Problem 11 - Master Resource Book in JEE Main Physics

Question

A point object is placed at a distance of \(30 \mathrm{~cm}\) from a convex mirror of a focal length \(30 \mathrm{~cm}\). The image will form at [al infinity (b) pole (c) \(15 \mathrm{~cm}\) behind the mirrot (d) no image will be formed

Step-by-Step Solution

Answer

The image forms at the pole.

Step 1: Identify given values

The distance of the object from the mirror, denoted as \( u \), is \( 30 \text{ cm} \). The focal length of the convex mirror \( f \) is also \( 30 \text{ cm} \).

Key Concepts

Focal Length

The focal length of a mirror is an essential concept in understanding how it manipulates light. In the context of a convex mirror, the focal length is the distance between the mirror's surface and its focal point. Convex mirrors have a positive focal length, which implies that the focal point is located on the side opposite to the object.

To determine the behavior of a convex mirror, you can use this simple equation:

• The mirror formula: \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \)
• Here, \( f \) is the focal length, \( v \) is the image distance, and \( u \) is the object distance.

In our exercise, the given focal length is \( 30 \text{ cm} \). Understanding this helps predict the nature and position of the image formed by the mirror. Because convex mirrors diverge light rays, the focal length aids in calculating where the virtual image will appear.

Image Formation

Image formation by convex mirrors is unique because they always form virtual, diminished, and erect images of real objects. These properties are due to the fact that convex mirrors reflect light in a way that the reflected rays seem to originate from a focal point behind the mirror.

In the specific exercise, the object is placed at \( 30 \text{ cm} \), which coincidentally is exactly the focal length of the mirror. The image formed in this situation is virtual and located between the focal point and the mirror.

Key properties of convex mirror image formation:

• The image is always virtual, which means it cannot be projected on a screen because the rays only appear to meet behind the mirror.
• It is reduced in size compared to the actual object, making it useful for purposes like rear-view mirrors in vehicles.
• The image is erect, maintaining the same orientation as the object.

Understanding these properties helps in predicting how and where images will form when viewing objects through convex mirrors.

Distance from Mirror

Distance from the mirror affects how the image is perceived in a convex mirror. In this exercise, the object is placed \( 30 \text{ cm} \) away, coinciding with the focal length. This scenario has a unique effect on image formation.

When calculating the image distance \( v \) using the mirror formula: \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), plugging in our values, we find that the image forms closer to the mirror than the focal length. Plugging in \( f = 30 \text{ cm} \) and \( u = -30 \text{ cm} \) into the equation, you solve for \( v \). The negative sign for \( u \) reflects that the object distance is measured opposite to the direction of the incident light.

Convex mirrors tend to form images closer and far smaller than the objects themselves, making them practical for views like in security mirrors. As distance from the mirror increases, the size of the image further diminishes, providing a greater field of view.