| Aim | To find the refractive index of a liquid by using (i) concave mirror, (ii) convex lens, and plane mirror. |
| Apparatus Required | A liquid sample Concave mirror Convex lens Plane mirror Light source (such as a lamp or laser) A ruler or measuring tape A protractor or an angle measuring device |
| Theory | The refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. The refractive index of a liquid can be determined by measuring the focal length of a concave mirror or convex lens or by measuring the apparent and actual depths of an object viewed through a plane mirror submerged in the liquid. |
| Procedure | (i) Using concave mirror: Place the concave mirror on a stand and align it with a light source. Fill a container with the liquid sample and place it in front of the concave mirror. Adjust the distance between the concave mirror and the container until a sharp image of the light source is formed on the screen. Measure the distance between the concave mirror and the screen, and the distance between the light source and the concave mirror. Calculate the focal length of the concave mirror using the formula: f = r/2, where r is the radius of curvature of the concave mirror. Calculate the refractive index of the liquid using the formula: n = (1 + d/f), where d is the thickness of the liquid. (ii) Using convex lens: Place the convex lens on a stand and align it with a light source. Fill a container with the liquid sample and place it in front of the convex lens. Adjust the distance between the convex lens and the container until a sharp image of the light source is formed on the screen. Measure the distance between the convex lens and the screen, and the distance between the light source and the convex lens. Calculate the focal length of the convex lens using the formula: 1/f = (n – 1)(1/R1 – 1/R2), where R1 and R2 are the radii of curvature of the lens surfaces and n is the refractive index of the lens material. Calculate the refractive index of the liquid using the formula: n = f/(f – d), where d is the thickness of the liquid. (iii) Using plane mirror: Fill a container with the liquid sample and place it on a stand. Place a plane mirror at the bottom of the container. Place a ruler or measuring tape vertically in front of the container. Observe the apparent depth of the ruler through the liquid by looking at the ruler from the side of the container. Measure the actual depth of the ruler using the ruler or measuring tape. Calculate the refractive index of the liquid using the formula: n = (real depth/apparent depth). |
| Observation and Result | Observation: The focal length of the concave mirror or convex lens is dependent on the refractive index of the liquid. The apparent depth of the ruler viewed through the plane mirror is dependent on the refractive index of the liquid. The thickness of the liquid can be accurately measured using a ruler or measuring tape. Result: The refractive index of the liquid can be accurately determined using the concave mirror, convex lens, or plane mirror method. The accuracy of the measurement can be improved by taking multiple measurements and calculating the mean and standard deviation. |
