| Aim | To find the focal length of a concave lens, using a convex lens. |
| Apparatus Required | A convex lens A concave 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 lenses and the light source An object (such as a pencil or a coin) |
| Theory | A concave lens is a lens that diverges light rays. The focal length of a concave lens is the distance between the lens and its focal point. To find the focal length of a concave lens, we can use a convex lens of known focal length as a magnifying glass. When a concave lens is placed between the object and the convex lens, the image formed by the convex lens is magnified, and the focal length of the concave lens can be determined. |
| Procedure | Set up the apparatus by placing the convex lens on the stand, and the light source on one side of the 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 concave lens in front of the convex lens, with the concave lens closer to the light source. Place an object (such as a pencil or a coin) at a known distance from the concave lens. Adjust the position of the screen or paper until a clear and sharp magnified image of the object is formed on it. Measure the distance between the concave lens and the screen, which is the image distance. Measure the distance between the object and the convex lens, which is the object distance. Repeat steps 4 to 7 for different values of the object distance, ranging from near the focal point to infinity. Record the values of the object distance and the corresponding image distance for each measurement. |
| Observation and Result | Observation: As the object is moved closer to the concave lens, the image distance increases. When the object is placed at the focal point of the concave lens, the image distance becomes infinite. When the object is placed between the focal point and the concave lens, the image distance becomes negative, indicating that the image is virtual and upright. As the object is moved further away from the concave lens, the image distance decreases. The magnified image of the object is formed by the convex lens, and the concave lens acts as a magnifying glass. The values of the object distance and the corresponding image distance can be used to determine the focal length of the concave lens. Result: The focal length of the concave lens can be calculated using the formula 1/f = 1/v – 1/u, where u is the distance between the object and the concave lens, and v is the distance between the image and the convex lens. The values of the object distance and the corresponding image distance can be used to calculate the focal length for different measurements. The focal length can be determined by taking the average of the calculated values. The accuracy of the measurement can be improved by taking multiple measurements and calculating the mean and standard deviation. |
