Let's break down your questions one by one, starting with the effects of immersion in water on the focal lengths of concave and convex mirrors, as well as lenses.

Impact of Water on Focal Length

When we immerse optical devices like concave and convex mirrors or lenses in water, their focal lengths change due to the alteration in the refractive index of the medium surrounding them.

Concave and Convex Mirrors

For mirrors, the focal length is determined by the curvature of the mirror itself and is not directly affected by the medium in which they are placed. This is because mirrors reflect light rather than refract it. However, the effective focal length can appear to change when considering the overall optical system, especially if the surrounding medium has a different refractive index.

Lenses

In contrast, lenses do experience a change in focal length when placed in water. The focal length (f) of a lens is given by the lens maker's formula:

• 1/f = (n_lens - n_medium) * (1/R1 - 1/R2)

Here, n_lens is the refractive index of the lens material, n_medium is the refractive index of the surrounding medium (water in this case), and R1 and R2 are the radii of curvature of the lens surfaces. Since the refractive index of water (approximately 1.33) is greater than that of air (1.00), the focal length of the lens will increase when submerged in water. This means that the lens will converge light less effectively than it does in air.

Understanding Ammeter and Voltmeter Measurements

Next, let’s discuss the least count of ammeters and voltmeters, how they are measured, and which types are ideal for laboratory use.

Least Count Explained

The least count of an instrument is the smallest value that can be measured accurately with it. For an ammeter, this is typically determined by the smallest division on its scale. For example, if an ammeter has a scale where the smallest division is 0.1 A, then its least count is 0.1 A.

For a voltmeter, the least count is similarly defined. If a voltmeter has a scale with a smallest division of 0.2 V, then its least count is 0.2 V. The least count is crucial for determining the precision of measurements.

Ideal Instruments for Lab Use

In a laboratory setting, digital ammeters and voltmeters are often preferred due to their higher accuracy and ease of reading. An ideal ammeter should have a low range (e.g., 0-10 A) and a small least count (e.g., 0.01 A) for precise measurements of small currents. Similarly, an ideal voltmeter should have a range suitable for the experiments being conducted, often around 0-20 V, with a least count of 0.1 V or less for accurate voltage readings.

Focal Length of a Plane Mirror

Now, let’s address the focal length of a plane mirror. A plane mirror has a unique characteristic: its focal length is considered to be infinite. This is because the light rays reflecting off a plane mirror do not converge or diverge; they simply reflect back at the same angle they hit the mirror.

Why Infinite Focal Length?

To understand this concept, think of how a plane mirror works. When parallel rays of light strike a plane mirror, they reflect back parallel to their original path. Since they do not converge to a point, we say that the focal point is at infinity. In practical terms, this means that a plane mirror does not focus light like concave or convex mirrors do.

In summary, the immersion of lenses in water affects their focal lengths due to changes in refractive index, while mirrors remain unaffected in terms of focal length. The least count of ammeters and voltmeters is essential for precise measurements, with digital versions being ideal for lab use. Lastly, a plane mirror has an infinite focal length because it does not converge light.