Comprehensive Study on Image Formation by Convex and Concave Lenses

Principles of Image Formation by Lenses

Lenses perform the essential function of forming images through the process of refraction, which is the bending of light as it passes from one medium into another. The nature, position, and relative size of these images depend entirely on the type of lens used—whether convex or concave—and the specific position of the object relative to the lens's optical components, such as the optical centre $O$, the focal points $F$, and the centers of curvature $2F$.

Activity 9.12: Experimental Procedure for Convex Lenses

To systematically study how a convex lens forms images, a specific experimental setup is required. The first step involves taking a convex lens and determining its approximate focal length, using the method previously described in Activity 9.11. Once the focal length is known, five parallel straight lines are drawn with chalk on a long table. The distance between each successive line must be exactly equal to the measured focal length of the lens.

The lens is then placed on a lens stand and positioned on the central line so that the optical centre $O$ of the lens lies directly over it. The two lines on either side of the lens then represent the focus $F$ and the center of curvature $2F$. These positions are marked with the letters $F_1$ and $2F_1$ on the left side, and $F_2$ and $2F_2$ on the right side.

To begin the observation, a burning candle is placed far beyond $2F_1$ on the left side. The researcher must then move a screen on the opposite side of the lens to obtain a clear and sharp image of the flame. After identifying the nature, position, and relative size of this image, the activity is repeated by placing the candle at several distinct intervals: just behind $2F_1$, between $F_1$ and $2F_1$, at the focal point $F_1$, and finally between $F_1$ and the optical centre $O$. All observations regarding the image's characteristics are then tabulated.

Table 9.4: Nature, Position, and Relative Size of Images Formed by a Convex Lens

When an object is placed at infinity relative to a convex lens, the resulting image is formed at the focus $F_2$. This image is highly diminished and point-sized, possessing a nature that is real and inverted. As the object moves closer to the lens and is positioned beyond $2F_1$, the image shifts to a position between $F_2$ and $2F_2$. In this scenario, the image is diminished in size but remains real and inverted.

Placing the object exactly at $2F_1$ results in an image forming at $2F_2$. Interestingly, the relative size of the image is equal to the size of the object (same size), and the nature is real and inverted. When the object is moved between the focus $F_1$ and $2F_1$, the image is formed beyond $2F_2$, becoming enlarged while maintaining its real and inverted nature.

Upon placing the object at the focus $F_1$, the image is formed at infinity. In this state, the nature of the image remains real and inverted. Finally, if the object is placed between the focus $F_1$ and the optical centre $O$ of the lens, the image is formed on the same side of the lens as the object. This particular image is enlarged and changes its nature to virtual and erect, meaning it cannot be captured on a screen.

Image Formation by Concave Lenses

A separate activity is conducted to study the nature, position, and relative size of the image formed by a concave lens. This involves placing a burning candle in front of the concave lens and looking through the lens from the opposite side. Unlike the convex lens, the researcher will find it difficult or impossible to capture a real image on a screen, as the concave lens typically produces virtual images. The candle or object is moved to different positions to observe changes in the image's appearance directly through the lens medium.

Table 9.5: Nature, Position, and Relative Size of Images Formed by a Concave Lens

For a concave lens, the image characteristics are more limited in variety compared to the convex lens. When the object is at infinity, the image is formed at the focus $F_1$. The size of the image is highly diminished and point-sized, and its nature is virtual and erect.

When the object is placed at any position between infinity and the optical centre $O$ of the lens, the image is formed between the focus $F_1$ and the optical centre $O$. Regardless of the specific distance in this range, the image remains diminished in size and continues to be virtual and erect. This indicates that a concave lens will always produce an image that is virtual, erect, and smaller than the object, no matter where the object is placed.