal length ($f$).
- Formula:
$$P = \frac{1}{f \text{ (in meters)}}$$ - Unit: Its SI unit is the Dioptre, which is denoted by the symbol ‘D’.
- If the focal length of a lens is $1$ meter, its power will be $1 \text{ D}$.
Key Points:
- Convex Lens: Its power is always positive (+).
- Concave Lens: Its power is always negative (-).
- Other Options:
- Meter: The unit of distance.
- Watt: The unit of power (electrical or mechanical).
- Joule: The unit of energy or work.
Q. 2 How Ciliary Muscles Change Focal Length
The ability of the eye to adjust its focal length to see objects at different distances is called Accommodation. This process is managed entirely by the Ciliary Muscles.
- For Distant Objects: When you look at something far away, the ciliary muscles relax. This pulls the suspensory ligaments taut, making the eye lens thinner (less convex). A thinner lens has a longer focal length, allowing the light rays from a distance to focus perfectly on the retina.
- For Nearby Objects: When you look at something close (like your phone), the ciliary muscles contract. This releases the tension on the ligaments, allowing the lens to become thicker and more rounded. A thicker lens has a shorter focal length, which is necessary to bend light rays more sharply to hit the retina.
| Part | Function |
| Pupil | The opening that lets light into the eye. |
| Retina | The “screen” at the back of the eye where the image is formed. |
| Iris | The colored part that controls the size of the pupil. |
| Ciliary Muscles | The “engine” that changes the shape/focal length of the lens. |