to measure the focal length of a concave mirror
equipment
- Concave mirror
- An illuminated object
- Ray box
- A sheet of white cardboard
- A metre stick
method
- Set up apparatus as shown in diagram.
- Move the screen until a clear inverted image of the crosswire (illuminated object) is obtained.
- Measure the distance u (object distance from mirror) and v (image distance from mirror).
- Record these values.
- Repeat this procedure for different values of u.
- Plot a graph of 1/u against corresponding values of 1/v.
- Draw the straight line that best fits the points and produce it to intersect both axes.
conclusion
sources of error
Errors arise when:
There is a larger error in v than in u since u only involves error using the metre stick.
- deciding when the image on the screen is in sharpest focus.
- measuring the distances u and v with the metre stick.
There is a larger error in v than in u since u only involves error using the metre stick.
to verify snell's law and hence to measure the refractive index of glass
equipment
- A rectangular glass block
- A sheet of paper
- A pencil
- A ruler
- A protractor
method
- Place a glass block on a sheet of paper and mark it's outline.
- Shine a ray of light from a ray box as shown.
- Mark the position of the light ray as it enters and leaves the block and remove the block.
- Draw in the normals and measure i and r.
- Record the results and plot a graph of sin i against sin r.
- Draw a straight line though the origin.
conclusion
- Within experimental error all the points will lie on a straight line passing through the origin.
- The fact that the graph is a straight line through the origin verifies that Sin i ∝ Sin r.
- The slope of the line is the refractive.
sources of error
- The light beam will spread out, or diverge, making it difficult to mark its position.
- Larger angles will lead to a smaller percentage error.
to find the refractive index of a liquid
Equipment
- Beakers of various depths
- Retort stand
- Cork
- 2 pins
- Plane mirror
- Metre stick
method
- Set up the apparatus as shown. Focus on the search pin and the pin in the water
- Adjust the height of the search pin until there is no parallax between the two images, as shown in the diagram.
- Measure the distance from the search pin in the cork to the back of the mirror. This is the apparent depth. Measure the depth of the container. This is the real depth.
- Repeat for different depths of water.
conclusion
- Plot a graph of real depth against apparent depth.
- The slope of the line will be the refractive index.
sources of error
- Errors arise in both locating the position of no parallax and in measuring the distances with the metre stick.