| Aim | To find the focal length of a convex mirror, using a convex lens. |
| Apparatus Required | A convex mirror A convex lens A light source (such as a light bulb or a laser) A screen or a piece of paper to capture the image A ruler or measuring tape A stand to hold the mirror, lens, and the light source |
| Theory | A convex mirror is a curved mirror that diverges light rays. The focal length of a convex mirror is negative and is equal to half of the radius of curvature of the mirror. To find the focal length of a convex mirror, we can use a convex lens as a converging lens and place it in front of the convex mirror. When the object is placed in front of the convex lens, a real and inverted image is formed, which is further reflected by the convex mirror. The final image formed by the mirror is virtual and smaller in size. |
| Procedure | Set up the apparatus by placing the convex mirror on the stand, and the light source on one side of the convex lens. Place the screen or piece of paper on the opposite side of the convex lens, so that it can capture the image formed by the lens. Place the convex lens in front of the convex mirror, at a known distance d from the mirror. Switch on the light source and adjust its position so that it is focused on the lens. Place an object (such as a pencil or a coin) at a known distance u from the convex lens. Adjust the position of the screen or paper until a clear and sharp image of the object is formed on it. Measure the distance v between the screen and the lens. Repeat steps 5 to 7 for different values of u, ranging from near the focal point to infinity. Record the values of u and v for each measurement. |
| Observation and Result | Observation: As the object is moved closer to the convex lens, the image distance v decreases. When the object is placed at the focal point of the convex lens, the image distance becomes infinite. When the object is placed between the focal point and the convex lens, the image distance becomes negative, indicating that the image is virtual and upright. As the object is moved further away from the convex lens, the image distance increases. The final image formed by the convex mirror is virtual and smaller in size than the image formed by the convex lens. Result: The focal length of the convex mirror can be calculated using the formula f = -d/ (1 – d/f), where d is the distance between the convex lens and the object, and f is the focal length of the convex lens. The values of u and v for different measurements can be plotted on a graph, with u on the x-axis and v on the y-axis. The graph should show a curve that intersects the x-axis at the focal point of the convex mirror. The slope of the graph can also be used to determine the focal length of the convex mirror. |
