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Investigation of the chromatic aberration of a converging lens.

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Introduction

Investigation of the chromatic aberration of a converging lens

                                By Wen Yan Gao

Aim

To find the difference in focal length of a converging lens when used to produce images with red and blue light.

Planning

Background information

Chromatic aberration arises from dispersion- the property that the refractive index of glass differs with wavelength of light.

The focal length of a lens is determined by a combination of its geometry and the refractive index of the material from which it is made. The refractive index varies slightly with the wavelength of the light that is being transmitted. This means that the focal length of a lens will vary for different colours of light. For blue light (short wavelengths), the focal length is larger than that of red light (long wavelengths).

Prediction

It was predicted that the values of the focal length of the lens obtained from red and blue light are different, given that the standard focal length is 10cm.

Proposed Method:

The method is to use the lens formula.

1/f = 1/u +1/v

To use this formula both the object distance (u)

...read more.

Middle

3.The 12V power supply was switched on. The screen was moved until a sharpest (brightest) image was formed on the screen.

4.The position of the grill (a), lens (b) and screen (c) against the metre rule was recorded.

5.Values for u and v were calculated by finding the differences between the positions.

6. 1/u and 1/v were calculated.

7. A graph of 1/u against 1/v was plotted.

8. The above procedures were repeated with red and blue light filters respectively.

9. The above method was repeated three times for each measurement of three different colours of lights to ensure that the results were accurate

List of Apparatus:

        Light bulb (light source)

        Red and blue light filters (turn white light into red and blue respectively.)

Object grids        (to help to make the image clear)

        Converging lens (focal length=10cm)

        Lens holder (to hold the lens firmly)

        Image screen (so image can be cleared seen and observed)

        Metre rule (to measure u and v)

        12V power supply (supplies power to the light bulb.)

Variables to be considered (fixed and changing)

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Conclusion

The values were suitable for producing a good line.

Possible sources of errors

The range of data was not big enough.

The image was not clear enough

The apparatus were not positioned in a straight line.

Implementing

Place the light and screen at opposite ends of the optical bench, and move a converging lens until a sharp image is formed on the screen. The distance between lens and screen is (nearly) the focal length of the lens.

Short (blue appearing) wavelengths are refracted more than long (red appearing) wavelengths. Consequently, lenses like the one shown above will not image light all in one place.

Chromatic aberration arises from dispersion- the property that the refractive index of glass differs with wavelength.

positioned along a straight line

Data Evaluation

Identification of Sources of Error

Possible sources of errors

The range of data was not big enough.

The image was not clear enough

The apparatus were not positioned in a straight line.

Variations in readings

Each set of readings was taken twice to ensure the accuracy.

As seen from the graph, there is no anomalies

The actual outcome was very similar to the expected result.

Summary

The chromatic aberration can be corrected by making the lens by different glass.

...read more.

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